Semi-DIY Power Station for Home Blackout Backup

Version 13. A complete build guide: decisions, calculations, parts, wiring, settings, testing, housing and safe use. What changed in each version is listed in section 24.

Interactive 3D wiring model (12 / 24 / 48 V, 1 to 3 batteries of 50–400 Ah, inverter 1000–5000 W, single box or modular stack, inverter on the floor or wall-mounted with automatic choice of the smallest box, internal or external charger, solar and 12 V/USB on or off, runtime slider, prices and a prebuilt comparison): open the 3D model

Safety: sizing rules, fuses, cables, battery handling, 230 V safety, fire and emergency procedures are in the separate safety guide. Read it before buying parts.

Prices are approximate German/EU street prices as of autumn 2026 and change often. Check current prices before buying. Where a component manual differs from this guide, follow the manual. I'm not an electrician. Anything connected to your house wiring must be done by a licensed electrician (Elektrofachkraft).

Contents

1. Recommended setup at a glance
2. 12 V or 24 V
3. How the AC side works: switchover options
4. Calculate your blackout loads
5. Size every component
6. Bill of materials
7. Tools and consumables
8. Battery bank: one battery or several
9. Wiring diagram
10. Cable and fuse table
11. Housing: enclosure, layout and mounting
12. Build steps
13. Settings for every device
14. Commissioning and testing
15. Using it in a blackout
16. Earthing, RCDs and gas boilers
17. Fire safety
18. Legal and insurance notes for Germany
19. Maintenance and storage
20. Common mistakes
21. Troubleshooting
22. Alternatives and upgrade paths
23. Checklists and quick reference card
24. Revision notes

1. Recommended setup at a glance

2. 12 V or 24 V

Why voltage matters

Power = voltage × current. At 12 V every watt needs twice the current of 24 V. Current decides cable size, fuse size, heat and losses. Cable losses scale with current squared, so 24 V has about a quarter of the cable losses of 12 V at the same power.

Comparison

At 48 V the maximum charge voltage is 58.4 V, still below the 60 V DC limit for extra-low voltage. It's safe to touch, but arcs are stronger: use DC-rated parts for at least 60 V (switch, fuses, breakers).

Decision rule

flowchart TD
  A{Is your main output 230 V AC?} -->|No, mostly 12 V devices under 1 kW| V12[12 V]
  A -->|Yes| B{More than about 5 kWh or 3 kW, or a future home battery?}
  B -->|No| V24[24 V: recommended for blackout backup]
  B -->|Yes| V48[48 V]

• 12 V if most of what you power runs on 12 V and AC loads stay under about 1 kW.
• 24 V if your main output is 230 V AC or you need more than about 1.2 kW. For home blackout backup, choose 24 V.
• 48 V if you plan more than about 5 kWh or more than 3 kW of AC load, or want to grow into a home battery later. The 3D model supports 48 V with 50–200 Ah batteries and 1500–5000 W inverters.

Don't start at 12 V planning to "upgrade later" by putting batteries in series. You'd have to replace the inverter, charger, MPPT and DC-DC converter.

3. How the AC side works: switchover options

The question is what happens to your fridge and router in the moment the grid fails.

Option B1: separate inverter and charger, manual

flowchart LR
  W1[Wall socket] --> C[Charger] --> B[Battery] --> I[Inverter] --> O[Box outlet]
  W2[Wall socket] --> F[Fridge, normally]
  O -. replug by hand during an outage .-> F

The charger and inverter are two independent devices with no connection between their AC sides. Normally the fridge is plugged into the wall. When the grid fails, the fridge stops, and someone has to unplug it from the wall and plug it into the box. At night or while you're away, it stays off.

Option B2: separate inverter and charger, always-on

flowchart LR
  W[Wall socket] --> C[Charger] --> B[Battery] --> I[Inverter] --> D[Fridge and router, permanently]

Your devices are permanently plugged into the inverter, and the charger keeps the battery full. When the grid fails, the charger simply stops and the battery takes over. No switchover, no interruption.

Option B3: separate inverter + automatic transfer switch (ATS)

flowchart LR
  W[Wall socket] --> ATS{Automatic transfer switch}
  I[Inverter, always on] --> ATS
  ATS --> D[Fridge and router]

The ATS (€30–100) feeds your devices from the grid and switches to the inverter when the grid fails. Switching takes about 10–50 ms, usually fine for fridges, sometimes too slow for PCs. A plug-in ATS is fine. A hard-wired one is a job for an electrician. The inverter's idle draw still runs 24/7.

Option A: inverter/charger with automatic switchover (e.g. Victron MultiPlus)

flowchart LR
  W[Wall socket] --> AIN[AC in]
  subgraph MP[MultiPlus inverter/charger]
    AIN --> R{Transfer relay}
    AIN --> CH[Charger]
    INV[Inverter] --> R
  end
  CH --> B[Battery]
  B --> INV
  R --> AOUT[AC out] --> D[Fridge, router, lights]

One device with an AC input (plugged into the wall) and an AC output (your devices):

1. Grid on: an internal relay connects input to output. Devices run on the grid, the battery is charged, and the inverter is idle.
2. Grid fails: the unit detects it within milliseconds, opens the relay (which also isolates you from the grid, so nothing feeds back) and starts the inverter. The gap is about 20 ms, too short for a fridge, router or most PCs to notice.
3. Grid returns: the inverter synchronizes to the grid's phase, closes the relay and hands over without a gap.
4. PowerAssist: if your load exceeds the set wall-input limit, the battery adds power.

