Battery calculator
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.
Calculation breakdown
| Nominal battery energy12.8 V × 100 Ah | 1,280 Wh |
|---|---|
| Usable energy at 90% depth of discharge | 1,152 Wh |
| Load | 120 W |
| Power drawn from the batteryLoad at 90% conversion efficiency | 133 W |
| Discharge current0.1 C | 10.4 A |
Runtime = usable energy ÷ power drawn from the battery = 1,152 Wh ÷ 133 W = 8.64 h (8 h 38 min)
Your capacity, voltage, load and efficiency, run against each chemistry's typical usable depth of discharge and Peukert behaviour. Change any input above and these update with it.
| LiFePO4 (lithium iron phosphate)yours | 8 h 38 min 90% usable depth of discharge |
|---|---|
| Lithium-ion (NMC / NCA) | 7 h 41 min 80% usable depth of discharge |
| AGM (sealed lead-acid) | 4 h 28 min 50% usable depth of discharge |
| Gel (sealed lead-acid) | 4 h 28 min 50% usable depth of discharge |
| Flooded lead-acid | 4 h 50% usable depth of discharge |
Method
Runtime is an energy budget. Start from the pack's nominal energy, remove what you cannot or should not use, then divide by the power actually drawn from the battery.
The load you enter is measured at the inverter output, so the battery has to supply more than that. Dividing by conversion efficiency and adding the inverter's idle draw gives the real DC power leaving the battery.
For lead-acid chemistries a Peukert correction is applied, because delivered capacity falls as discharge current rises above the rate the battery was rated at. It is deliberately not applied to LiFePO4 or lithium-ion, where capacity holds up well across normal discharge currents.
Nominal energy (Wh) = Voltage (V) × Capacity (Ah) Battery power (W) = Load (W) ÷ efficiency + idle draw (W) Effective capacity (Ah) = Capacity × (I_rated ÷ I_actual) ^ (n − 1) [lead-acid only] Usable energy (Wh) = Voltage × Effective capacity × health × depth of discharge Runtime (h) = Usable energy (Wh) ÷ Battery power (W)
Symbols
Worked examples
Example
A 12.8 V 100 Ah LiFePO4 battery, 90% usable depth of discharge, feeding a 120 W mains load through an inverter that is 90% efficient.
Nominal energy = 12.8 V × 100 Ah = 1,280 Wh Usable energy = 1,280 Wh × 90% = 1,152 Wh Battery power = 120 W ÷ 0.90 = 133.3 W Runtime = 1,152 Wh ÷ 133.3 W = 8.64 h
Result: About 8 h 38 min.
Example
An AGM battery rated at the 20 hour rate, limited to 50% depth of discharge, driving the same 120 W load through the same inverter. The battery supplies about 11.1 A, well above its 5 A rated current, so a Peukert correction applies.
Battery power = 120 W ÷ 0.90 = 133.3 W, so current ≈ 11.1 A Effective capacity = 100 Ah × (5 A ÷ 11.1 A) ^ 0.1 ≈ 92.3 Ah Usable energy = 12 V × 92.3 Ah × 50% ≈ 554 Wh Runtime = 554 Wh ÷ 133.3 W ≈ 4.15 h
Result: About 4 h 9 min — less than half the LiFePO4 result, from a battery with the same headline rating.
Detail
Choosing a chemistry loads the values below. They are typical published figures and starting points for planning, not specifications for your battery.
| Chemistry | Nominal 12 V pack | Typical usable DoD | Peukert correction | Capacity rated at |
|---|---|---|---|---|
| LiFePO4 (lithium iron phosphate) | 12.8 V | 90% | Not applied | 1 h rate |
| Lithium-ion (NMC / NCA) | 10.8 V | 80% | Not applied | 1 h rate |
| AGM (sealed lead-acid) | 12 V | 50% | Applied, n ≈ 1.1 | 20 h rate |
| Gel (sealed lead-acid) | 12 V | 50% | Applied, n ≈ 1.1 | 20 h rate |
| Flooded lead-acid | 12 V | 50% | Applied, n ≈ 1.25 | 20 h rate |
These are starting points drawn from typical published figures, not guarantees. Every one of them is editable in the calculator — use your battery's datasheet where you have it.
Detail
Detail
Runtime estimates fail most often because of the load, not the battery. Appliance labels usually show maximum draw rather than average consumption, and anything with a thermostat or a motor cycles on and off.
A plug-in energy meter over 24 hours gives a far better average than a nameplate rating. Where a load cycles, enter its average power, not its peak.
Limits of the model
Assumptions
The load is constant for the whole discharge.
Nominal voltage is constant; real pack voltage sags under load and falls as the battery empties.
The battery starts fully charged, and the depth of discharge you enter is what you are willing to use.
Temperature is assumed to be around room temperature.
Conversion efficiency is a single flat figure, applied to the load.
Not covered
This is an estimate, not a guarantee. Real runtime depends on the specific cells, their age, the temperature and the load profile.
Cold weather reduces available capacity substantially, particularly for lead-acid, and many lithium batteries will not charge below freezing.
Inverter efficiency varies with load; it is typically worst at very low loads.
The Peukert model describes lead-acid behaviour approximately, and manufacturers' exponents vary between cells.
Surge loads such as motor start-up are not modelled here; size the inverter for those separately.
Sources
Next steps
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.
The label capacity is not the energy available to your load. Set the starting and minimum state of charge, battery health and conversion efficiency to expose the usable window.
Lead-acid batteries deliver less than their rated amp-hours when discharged faster than the test rate. Peukert's law estimates that reduction from the battery's rated current and exponent.
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.
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.
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.