Battery Life Estimator
Estimate how long a battery will last from its capacity, voltage and load, in mAh, Ah or Wh.
Enter a capacity and a load — the runtime updates as you type.
Estimated runtime8 h 30 min
- Runtime in hours
- 8.5 h
- Usable energy
- 15.72 Wh
- Usable charge
- 4.25 Ah
- Load power
- 1.85 W
- Load current
- 0.5 A
- Discharge rate
- 0.10 C
This is an estimate from the standard constant-load model
(runtime = usable capacity ÷ load). Real batteries deliver less at high discharge
rates, in the cold, and as they age — always leave a safety margin.
How it works
The calculator uses the standard constant-load battery runtime model:
runtime (h) = capacity × efficiency ÷ load. When the capacity and the load are in
matching units the maths is direct — 5,000 mAh with a 500 mA draw is 10 hours at 100 %
efficiency, 8.5 hours at 85 %. When you mix them (amp-hours against watts, or watt-hours
against amps) the battery voltage converts between the two, because
watt-hours = amp-hours × volts and watts = amps × volts. A 12 V,
100 Ah battery therefore stores 1,200 Wh; at 50 % usable depth of discharge that is 600 Wh,
which runs a 60 W fridge for 10 hours.
The efficiency box is where reality gets folded in. Lead-acid batteries should not be drained below about 50 %, lithium (LiFePO₄, Li-ion) packs tolerate 80–90 %, and an inverter or DC-DC converter throws away another 5–15 %. The estimate ignores Peukert's law, which makes lead-acid batteries lose extra capacity at high discharge currents, so treat high-current lead-acid results as optimistic. Everything is computed in your browser with plain JavaScript: nothing is uploaded, there are no sign-ups and no ads.
Frequently asked questions
How do you calculate battery life from mAh?
Divide the capacity by the load in the same units: runtime (hours) = capacity (mAh) x efficiency / load (mA). A 5,000 mAh battery with a 500 mA draw lasts 10 hours in theory, or about 8.5 hours at a realistic 85% usable capacity.
How do I convert amp-hours to watt-hours?
Multiply by the battery voltage: watt-hours = amp-hours x volts. A 12 V, 100 Ah battery stores 1,200 Wh. The calculator does this for you, so you can enter an Ah capacity with a load in watts, or a Wh capacity with a load in amps.
What usable capacity percentage should I use?
Around 50% for lead-acid batteries, which should not be discharged deeper, and 80-90% for lithium (LiFePO4 or Li-ion). Subtract another 5-15% if an inverter or DC-DC converter sits between the battery and the load.