Which to choose

flowchart TD
  Q1{Must the fridge keep running with nobody at home?} -->|No| B1[B1: manual replug]
  Q1 -->|Yes| Q2{Budget for 400 to 700 euro more?}
  Q2 -->|Yes| A[A: MultiPlus with automatic switchover]
  Q2 -->|No| Q3{Accept about 50 to 70 euro per year running cost?}
  Q3 -->|Yes| B2[B2: always-on inverter]
  Q3 -->|No| B3[B3: inverter plus automatic transfer switch]

3.1 Charger inside the box or external

The charger doesn't have to live in the box. Instead, fit a DC charge input: an Anderson SB50 panel socket wired through a fuse to the busbars. Any LiFePO4 charger with a matching SB50 plug then charges the whole bank from outside.

flowchart LR
  EC[External LiFePO4 charger] -->|SB50 plug, blue| IN[Charge input socket]
  IN -->|+ through fuse| PB[+ busbar]
  IN -->|−| NB[− busbar, load side of the shunt]

Rules:

• Different colours for different inputs: charge input blue, solar input grey, battery links red. Anderson housings of different colours can't mate, so solar panels (up to ~85 V) can never be plugged into the charge input.
• Fuse the charge input next to the + busbar: 20 A (24 V, 15 A charger), 25 A (12 V, 20 A charger), 15 A (48 V, 10 A charger). Cable as for the internal charger.
• The battery switch must be on to charge, because the input lands on the busbars. The shunt counts the charge.
• Connect the charger to the box first, then plug it into 230 V. Unplug 230 V first.
• Optional: an SB50 charge socket on each battery module (wired after its fuse) lets you charge a module on its own, for example before connecting it the first time.
• Car charging: the same input accepts a DC-DC charger (e.g. Victron Orion XS) from a vehicle.

4. Calculate your blackout loads

Use the slider in the 3D model to try different average loads quickly. For a real plan, measure.

4.1 Measure, don't guess

Buy a plug-in energy meter (about €15) and measure each device for 24 hours. Fridges and freezers cycle, so a 24-hour reading is far more accurate than the label.

4.2 Load worksheet

Not practical for this system: electric heating, heat pumps, electric water heaters, ovens, induction hobs, washing machines (heating phase).

4.3 Formulas

Usable energy (Wh) = Battery Wh × DoD × Inverter efficiency
                   = 2560 × 0.90 × 0.90 ≈ 2,070 Wh per battery

Runtime (h)        = Usable energy / (Average load W + Inverter idle W)

Batteries needed   = (Daily Wh incl. idle × days of autonomy) / 2,070   → round up

Corrections for real life:

• Cold: at 0–5 °C a battery delivers about 5–10% less. Keep it indoors.
• Low loads: inverter efficiency drops at very light loads (about 85% at 10% load). Idle draw is already counted separately.
• Ageing: plan with about 90% of nominal capacity after a few years.
• DC loads (USB-C, 12 V) bypass the inverter and are about 5–10% more efficient.

4.4 Worked examples

Example: fridge + freezer + router + lights + boiler ≈ 2.6 kWh per day including idle. For 2 days: 2 × 2.6 / 2.07 = 2.5, so 3 batteries, or 2 batteries plus solar.

Stretch tip: a full freezer stays frozen for about 24–48 hours without power. Running it in blocks (2 h on, 4 h off) saves about two thirds of its energy.

4.5 Inverter sizing check

Continuous rating ≥ devices running at the same time × 1.25
Surge rating      ≥ largest start-up surge + everything else running

Example: fridge (100 W, 600 W surge) + freezer (100 W) + boiler (120 W) + lights and router (60 W) = 380 W continuous, 880 W surge. A 1200 W inverter already covers this. Choose 2000 W only if you also want a kettle, microwave or power tools.

5. Size every component

24 V values first, 12 V values in brackets.

5.0 Battery capacity options

Typical LiTime-class LiFePO4 batteries with built-in BMS. Sizes and prices are approximate; check the datasheet of the battery you buy.

Notes:

• "Amps" here means capacity in amp-hours (Ah). Energy = nominal voltage × Ah (25.6 V or 12.8 V).
• The BMS limits the inverter. Continuous inverter power ≈ BMS current × battery voltage × 0.9. Two batteries in parallel double the BMS current.
• 24 V 300 Ah and 48 V 200 Ah single batteries are made by fewer brands and are heavy (57 kg and 85 kg). Two smaller batteries in parallel are easier to find and carry.
• 48 V rack batteries often have a built-in breaker and a CAN/RS485 port so the inverter can read their state of charge. Check that your inverter supports the battery's protocol, or run them stand-alone with the shunt.
• Bigger battery or more batteries? For the same total energy, one big battery is cheaper and needs fewer cables. Several smaller ones are easier to lift, give more BMS current and keep running if one fails.

5.1 DC current

I_DC = P_AC / (V_min × η)
24 V: 2000 W / (24 V × 0.9) = 93 A
12 V: 1500 W / (12 V × 0.9) = 139 A

Use minimum battery voltage (24.0 V / 12.0 V), because current is highest when the battery is nearly empty.

5.2 BMS check

BMS continuous rating (all batteries together) ≥ I_DC × 1.1
24 V, 1 battery: 100 A ≥ 102 A → borderline at a full 2000 W, fine for typical loads
24 V, 2 batteries: 200 A ≥ 102 A → comfortable
12 V, 1 battery: 200 A ≥ 153 A → OK

See the table in section 5.3 for other inverter sizes.

5.3 Battery fuses and breaking capacity

Fuse ≥ I_DC × 1.25  (next standard size: 60, 80, 100, 125, 150, 175, 200, 250, 300 A)
Cable: smallest cross-section whose derated ampacity is ≥ the fuse rating

Main fuse and cable by inverter size (used in the 3D model):

The main cable includes battery → fuse → switch → busbars → inverter, and every battery-to-collector cable. Check that the batteries' combined BMS current is above the full-load current (section 5.0); if not, the BMS switches off before the inverter reaches full power.

A LiFePO4 battery can deliver several thousand amps into a short, so the battery fuse must be able to break that current:

With several batteries, every battery gets its own fuse at its + terminal (see section 8).

