Battery runtime calculator
How long a battery lasts under a given load.
Fill in the fields and the result will appear here automatically.
Estimate run time under a constant-power load using rated Ah and nominal battery voltage. Amp-hours describe charge; multiplying by voltage gives energy in Wh. Depth of discharge selects the usable fraction of nominal energy, while η is the fraction reaching the load after conversion. Both percentages are entered explicitly and are not selected automatically from battery chemistry.
How it works
Formula and logic
Nominal energy E₀ = C·U Wh. Useful energy E = C·U·DoD·η/10 000. Duration t = E/P hours, because Wh/W = h. Doubling power with unchanged inputs halves run time.
Example
100 Ah × 12 V = 1 200 Wh. At 80% depth of discharge and η = 90%, useful energy is 864 Wh; a 200 W load gives 4.32 h, about 4 h 19 min. A 400 W load gives 2.16 h.
Fields and units
- Capacity — Ah
- Voltage — V
- Load — W
- Depth of discharge — %
- Conversion efficiency — %
How to use
- — Enter capacity in Ah and nominal voltage in V.
- — Enter constant load power in W, on the side of the conversion to which η applies.
- — Set depth of discharge and energy efficiency, each greater than 0 and at most 100%.
- — Compare nominal and useful energy; a small positive duration is displayed without becoming a false zero.
Method and limitations
- Calculation method
- Formula and logic
- Data or methodology source
- Victron Lithium Battery Smart: nominal Ah, Wh and temperature dependence — specified series Victron BMV-700H: capacity depends on discharge rate; Peukert-model limits
- Limitation
- Nominal-energy and constant-load estimate. Battery current limits, inverter ratings, starting power and protection cutoff are not checked.
FAQ
Why can't I divide Ah directly by watts?
Ah measures charge, while W measures power. To obtain hours, first multiply Ah by voltage to obtain Wh. For a current load, the corresponding balance would instead use Ah/A.
What depth of discharge is permissible?
Use the conditions specified for the actual battery. This tool does not recommend a universal 50% or 80%, predict cycle life, or determine cutoff voltage.
Why can measured run time differ?
Battery voltage varies, and available capacity depends on current, temperature and ageing. Lead-acid batteries can exhibit a substantial Peukert effect. Discharge curves and variable loads are not modelled.
Is this η the same as the charge-time efficiency?
Here η compares energy before and after conversion to the load. The charge-time page uses retained charge in Ah. Those measurements are not interchangeable without checking what each represents.