Battery Runtime

Free Battery Runtime calculator with clear step-by-step results.

About Battery Runtime

Converts a battery's amp-hour rating into real runtime. Nameplate capacity is derated by the depth of discharge you are willing to use and by inverter losses, then divided by the load in watts. The DC current figure tells you whether your cabling and fusing are adequate for that draw.

Formulas

Usable energy

Usable Wh = Ah x V x depth of discharge x inverter efficiency

Runtime

Hours = usable Wh / load watts

Worked example

With the default values below, Battery Runtime returns Runtime at this load of 2.88. Change any input above and every figure here is recalculated in your browser.

Inputs used

Battery capacityminimum 1 Ah: 100 Ah
System voltageminimum 1 V: 12 V
Continuous loadminimum 1 W: 300 W
Usable depth of dischargeaccepts 5–100 %: 80 %
Inverter efficiencyaccepts 30–100 %: 90 %

Results

Runtime at this load: 2.88 h
Runtime in minutes: 173 min
Usable energy in the bank: 864.0 Wh
DC current the load draws: 27.78 A

Step by step

1Values used
2Usable energy
3Runtime
4Runtime at this load= 2.88 h
5Runtime in minutes= 173 min
6Usable energy in the bank= 864.0 Wh
7DC current the load draws= 27.78 A

Assumptions and limits

The result is derived only from the 5 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax or compounding frequency into account.
Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
Battery installations involve fire and electrical shock hazards. Sizing, fusing and ventilation must follow the manufacturer's specification and local electrical code.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Battery installations involve fire and electrical shock hazards. Sizing, fusing and ventilation must follow the manufacturer's specification and local electrical code.

Frequently Asked Questions

Why can't I use the full amp-hour rating?

Lead-acid banks lose most of their cycle life below 50% discharge, and even lithium packs reserve headroom. The depth-of-discharge field is what keeps the estimate honest.

Does a bigger load reduce effective capacity?

Yes, for lead-acid. Peukert's effect means a heavy draw yields fewer usable amp-hours than the 20-hour rating suggests. Lithium chemistries are far less affected.