5.4 Cable cross-section and voltage drop

ΔV = (2 × L × I × ρ) / A
ρ (copper) = 0.0175 Ω·mm²/m, L = one-way length (m), A = cross-section (mm²)

24 V, 35 mm², 1 m, 95 A:  ΔV = 0.095 V (0.4%)
12 V, 70 mm², 1 m, 140 A: ΔV = 0.070 V (0.6%)

Targets: under 1–2% for main and inverter cables, under 3% for branches.

Approximate ampacity, fine-stranded copper, single core, free air at 30 °C. Inside a closed box, derate by about 20%.

Use fine-stranded, flexible cable (welding cable, H07RN-F or H07V-K), ideally tinned copper.

5.5 AC charger

Charge time (h) ≈ (Ah × number of batteries) / charger A × 1.05
24 V: 100 Ah / 15 A ≈ 7 h per battery
12 V: 200 Ah / 20 A ≈ 10 h per battery

LiFePO4 usually accepts up to 0.5C (50 A for a 24 V 100 Ah battery). With 2–3 batteries, a 25–30 A charger or a MultiPlus (35–70 A) makes recharging between outages much faster. Check that the total charge current stays within the BMS charge limit of all batteries together.

5.6 Solar MPPT (optional)

MPPT output current = PV watts / charging voltage
24 V: 500 W / 27 V ≈ 18.5 A → 100/20
12 V: 400 W / 13.5 V ≈ 30 A → 100/30

Cold check: Voc_string × 1.15 < MPPT max PV voltage (100 V)
Example: 2 × 400 W in series, Voc 37 V each → 74 V × 1.15 = 85 V ✓

Start check: panel Vmp ≥ battery voltage + 5 V
24 V: one 400 W panel (Vmp ~31 V) is borderline, two in series are better

5.7 DC-DC converter (24 V, optional)

I_in = (20 A × 13.2 V) / (24 V × 0.9) ≈ 12 A → 15 A fuse, 4 mm²

5.8 Branch fuses

A fuse protects the cable. Rate it at or below the cable's ampacity and at or above about 1.25 × the device's maximum current. Place it as close to the busbar as possible, within about 18 cm.

6. Bill of materials

6.0 Price summary per configuration

Budget to mid-range parts, German street prices, autumn 2026, excluding tools (about €200–350 once). Calculated exactly like the 3D model, which shows the same breakdown for whatever you select. "All options" includes the MPPT, two 430 W solar panels with cables and mounts, and the 12 V/USB outputs.

24 V with the default 2000 W inverter:

12 V with the default 1500 W inverter:

48 V with the default 3000 W inverter:

One 48 V 50 Ah battery (50 A BMS) can't supply a 3000 W inverter; use 1500 W or two batteries.

Modular version: add about €60–110 for 1 module, €90–160 for 2 and €120–210 for 3 (module cases, Anderson plugs, link cables, rolling base).

Breakdown for the recommended setup (24 V, one 100 Ah battery, 2000 W, all options):

Premium brands (Victron battery, inverter and charger) can roughly double the total.

6.0b Compared with prebuilt power stations

Street prices seen in German and Austrian shops, September–October 2026. Sales often cut them by 20–40%, so check current prices. Solar input is the maximum the unit accepts.

How DIY compares per kWh with the cheapest similar prebuilt option (mid-range prices; with solar, the same two panels are added to both sides):

Conclusion: below about 3 kWh, a prebuilt unit on sale costs about the same or less than DIY. From about 5 kWh, DIY gets clearly cheaper per kWh (about 10–20% less), and from 10 kWh about 30–45% less. Prebuilt units add a warranty, an app, built-in UPS switchover on most models and no assembly; DIY adds repairability, standard parts and easy expansion.

6.1 Core system, 24 V, 1 battery (option B: separate inverter + charger)

6.2 Optional modules

6.3 Cable and connectors (1 battery, all modules)

6.4 Each additional battery (inside the same box)

6.5 12 V version: what changes

6.6 Where to buy and how to search

Search terms are in German because they find more results in German shops. The links open a search on idealo (price comparison across many shops) and Amazon.de; specialist shops often have better advice and genuine parts. The 3D model shows the same links next to every part in its cost breakdown and in each component card, with the search terms adjusted to your voltage, battery and inverter.

Tips:

• Victron parts: buy from authorised dealers; the price difference to marketplace offers is small and you get warranty support.
• Batteries: check the datasheet for parallel use, low-temperature protection and BMS current before buying, and buy all batteries of a bank at the same time.
• Fuses and connectors: genuine Class T, MRBF and Anderson parts matter; cheap copies can fail to break a short circuit or overheat.
• Cable: fine-stranded, tinned copper; buy by the metre with matching lugs from the same supplier.

7. Tools and consumables

8. Battery bank: one battery or several

8.1 One battery

flowchart LR
  BP[Battery +] --> F[125 A fuse on the terminal] --> SW[Battery switch, side wall] --> PB[+ busbar]
  BN[Battery −] --> SH[Shunt, battery side to load side] --> NB[− busbar]
  F -. sense wire with 1 A fuse .-> SH

The fuse sits directly on the battery terminal, so not a single centimetre of cable is unprotected. The switch is mounted through the side wall, so you can turn everything off from outside.

8.3 The shunt's voltage-sense wire

Monitors like the Victron SmartShunt have a thin red lead (often labelled Vbatt+ or B+). It measures battery voltage and powers the shunt electronics.

• Connect it on the battery side of the switch (the switch input stud, or the + collector with several batteries). The shunt keeps counting even when the switch is off, which keeps the state of charge accurate. Its own draw is only a few milliamps.
• Fuse it with 1 A right where it connects to battery +. Most shunts include this fuse in the lead; if yours doesn't, add an inline fuse.
• Cable: 0.75 mm², as short as practical.
• Without it the shunt can't show voltage or state of charge, and many models won't switch on at all.
• Optional extra input on some shunts: battery temperature sensor or midpoint monitoring. Not needed here.

