Battery calculator
Energy targets are quoted in watt-hours, but batteries are sold in amp-hours. Divide by your system voltage to find the capacity you actually need to buy.
Calculation breakdown
| Capacity in milliamp-hours | 100,000 mAh |
|---|---|
| Energy | 1,280 Wh |
| Nominal voltage | 12.8 V |
Capacity (Ah) = Energy (Wh) ÷ Voltage (V) = 1,280 Wh ÷ 12.8 V = 100 Ah
Method
Watt-hours and amp-hours are linked by voltage. Dividing energy by the nominal system voltage gives the charge capacity a bank must be rated for.
This is the step that decides whether a system is built at 12 V, 24 V or 48 V. The same 5 kWh of storage needs roughly 390 Ah at 12.8 V but under 100 Ah at 51.2 V, and the lower current is what keeps cable and fuse sizes sensible.
Capacity (Ah) = Energy (Wh) ÷ Voltage (V) Energy (Wh) = Energy (kWh) × 1000
Symbols
Worked examples
Example
You want 2 kWh of nominal storage on a 24 V lead-acid system.
Energy = 2 kWh × 1000 = 2,000 Wh Capacity = 2,000 Wh ÷ 24 V Capacity = 83.33 Ah
Result: About 83 Ah nominal. Because lead-acid is normally limited to 50% depth of discharge, you would need roughly double that to actually use 2 kWh.
Example
A power station is advertised at 1,280 Wh and you want to know what battery it is built from.
Capacity = 1,280 Wh ÷ 12.8 V Capacity = 100 Ah
Result: A 12.8 V 100 Ah LiFePO4 pack.
Detail
These are nominal capacities. Usable capacity is lower once depth of discharge and conversion losses are applied.
| Energy | 12 V | 12.8 V | 24 V | 25.6 V | 48 V | 51.2 V |
|---|---|---|---|---|---|---|
| 500 Wh | 42 Ah | 39 Ah | 21 Ah | 20 Ah | 10 Ah | 10 Ah |
| 1,000 Wh | 83 Ah | 78 Ah | 42 Ah | 39 Ah | 21 Ah | 20 Ah |
| 2,000 Wh | 167 Ah | 156 Ah | 83 Ah | 78 Ah | 42 Ah | 39 Ah |
| 5,000 Wh | 417 Ah | 391 Ah | 208 Ah | 195 Ah | 104 Ah | 98 Ah |
| 10,000 Wh | 833 Ah | 781 Ah | 417 Ah | 391 Ah | 208 Ah | 195 Ah |
A higher system voltage delivers the same energy at a lower amp-hour rating, which is why large banks are built at 48 V.
Detail
The amp-hour figure this calculator returns is the rated size of the bank. How much of it you can actually use depends on the chemistry: lead-acid banks are commonly limited to about half their rating, while LiFePO4 banks are often specified to 80–100%.
Limits of the model
Assumptions
Nominal voltage is treated as constant, matching how capacity ratings are published.
The result is nominal capacity before any depth-of-discharge or efficiency derating.
Not covered
It does not account for usable depth of discharge, conversion losses, ageing or temperature.
Real banks are built from whole batteries, so the practical answer is usually rounded up to the next available size.
Sources
Next steps
Amp-hours describe charge, not energy. Multiply by the pack's nominal voltage to get watt-hours, the figure you need for runtime, shipping limits and solar sizing.
Enter your battery and your load to estimate runtime. The calculator applies depth of discharge, battery health, conversion losses and — for lead-acid only — the Peukert effect, and shows each step of the energy budget.
Charging is fast while current is constant and slower near full. This estimate separates those stages, accounts for loads using charger current, and shows a range when lead-acid absorption time is genuinely variable.
Turn daily energy use into the nominal kWh and amp-hours your bank needs. The model accounts for days without charging, conversion loss, battery health, usable depth of discharge and whole-battery rounding.
Last meaningful update: 2026-08-19. This date changes only when the model, the sources or the guidance change.