How to size a LiFePO4 battery bank.

Updated 24 October 2026 · Calculator v1.1.0, formula v1.0.0

Short answer: work out the energy your loads take from the battery each day, multiply by the number of days the bank must carry them without charging, then divide by the share of the battery you actually plan to use. Divide that energy by the battery’s nominal voltage to get amp-hours, and round up.

Step 1 — Daily energy at the battery

Add up three separate things, in watt-hours per day (Wh/day):

1. AC appliance energy — what your appliances on the inverter use. Use a plug-in energy meter or the appliance’s rated energy where you can; nameplate watts × hours is a rough guide. Divide this figure by the inverter efficiency (for example 0.90) to get the energy drawn from the battery.
2. Direct DC energy — lights, pumps, fans and DC fridges wired to the battery without the inverter. Add as-is.
3. Inverter overhead — the inverter’s no-load or standby draw (watts, from its datasheet) × hours per day it is switched on.

Example: 3,500 Wh/day of AC appliances at 90% efficiency is 3,888.9 Wh at the battery. Add a 10 W inverter overhead for 24 hours (240 Wh) and you get 4,128.9 Wh/day.

Step 2 — Reserve energy

Multiply by the number of days the bank must run the loads with no charging at all (cloudy days, no generator, no shore power). Two days in the example gives 8,257.8 Wh. This is the energy your loads will actually consume.

Step 3 — Nameplate energy

You do not plan to empty a battery completely, and its available capacity falls with age and temperature. Divide reserve energy by both allowances:

• Usable depth of discharge (DoD) — the share of the rated capacity you plan to use, for example 80%.
• Available capacity after derating — an allowance for ageing and temperature, for example 90%.

8,257.8 ÷ (0.80 × 0.90) = 11,469.1 Wh of nameplate energy.

Step 4 — Amp-hours at the real nominal voltage

Divide by the battery’s actual nominal voltage—not the class name. A “48V” LiFePO4 bank is usually 51.2 V nominal (16 cells × 3.2 V); a “12V” bank is usually 12.8 V. 11,469.1 ÷ 51.2 = 224.0 Ah, which rounds up to 225 Ah. Always round up: rounding down would leave the plan short.

What changes the answer most

Same example (3,500 Wh/day AC, 51.2 V), changing one input at a time:

Reserve days scale the result directly, and an inverter left on all day can cost more capacity than many people expect. Switching the inverter off when it is not needed—or running small loads on DC—often saves more than a larger battery would cost.

What this estimate leaves out

• Cable and connection losses you have not entered, battery self-discharge and any extra safety margin.
• Charging: this is the energy for the no-charge period, not a solar or charger design.
• Chemistry detail: the model is intended for LiFePO4-type lithium banks. Lead-acid banks are usually planned around a much lower usable depth of discharge and lose capacity at high discharge rates; use the manufacturer’s figures.
• Current limits: a battery’s Ah rating does not tell you whether it can deliver your inverter’s current. Check continuous and surge discharge ratings—see What size battery for my inverter?

Hardware to compare once you know the size

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