8.2 Two or three batteries in parallel (same box)

flowchart LR
  B1P[Battery 1 +] --> F1[Fuse] --> PC[+ collector]
  B2P[Battery 2 +] --> F2[Fuse] --> PC
  B3P[Battery 3 +] --> F3[Fuse] --> PC
  PC --> SW[Battery switch, side wall] --> PB[+ busbar]
  B1N[Battery 1 −] --> NC[− collector]
  B2N[Battery 2 −] --> NC
  B3N[Battery 3 −] --> NC
  NC --> SH[Shunt] --> NB[− busbar]

Rules:

1. Identical batteries: same model and capacity, ideally the same batch. The manufacturer must allow parallel use (check the maximum number, often 4).
2. One fuse per battery, mounted directly on its + terminal.
3. Equal cable length and size from every battery to the collector busbars. In a compact box the batteries sit at different distances from the collectors, so cut every battery cable to the length of the longest run and loop the extra neatly on the shorter ones.
4. The − collector goes to the battery side of the shunt, so the shunt measures the whole bank. Nothing else lands on the battery side.
5. Equalize before the first connection: charge every battery to 100% on its own and let them rest a few hours. Voltages should match within 0.05 V.
6. Collector rating: at least the total possible current (24 V: ≥250 A).
7. Adding a battery later: fully charge the new battery and the existing bank separately, then connect. An older bank and a new battery work together, but the bank behaves like its weakest battery.
8. Weight: about 22 kg per battery. A box with 3 batteries weighs about 85–90 kg (see section 11).

Series (2 × 12 V → 24 V): only if the manufacturer explicitly allows it. It needs balancing between batteries. Prefer a single 24 V battery.

9. Wiring diagram

Shown for option B with all optional modules and 2 batteries. With one battery, the collectors disappear: the battery's terminal fuse connects straight to the switch, and battery − straight to the shunt.

flowchart LR
  B1[Battery 1] -- "+ via 125 A fuse" --> PC[+ Collector]
  B2[Battery 2] -- "+ via 125 A fuse" --> PC
  B1 -- "−" --> NC[− Collector]
  B2 -- "−" --> NC
  PC -- "35 mm²" --> SW[Battery switch<br>side wall]
  SW -- "35 mm²" --> PB[+ Busbar]
  NC -- "35 mm²" --> SH[Shunt]
  SH -- "35 mm²" --> NB[− Busbar]
  PC -. "sense wire 0.75 mm², 1 A fuse" .-> SH

  PB -- "35 mm²" --> INV[Inverter]
  NB -- "35 mm²" --> INV
  INV -- "3×1.5 mm²" --> AC[230 V outlet]

  ACIN[Fused IEC inlet] --> CHG[AC charger]
  CHG -- "4 mm², 20 A fuse" --> PB
  CHG --> NB

  PV[Solar input, optional] -- "4 mm² solar" --> MPPT[MPPT 100/20]
  MPPT -- "6 mm², 30 A fuse" --> PB
  MPPT --> NB

  PB -- "4 mm², 15 A fuse" --> DCDC[DC-DC 24→12 V, optional]
  NB --> DCDC
  DCDC --> FB[12 V fuse block]
  FB -- "10 A" --> USB[USB-C 100 W]
  FB -- "15 A" --> S12[12 V socket]
  FB -- "1 A" --> VM[Voltmeter]

With a MultiPlus (option A), the inverter and charger become one device: one pair of DC cables to the busbars, wall plug to its AC input, your outlet strip on its AC output.

Key rules

• Only battery negatives (directly or via the − collector) connect to the battery side of the shunt.
• Every + branch gets a fuse close to the busbar. Negatives don't need fuses.
• One DC system ground point: follow the inverter manual. Many recommend connecting the − busbar once to the enclosure/PE earth stud, at one point only.

10. Cable and fuse table

24 V

The 24 V main-cable rows above assume a 2000 W inverter. For other sizes use the table in section 5.3.

12 V differences

Complete cable list (recommended setup)

Every cable for the recommended setup: 24 V, one 100 Ah battery, 1500 W inverter, internal charger, solar and 12 V/USB, all in one box. Main cables are 25 mm² because the inverter is 1500 W; with a 2000 W inverter use 35 mm² as in the table above. Cables are numbered in build order (section 12). Cut lengths come from the 3D model layout plus about 10% slack, so lay each cable in your box before cutting it.

For any other setup, open the 3D model and expand Cable list in the overview card. It lists every cable, length and termination for what you select, and each part's card shows the cables connected to it.

Cable to buy

Totals of the cut lengths above. Add about 0.5 m per size for mistakes and re-crimps.

The cable from the solar panels to the SB50 input is extra and depends on where the panels are.

Lugs and ferrules to buy

Stud sizes are typical: M8 on battery terminals, busbars and inverter, M10 on the battery switch and SmartShunt. Check them on your own parts before buying lugs. Stack at most 2 to 3 lugs per stud, biggest lug at the bottom.

11. Housing: enclosure, layout and mounting

11.1 Choose the enclosure type

Rule of thumb: 1 battery → portable box. 2–3 batteries → stationary, or modules.

Inside dimensions chosen by the 3D model (24 V, 2000 W inverter, internal charger, with clearances; check against your actual parts). "Wall" means the inverter is wall-mounted.

An external charger saves a few more centimetres in most setups (for one 100 Ah battery with solar and 12 V/USB: 63 × 59 × 31 cm with the inverter on the floor). 12 V sizes are within a few centimetres of these. Choose an enclosure about 5 cm larger in each direction for cable glands, handles and mounting. Above about 50 kg, plan for a stationary setup or split the batteries into separate modules.

