Battery Bank Size Calculator

How It Works

The formula

The three factors in the denominator each shrink what a battery gives you back, so they divide rather than multiply. Depth of discharge is how much of the bank you are prepared to use. Round-trip efficiency is the share of the energy that survives the charge and discharge cycle. The temperature derate is how much capacity the cold takes away. Together they typically leave 60% to 76% of the nameplate available as real energy, which is why a bank sized on kilowatt-hours alone always comes out too small. How many days to choose, and the van versus cabin worked examples, are in how to size an off-grid battery bank.

Worked example

A cabin using 5 kWh a day wants 2 days of autonomy on a 48 V lithium bank, at 0.8 DoD and 95% round-trip efficiency, in a space that stays above 20°C:

• Usable energy needed = 5 × 2 = 10 kWh
• Derating product = 0.8 × 0.95 × 1.0 = 0.76, so 76% of nameplate is usable
• Bank capacity = 10 ÷ 0.76 = 13.16 kWh
• At 48 V that is 13,158 ÷ 48 = 274 Ah
• Built from 100 Ah 12 V blocks: 4 in series to make 48 V, 3 strings in parallel to clear 274 Ah, so 12 batteries
• That installs 300 Ah, or 14.4 kWh, which delivers 10.9 kWh usable and 2.19 days of real autonomy

Rounding up to whole batteries always overshoots, and that overshoot is free margin worth keeping. Note how the answer moves if the same cabin uses lead-acid: at 0.5 DoD and 80% round trip the derating product falls to 0.40, the bank has to be 25 kWh rather than 13.16 kWh, and the battery count doubles from 12 to 24. Chemistry changes the size of the bank far more than the daily load does.

How many days of autonomy to choose

Two days is the common answer for a system with a generator or a grid connection to fall back on, three to five days for a genuinely remote installation with no backup, and one day for a van or boat that moves regularly and can charge from the alternator. More days is not automatically better: a bank sized for five days of December weather spends the other eleven months mostly full, ages on the shelf rather than in use, and costs several times what a generator and 40 litres of fuel would have. Beyond about three days, a backup charging source is usually the cheaper answer.