Battery calculator
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.
Calculation breakdown
| Energy in kilowatt-hours | 1.28 kWh |
|---|---|
| Capacity in amp-hours | 100 Ah |
| Nominal voltage | 12.8 V |
Energy (Wh) = Voltage (V) × Capacity (Ah) = 12.8 V × 100 Ah = 1,280 Wh
Method
A battery's amp-hour rating tells you how much charge it can deliver, not how much energy it stores. Two 100 Ah batteries at 12 V and 48 V hold completely different amounts of energy, which is why capacity alone cannot be compared across systems.
Converting to watt-hours removes the voltage from the comparison. Watt-hours are also the unit that runtime, solar production and airline battery limits are expressed in.
Energy (Wh) = Voltage (V) × Capacity (Ah) Capacity (Ah) = Capacity (mAh) ÷ 1000
Symbols
Worked examples
Example
A 100 Ah lithium iron phosphate battery sold as a 12 V drop-in replacement. Its nominal voltage is 12.8 V because it contains four 3.2 V cells in series.
Energy = 12.8 V × 100 Ah Energy = 1,280 Wh
Result: 1,280 Wh, or 1.28 kWh.
Example
Power banks quote the capacity of the internal cells at 3.7 V, not the 5 V that comes out of the USB port.
Capacity = 20,000 mAh ÷ 1000 = 20 Ah Energy = 3.7 V × 20 Ah Energy = 74 Wh
Result: 74 Wh — comfortably under the 100 Wh limit airlines commonly apply to carry-on lithium batteries, though you should always check the airline's own rule.
Detail
Nominal voltage is what changes the answer, so the same capacity appears very differently across system voltages.
| Capacity | 12 V | 12.8 V | 24 V | 25.6 V | 48 V | 51.2 V |
|---|---|---|---|---|---|---|
| 20 Ah | 240 Wh | 256 Wh | 480 Wh | 512 Wh | 960 Wh | 1,024 Wh |
| 50 Ah | 600 Wh | 640 Wh | 1,200 Wh | 1,280 Wh | 2,400 Wh | 2,560 Wh |
| 100 Ah | 1,200 Wh | 1,280 Wh | 2,400 Wh | 2,560 Wh | 4,800 Wh | 5,120 Wh |
| 200 Ah | 2,400 Wh | 2,560 Wh | 4,800 Wh | 5,120 Wh | 9,600 Wh | 10,240 Wh |
| 300 Ah | 3,600 Wh | 3,840 Wh | 7,200 Wh | 7,680 Wh | 14,400 Wh | 15,360 Wh |
Values are generated with the same function that powers the calculator, so the table and the tool can never disagree.
Detail
Limits of the model
Assumptions
The capacity you enter is the battery's rated capacity at its rated discharge rate.
Nominal voltage is treated as constant across the discharge, which is the standard convention for capacity ratings.
Not covered
This is the total nominal energy of the pack. It is not the energy you can actually use — that depends on the allowed depth of discharge and on conversion losses.
Delivered capacity falls at high discharge currents, especially for lead-acid batteries.
Capacity declines as a battery ages; the label figure describes a new battery.
Sources
Next steps
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.
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.
Current depends on more than watts and volts. Pick your system type so the calculation uses the right relationship — power factor for AC, and √3 for three-phase.
Last meaningful update: 2026-08-19. This date changes only when the model, the sources or the guidance change.