11.2 Compact layout

The layout uses two rows with a wiring strip between them, so every main cable is short:

flowchart TB
  BW[Back wall: AC inlet and solar input, low] --> R1
  subgraph R1[Back row]
    BAT[Batteries] --- C12[DC-DC and fuse block, if they fit here]
  end
  R1 --> ST
  subgraph ST[Wiring strip]
    COL[+ and − collectors] --- SH[Shunt] --- BUS[+ and − busbars]
  end
  ST --> R2
  subgraph R2[Front row]
    CHG[Charger and MPPT] --- INV[Inverter, or wall-mounted on the right side wall]
  end
  R2 --> FP[Front panel: outputs high, inputs low]
  SWL[Battery switch, left side wall] -.- ST
  FAN[Exhaust fan, high, left side wall] -.- R2
  VENT[Air intake, low, next to the inverter] -.- R2

How the parts are arranged:

• Inverter mount (floor or wall): the inverter either stands on the floor in its own column, or is wall-mounted on the right side wall on a metal plate with its terminal end towards the busbars and the terminals facing into the box. Two wall positions are used:
• Beside the floor parts (12–13 cm above the floor): only the busbars run underneath; charger, MPPT and 12 V parts stand to its left. The box gets about 1 cm taller but loses the inverter's floor column, typically 10 cm of width.
• Above the floor parts (about 17 cm above the floor): charger, MPPT and 12 V parts can stand underneath it. The box gets about 5 cm taller; this pays off only when the floor is very crowded.
• Rearranging to fit: with a wall-mounted inverter the sockets and USB move to the left part of the front panel so the inverter can run almost to the front wall, the battery cables run in a lane just in front of the batteries (behind the inverter), and the charger and 12 V cables run in the gap between the busbars and the front row (under the inverter).
• How the model chooses: for every setup it builds the floor and both wall variants, lengthwise and sideways batteries, and columns in front, behind or split, then keeps the smallest inside volume. Use the Floor or Wall buttons to force one.

• Where it helps most: one 100–300 Ah battery with solar and 12 V/USB (6–8% less volume) and the modular stack. With 2–3 batteries the battery row sets the size and the floor layout usually stays smallest.

• Batteries always go in the back row. With 2–3 batteries they're turned sideways when that gives a smaller box (the 3D model picks automatically).

• Columns: the charger and MPPT share one footprint (charger nearest the strip, MPPT behind it). The DC-DC converter and fuse block share another. The inverter has its own.
• Columns are placed in whichever row is shorter, so both rows end up about the same width.
• Devices face the strip: every DC terminal points toward the busbars.
• Wall connections: sockets and USB in a row high on the front panel. The AC inlet and solar input low, on the wall next to the charger column. The battery switch through the left side wall, at the end of the strip.

Clearances

11.3 Making it safe

• Non-combustible enclosure, or line wood with steel sheet or fire board.
• Cover everything live: terminal covers on battery posts and busbars. With the lid open, no bare copper should be touchable.
• Battery switch through the side wall, at hand height, with a red knob and a label. This is your emergency off, so it must work without opening the lid.
• Ventilation: intake grille with dust filter low on the wall next to the inverter; exhaust fan high on the opposite side wall, switched by a thermostat at about 40 °C. Low-in, high-out gives natural airflow even when the fan is off. LiFePO4 doesn't release gas in normal use.
• Location: 5–30 °C, dry. In a basement, raise it about 10 cm in case of flooding. Not in escape routes, not next to flammable storage.
• Smoke or heat detector above or near the setup.
• Strap every battery down to the base board, so nothing slides when the box is carried or tipped.
• Separate AC and DC wiring, cable glands where cables pass through walls, everything labelled.
• Keep children out: lockable lid or cabinet.

11.4 Making it practical

• All controls outside: sockets, USB, display, AC inlet and solar input on the walls, battery switch on the side wall. Normal use never requires opening the box.
• Outputs high, inputs low: easy to plug into, and cables from inside don't cross each other.
• Removable mounting board, so you can work on the electronics at a table.
• DIN rail for small parts (fuse block, DC-DC, ATS).
• Wheels and handles above about 25 kg. Two people for lifting above 40 kg.
• Fixed, labelled extension leads from the backup outlet strip to the fridge, freezer and router.
• Monitoring without opening: BMS, shunt and MultiPlus apps over Bluetooth, or a Cerbo GX for remote access.
• Service access: every fuse reachable without removing other parts. A spare fuse set taped inside the lid.

11.5 Building the boxes

Choose the material

Fire lining for wooden boxes: 0.5–1 mm galvanised steel sheet or 6–10 mm calcium silicate board under and around the batteries, and behind the inverter. Fix it with screws, not glue alone.

Size the box

1. Take the inside dimensions from the 3D model (intro card, or section 11.1).
2. Add 5 cm in width and depth for cable glands, latches and tolerance.
3. Height: tallest part plus 8–10 cm for cables and fuse access above the battery terminals. The lid must never touch a terminal.
4. For a modular stack, every module gets the same footprint so they stack flush.

Example cut list, single box for 24 V 1 × 100 Ah with all options and the inverter wall-mounted (inside 57 × 64 × 32 cm → box 62 × 69 × 37 cm inside), 15 mm plywood:

Openings and cut-outs

Always use the cut-out sizes from each part's datasheet. Typical values:

Ventilation rule of thumb: about 50 cm² free intake area and one 80–120 mm fan per 2000 W of inverter, intake and exhaust on opposite sides, mesh against insects.

Mounting inside

• Removable mounting board: all parts are screwed to it, and it's bolted to the case base with 4 bolts. You build and test everything on the bench, then drop it in.
• Batteries: non-slip rubber mat underneath, two straps or a steel bracket across the top, bolted through the board.
• Busbars and shunt: on insulated standoffs; transparent covers over all bare copper.
• Small parts: DIN rail for the fuse block, DC-DC converter and ATS.
• Wall-mounted inverter: bolt it to a 2–3 mm aluminium or steel mounting plate, and bolt the plate through the side wall with M6 bolts and large washers or a backing plate on the outside (a 2000 W inverter weighs 8–12 kg). In a plywood box use 15 mm walls at least. Keep 5 cm free above and around the fins, put the air intake just below it and the exhaust fan high on the opposite wall. Fit rubber spacers if the inverter fan hums against the wall.
• Cables through walls: always through cable glands or grommets. No bare plywood or metal edges against insulation.
• Lid: hinges plus a stay or gas strut, a lockable latch, foam gasket against dust, and a 2–3 mm polycarbonate sheet under the lid above the terminals.

