Battery Runtime Calculator
Estimate how long a battery can power a DC or constant load: runtime ≈ usable watt-hours ÷ load watts. Enter capacity in Wh, or in Ah and volts. Optional efficiency for inverter losses. Client-side only.
Printed watt-hour rating if you have it. Leave empty to use amp-hours × volts below.
Or enter amp-hours and nominal voltage
Amp-hour rating at the nominal voltage (e.g. 100 Ah for a 12 V deep-cycle bank).
Pack or cell circuit voltage the Ah spec refers to (e.g. 12, 24, 48, or 3.7 for a single Li-ion cell).
Average power drawn from the battery (before losses you already folded into efficiency, if any).
Optional. Use values below 100% to roughly account for inverter losses, BMS cut-off, or not using the full SOC window. Leave blank for 100%.
Table of Contents
What this calculator estimates
Runtime is how long a battery can supply a given average load before its usable energy is depleted. For a simple model, treat the battery as a bucket of energy measured in watt-hours (Wh).
One watt-hour is one watt for one hour. If you know amp-hours (Ah) at a nominal voltage V, energy in watt-hours is Wh = Ah × V.
Constant load in watts draws energy at that rate: in an ideal world, hours of runtime = stored Wh ÷ load W. Real systems rarely use every Wh on the label — temperature, age, high currents, and cutoff voltages all reduce what you get.
Use the optional efficiency field as a single fudge factor (for example 85% through a small inverter) so usable Wh = rated Wh × efficiency ÷ 100. This is a planning estimate, not a substitute for a measured discharge test on your exact hardware.
Formula
Runtime from watt-hours
h ≈ Wh_usable ÷ P_load
Wh_usable is often the nameplate Wh multiplied by your efficiency percent ÷ 100. P_load is average power in watts.
From amp-hours and voltage
Wh = Ah × V — then h ≈ Wh_usable ÷ P_load
Example: 12 V, 100 Ah → 1,200 Wh. At 200 W and 90% efficiency → 1,200 × 0.9 ÷ 200 = 5.4 h.
Quick Reference
| Capacity | Load | Eff. % | Runtime (illus.) | Scenario |
|---|---|---|---|---|
| 100 Wh | 10 W | 100 | 10 h | Small power bank–sized pack |
| 1,200 Wh (12 V × 100 Ah) | 150 W | 100 | 8 h | Deep-cycle lead at nameplate |
| 2,400 Wh (48 V × 50 Ah) | 800 W | 85 | 2.55 h | Home battery through inverter * 0.85 |
| 18.5 Wh (3.7 V × 5 Ah) | 2 W | 100 | 9.25 h | Single 18650–class cell, rough |
Real-World Examples
120 W fridge on a 12 V bank
120 Ah at 12 V is 1,440 Wh. Load 120 W, efficiency 90% → 1,440 × 0.9 ÷ 120 = 10.8 h of average runtime (actual motor starts spike higher).
Laptop from a DC pack
Power station labeled 500 Wh, laptop draws about 40 W average → 500 ÷ 40 = 12.5 h before the station is empty if losses are small.
UPS ballpark
UPS batteries are quoted in VA and runtime curves depend on load; this tool is better for simple Wh/W planning. For UPS, prefer vendor runtime charts for short, high-rate discharges.
Solar storage day buffer
10 kWh usable (after your own derate), house draws 500 W average overnight → 10,000 ÷ 500 = 20 h — a sanity check, not including recharge from PV.
FAQ
If the label lists Wh (common on USB power stations), use Wh. For many lead and lithium “12 V 100 Ah” style specs, use Ah and the same nominal voltage the Ah rating is defined at.
Convert to amp-hours: Ah = mAh ÷ 1,000. A 10,000 mAh cell is 10 Ah. Then Wh = Ah × V; for example 10 Ah × 3.7 V = 37 Wh.
Peukert effect on lead batteries at high current, voltage sag under load, temperature, state-of-health, and never discharging to true zero all reduce usable energy. Treat the answer as an upper-bound planning number unless you have test data.
Enter the DC-side power the battery must supply after your inverter, or include inverter loss in the efficiency field and use the AC load power multiplied by estimated inverter efficiency — whichever way you prefer to bookkeep it.
If the watt-hour field is filled with a valid number, that value wins and Ah/voltage are ignored for the calculation until you clear Wh — same idea as our other dual-path calculators.
Battery energy ties to kWh over time: if you know average watts and needed hours, the Watts to kWh Calculator gives energy; this tool goes the other way from stored energy to hours at a load.