Off-Grid Solar Calculators

Battery storage

Battery Bank Sizer for RV/Van

Size an RV, van or cabin battery bank from daily watt-hours and days of autonomy. Rated Wh = (daily Wh × days) ÷ (DoD × round-trip efficiency), then Ah = Wh ÷ V. This is stored energy, not a solar-array size—cover one day of use with panels separately.

Inputs

Wh

Total watt-hours you expect to use in a typical 24 hours (lights, fridge, laptop, etc.).

days

How many days you want to run with little or no solar charging.

Usable fraction of the rated bank. LiFePO4 can be used much deeper than lead-acid.

System voltage of the house battery bank.

Combined charging, wiring and inverter losses. 90% is a conservative off-grid default.

Results

Usable energy to cover
3,000Wh

Daily use multiplied by days of autonomy, before chemistry and losses.

Rated bank size
3,509Wh

Nameplate watt-hours after DoD and round-trip efficiency.

Rated capacity
292Ah

Nameplate amp-hours at the chosen system voltage.

Recommended gear

Engineering methodology

Usable energy to cover = daily watt-hours × days of autonomy. Rated watt-hours = usable energy ÷ (DoD × round-trip efficiency). Rated amp-hours = rated watt-hours ÷ battery voltage, again from Wh = Ah × V. Round-trip efficiency accounts for charge, wiring and inverter losses (often around 85–95%).

Days of autonomy are not extra solar

Rated Wh = (daily Wh × days) ÷ (DoD × round-trip efficiency). Two days means you store two days of use after chemistry and losses—not that the array is twice as large. Size panels from one day’s Wh; size the bank from days without sun.

12V 200Ah vs 24V 100Ah

Both are 2,400 Wh nameplate (Ah × V). Usable energy then depends on DoD. Higher voltage cuts current for the same watts, which shrinks DC cable and fuse sizes. Pick voltage for the inverter and MPPT, then convert Wh back to Ah.

Frequently asked questions

How many days of autonomy should an RV or cabin battery bank have?

Two days is a common off-grid starting point: one day of use plus a cloudy day. Winter or heavily shaded sites often need more. This is an energy budget, not a generator or solar-array design.

Why does 12 V need more amp-hours than 48 V for the same watt-hours?

Watt-hours are energy. Amp-hours = watt-hours ÷ volts. A 2.4 kWh bank is 200 Ah at 12 V and 50 Ah at 48 V. Voltage does not change the energy you must store; it only changes the Ah number on the label.

What round-trip efficiency should I enter?

About 90% is a conservative mixed-load default. Efficient LiFePO4 DC loads can approach 95%. Inverter-heavy AC use is often closer to 85%. The factor is applied as rated Wh = (daily Wh × days) ÷ (DoD × efficiency).

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All battery storage tools