Handles, wheels and labels

• Handles: bolted through with backing plates, rated for at least the full weight. Two on opposite sides.
• Castors: 75–100 mm, two lockable, each rated for at least half the total weight.
• Labels: polarity at every terminal, fuse rating at every fuse, system voltage on the outside, "Battery switch / emergency off" next to the knob, and "Do not connect to house wiring" next to the 230 V outlet.

Box build steps

1. Plan with the 3D model; write down the inside dimensions and wall openings.
2. Cut the panels; drill and cut every opening before assembly.
3. Glue and screw the box together; add aluminium corner profiles.
4. Fit the fire lining.
5. Seal or paint the wood.
6. Fit handles, castors (or rolling base), lid hardware and latches.
7. Mount the panel parts (sockets, inlets, switch, grille, fan) with their gaskets.
8. Build the electronics on the mounting board, test it on the bench, then bolt it in.
9. Strap in the batteries last, with the switch off and the battery fuses out.
10. Label everything, then follow section 14 to commission.

11.6 Modular design

For systems over about 35 kg, split the system into parts that one person can lift. In the 3D model, press Modular.

flowchart TB
  subgraph STACK[Modular stack]
    direction TB
    E[Electronics module, about 15 kg, one port per battery]
    M2[Battery module 2, about 26 kg]
    M1[Battery module 1, about 26 kg]
    RB[Rolling base, 4 castors]
    E ~~~ M2 ~~~ M1 ~~~ RB
  end
  M1 -->|own link cable, same length| E
  M2 -->|own link cable, same length| E

Electronics module: inverter, charger, MPPT, DC-DC converter, fuse block, battery switch (side wall), shunt and main busbars, collector busbars only with 2 or more battery modules, all outlets and inlets, ventilation, and one battery port per module (plus a spare if you plan to expand). About 25 cm high.

Each battery module: one battery strapped down, its fuse on the + terminal inside the module, and one Anderson socket on the same side as the electronics ports. Use SB175 when the battery fuse is up to 175 A, SB350 above that.

Wiring:

• Star wiring: one link cable per module straight to its own port, no daisy chain.
• Inside the electronics module with one battery module: port + straight to the battery switch input, port − straight to the battery side of the shunt. No collector busbars needed.
• With 2 or more modules: every port + to the + collector, every port − to the − collector; the + collector to the switch, the − collector to the battery side of the shunt. Alternative for 2 modules: stack both lugs on the switch input stud and on the shunt's battery-side stud (at most 2 lugs per stud) and skip the collectors.
• The collectors can't be replaced by the main busbars: the main + busbar sits after the switch and the main − busbar after the shunt, so batteries joined there could not be switched off and their current would bypass the shunt.
• Main-size cable (section 5.3), the same length for every module.

Rules:

• Plug and unplug only with the battery switch off and no load.
• Charge every new or replaced module to 100% on its own before plugging it in.
• Stack with corner blocks or locating pins, and strap the stack together for moving.

Sizes from the 3D model (internal charger; heights include a 5 cm base). The model keeps the smallest stack volume:

With all options the electronics now sit in one row (inverter, charger with MPPT, 12 V parts side by side), which makes every module 8 cm wider but 18 cm shallower than the two-row layout of version 7: about 23% less stack volume (for 2 × 100 Ah: 67 × 37 × 96 cm instead of 59 × 55 × 96 cm). For 24 V 200 Ah batteries, a wall-mounted inverter gives the smallest stack.

Modular build steps: build the electronics module first (section 12, steps 1–8 without batteries), then each battery module (battery, terminal fuse still out, short cables to its socket), make the link cables, charge every module to 100%, stack, plug in with the switch off, insert the fuses and commission (section 14).

11.7 Toolbox stack (stackable tool-brand boxes)

The modular design also works with stackable toolbox systems: one toolbox per battery plus one for the electronics. In the 3D model, choose Housing: Toolbox stack; the intro card then checks whether your setup fits the two toolboxes below.

How the boxes connect:

flowchart LR
  subgraph BB[Battery toolbox]
    B[Battery] -->|+| F[Fuse on the + terminal]
    F --> S1[SB175 socket, left side]
    B -->|−| S1
  end
  subgraph EB[Electronics toolbox]
    S2[SB175 port, left side] -->|+| SW[Battery switch] --> PB[+ busbar]
    S2 -->|−| SH[Shunt, battery side] --> NB[− busbar]
  end
  S1 ==>|external link cable, main cable size, plugs both ends| S2

• Electrical: a panel-mounted Anderson SB175 socket on each box (SB350 above a 175 A fuse), on the same side and height, and one external link cable with plugs at both ends, 40–60 cm with a small loop, clipped to the outside.
• Mechanical: the boxes' own latches hold the stack together. Don't drill through lids; boxes must separate and lid holes ruin the weather seal.
• Fuse inside the battery box on the + terminal, so the socket and cable are protected when the boxes are apart.
• Optional: an SB50 charge socket (blue) on the battery box to charge it on its own.

Connect and disconnect:

flowchart LR
  C1[Battery switch off] --> C2[Latch the boxes together] --> C3[Plug in the link cable] --> C4[Battery switch on]
  D1[Loads off, battery switch off] --> D2[Unplug the link cable] --> D3[Unlatch the boxes]

What goes where:

Mount everything on an internal aluminium or plywood plate bolted through the walls with backing plates; plastic walls must never carry the parts.

Toolboxes checked (inside dimensions and ratings from manufacturer and retailer data, autumn 2026):

What fits (3D model, 24 V, external charger, 1500 W):

So a toolbox stack works best for 24 V 50 Ah modules (1.28 kWh each) with an external charger and no extras; add more 50 Ah battery boxes for more energy. For a 100 Ah battery, use a larger box, the modular stack with custom boxes, or a two-tier box.

Two-tier single box (alternative): one tall box with the battery on the floor and an aluminium deck above it on four corner posts carrying the electronics. The battery cables pass through a grommeted gap in the deck. With an external charger it needs about 44 × 34 × 56 cm (24 V 50 Ah) or 63 × 36 × 56 cm (24 V 100 Ah) inside, which is taller than most toolboxes.

Toolbox price: a stackable storage box set can cost about €50 (the 3D model uses €50 for the set plus €15–30 per box for base plates, mounting plates and fire lining). Tool-brand XL boxes cost about €110–170 each if you need larger inside dimensions.

Toolbox-specific rules:

• Plastic softens at about 80–100 °C: fire lining under the battery and behind the inverter, 3 cm air gap to the walls.
• Cut-outs with a step drill or hole saw; 2 mm aluminium reinforcement plates behind every socket, switch and handle.
• Ventilation: intake low, thermostat fan high; this ends the IP65 rating of that box.
• Never mount anything on a lid.

12. Build steps

Before you start: battery switch off, every battery fuse removed. Remove watches and rings. Use insulated tools. Cover terminals you aren't working on.

Making a good lug (every main cable)

1. Cut cleanly with a cable cutter.
2. Strip exactly the depth of the lug barrel.
3. Twist strands lightly; don't trim any.
4. Insert fully and crimp with the matching die.
5. Pull test: it must not move.
6. Adhesive-lined heat shrink over the barrel.
7. Torque to spec (M8 battery terminals often 8–12 Nm), then add a paint-pen mark across nut and stud. No washers between lug and terminal.

Steps

flowchart TD
  S1[1. Mount the parts, switch off, fuses out] --> S2[2. Negative main]
  S2 --> S3[3. Positive main and shunt sense wire]
  S3 --> S4[4. Inverter]
  S4 --> S5[5. Charger or charge input]
  S5 --> S6[6. MPPT: battery side first, then solar]
  S6 --> S7[7. 12 V side and exhaust fan]
  S7 --> S8[8. 230 V output]
  S8 --> S9[9. DC system ground]
  S9 --> S10[10. Fuses in and power up]

Powering up the first time

flowchart TD
  P1[Check polarity with a multimeter] --> P2[Check torque marks and lug covers]
  P2 --> P3[Pre-charge the inverter through the resistor]
  P3 --> P4[Insert the first fuse, remove the resistor]
  P4 --> P5[Insert the remaining battery fuses]
  P5 --> P6[Battery switch on]
  P6 --> P7{BMS app: voltage correct, no alarms?}
  P7 -->|Yes| P8[Commissioning tests, section 14]
  P7 -->|No| P9[Switch off, find the fault]

1. Check polarity of every connection with a multimeter.
2. Check every nut has a torque mark and every lug is covered.
3. Pre-charge the inverter: its input capacitors draw a huge spike and cause a loud spark. Bridge one empty battery fuse holder (or the open switch) with the 25–50 Ω resistor for 5–10 seconds, until the voltage across the resistor is near 0 V. Then insert the fuse or close the switch, and remove the resistor.
4. Insert the remaining battery fuses.
5. Switch on, check the BMS app: voltage correct, no alarms, all batteries visible.
6. Continue with section 14.

13. Settings for every device

LiFePO4 values (per cell × 8 for 24 V, × 4 for 12 V, × 16 for 48 V). Your battery datasheet overrides these.

• AC charger: LiFePO4 profile. Disable recondition or desulfation modes.
• MPPT: battery type LiFePO4 or user-defined with the values above.
• Inverter: low-voltage alarm about 0.5 V above cut-off. Eco/search mode only if your fridge still starts reliably with it.
• MultiPlus: via VictronConnect + VE.Bus Smart dongle: LiFePO4 charge values, AC input current limit (e.g. 10 A), UPS function on, low-voltage cut-off, ground relay as required.
• BMS: confirm charge and discharge cut-offs and low-temperature protection. With several batteries, check that all report similar voltage and current.
• Shunt: capacity (total Ah of the bank), charged voltage 28.2 V (14.1 V at 12 V, 56.4 V at 48 V), tail current 4%. If the app shows no voltage, check the sense wire and its fuse first.

14. Commissioning and testing

Tick each item and note values in the test log (section 23).

1. [ ] Voltages correct at battery, busbars and inverter input.
2. [ ] Battery switch works from outside: off means 0 V on the + busbar.
3. [ ] BMS: no alarms, cell voltages within 0.05 V. With several batteries: voltages within 0.05 V of each other.
4. [ ] Charger: AC plugged in, charging current visible.
5. [ ] MPPT (if fitted): charging in daylight.
6. [ ] DC-DC (if fitted): 13.0–13.5 V at the 12 V socket. USB-C charges a phone and a laptop.
7. [ ] Inverter at no load: about 230 V AC, note the idle draw.
8. [ ] Small load: 60 W lamp for 10 minutes.
9. [ ] Fridge: starts reliably several times.
10. [ ] Full load: heater or kettle at the inverter's rating for 10 minutes. Then feel every lug, cable and fuse. Anything clearly warmer than the rest is a bad connection: switch off and redo it.
11. [ ] With several batteries: under load, the current per battery should be within about 10–15% of each other.
12. [ ] Fan: warm the thermostat (hair dryer) and check the fan starts; check air comes out at the top.
13. [ ] Switchover test (MultiPlus or ATS): with the fridge and router running, unplug the wall input. They must keep running. Plug it back in: no interruption.
14. [ ] RCD test if fitted (see section 16).
15. [ ] Drain test: run your real blackout loads until cut-off. Compare runtime with your calculation.
16. [ ] Recharge fully, note the charge time.
17. [ ] Re-torque main terminals after 1–2 weeks and check torque marks.

15. Using it in a blackout

Preparation

• Keep the bank between 80% and 100%, connected to the charger (or the MultiPlus permanently plugged in).
• Prepare labelled extension leads to the fridge, freezer, router and one lamp.
• Keep the quick reference card with the box.
• Keep a torch and a battery radio separately, independent of the box.

During an outage

flowchart TD
  O1{Switchover option?} -->|A, B2 or B3| O3[Nothing to do, devices keep running]
  O1 -->|B1 manual| O2[Plug fridge, router and lamp into the box]
  O2 --> O4[Start large loads one after another]
  O3 --> O4
  O4 --> O5[Check state of charge every few hours]
  O5 --> O6{Below 30 percent?}
  O6 -->|Yes| O7[Cut non-essential loads, run the freezer in blocks]
  O6 -->|No| O5
  O7 --> O8[Sunny? Put the solar panels out]

1. With option B1, plug the essential devices into the box. With A, B2 and B3, nothing to do.
2. Start large loads one after another (fridge, wait a minute, freezer).
3. Open fridge and freezer as little as possible. Run the freezer in blocks if the outage lasts.
4. Check state of charge every few hours. At 30%, cut non-essentials.
5. If sunny and you have solar: panels out.

What it can and can't run

16. Earthing, RCDs and gas boilers

• Many portable inverters have a floating output: N and PE are not connected. That's acceptable for single devices plugged in directly, but a residual-current device (RCD / FI) can't trip.
• With an N-PE bond on the inverter output (built in on some inverters, the MultiPlus does it with its ground relay in inverter mode), an RCD on the output works. That's the safer setup for several devices and extension leads.
• Gas boilers often need a proper N-PE reference for flame monitoring and won't ignite on a floating output. Some are also polarity-sensitive (plug orientation).
• Have an electrician set up and test the earthing. Don't improvise.
• Never connect the box to house wiring through a socket. It backfeeds into the grid and endangers line workers and you.

17. Fire safety

• Prevention is everything: good crimps, correct torque, correct fuses, ventilation, and the heat checks in section 14.
• Detection: smoke or heat detector in the room.
• If something smells hot or smokes: switch off the battery switch only if you can do so safely, leave the room, close the door, call 112.
• Small electrical fire not involving the battery (cable, inverter): CO2 extinguisher.
• Battery fire: leave it to the fire brigade. LiFePO4 is the least reactive lithium chemistry, but battery fires can reignite. An AVD (aqueous vermiculite) extinguisher made for lithium batteries is an option to keep nearby.

18. Legal and insurance notes for Germany

• Portable use with plug outputs (devices plugged into the box) is normal use of an emergency power unit.
• Connecting to house circuits requires an electrician-installed changeover switch that isolates the house from the grid (relevant standards include VDE 0100-551). Never do it yourself.
• Feeding into the grid isn't allowed with this system. Balcony solar rules (800 W feed-in) don't apply to it.
• Insurance: tell your home insurer about a larger self-built battery system and ask whether anything is required. Keep invoices and datasheets of the certified components.
• CE: a self-built device has no CE marking as a whole. Fine for private use, don't sell it.
• Rented flat: portable use is fine. Check your rental agreement for anything mounted permanently.

19. Maintenance and storage

Storage:

• 5–25 °C, dry, away from flammable materials.
• Long-term without the charger: about 50–60% charge, switch off.
• Blackout readiness: 80–100% on the charger. LiFePO4 handles this well.

20. Common mistakes

21. Troubleshooting

22. Alternatives and upgrade paths

23. Checklists and quick reference card

23.1 Decisions to make before buying

• ☐ Measured daily load (Wh) and peak load (W)
• ☐ Target autonomy (hours or days) → number of batteries
• ☐ Switchover option: A (MultiPlus), B1, B2 or B3
• ☐ Optional modules: solar yes/no, 12 V/USB yes/no
• ☐ Enclosure: portable box, modules or stationary
• ☐ Location: temperature, flooding, smoke detector, access

23.2 Shopping checklist

• ☐ Battery/batteries (identical, parallel-capable, low-temp cut-off)
• ☐ One battery fuse per battery, switch, busbars (+ collectors with 2–3 batteries), shunt
• ☐ Inverter + charger, or MultiPlus + dongle
• ☐ Optional: MPPT + solar connector; DC-DC + fuse block + outlets
• ☐ Branch fuses and holders
• ☐ Cables, lugs, heat shrink, glands, labels
• ☐ Enclosure, panel-mount battery switch, intake grille, fan + thermostat, straps, terminal and busbar covers
• ☐ Tools, pre-charge resistor, energy meter

23.3 Test log

23.4 Quick reference card

Print this and keep it with the box.

SYSTEM: 24 V LiFePO4, ___ batteries, ___ kWh, inverter ___ W
SWITCHOVER: MultiPlus / ATS / always-on / manual

BATTERY SWITCH: red knob on the LEFT SIDE of the box = emergency off

TURN ON:   Battery switch ON → BMS app check → inverter ON → devices
TURN OFF:  Devices off → inverter OFF → battery switch OFF

BLACKOUT ORDER: fridge → (1 min) → freezer → router → lights
DON'T RUN: heaters, kettles, ovens, washing machines

STATE OF CHARGE: 100% full | 30% cut non-essentials | inverter stops at 24.0 V

CHARGING: wall ~7 h per battery (15 A charger) | solar: battery first, then PV

FUSES: Battery 125 A each | Charger 20 A | MPPT 30 A | DC-DC 15 A
       USB-C 10 A | 12 V socket 15 A | Display 1 A | Fan 2 A

NEVER: plug into a house socket | charge below 0 °C | work on it switched on

SMOKE OR HOT SMELL: side-wall switch OFF only if safe → leave, close door → call 112

24. Revision notes

Version 13

Version 12

Version 11

Version 10

Version 9

Version 8

Version 7

Version 6

Version 5

Version 4 (prices and final review)

Version 3 (layout review)

Version 2

Changes from version 1, based on a review of the first guide: