Calculators · EV and electrical
Battery capacity, runtime and charging time calculator
Batteries are rated in amp-hours, but what powers your equipment is energy in watt-hours — amp-hours multiplied by voltage. These calculators convert between the two, estimate how long a battery will run a given load, how long a charger takes to refill it, and apply Ohm’s law for the voltage, current, power and resistance of a circuit. They apply to car, leisure, marine, solar and backup batteries alike.
Amp-hours to watt-hours
Multiplies capacity in amp-hours by the battery’s nominal voltage to give stored energy. It is the only fair way to compare batteries of different voltages, such as a 12 V and a 24 V system.
Wh = Ah × V
Runtime for a load
Estimates how long a battery can power a device, using only the share of capacity it is safe to discharge and allowing for inverter losses if the device runs on mains voltage. Lead-acid batteries should not be discharged below about 50%.
usable = Ah × V × DoD × efficiency runtime = usable / load
Charging time
Estimates how long a charger takes to bring a battery from one state of charge to another. Charging efficiency is about 85% for lead-acid and 95% for lithium; the final absorption stage of lead-acid charging adds extra time not modelled here.
Ah needed = capacity × (target − start) / 100 / efficiency time = Ah needed / charger current
Ohm’s law: power and resistance
From voltage and current, calculates power and resistance. Use it to work out what a 12 V accessory draws, or to check a heater or bulb rating against the current a circuit can carry.
P = V × I R = V / I
Worked example: a camper leisure battery
- A 100 Ah lithium battery at 12.8 V stores 1,280 Wh.
- Using 80% of it through a 90%-efficient inverter leaves 922 Wh for the load.
- A 200 W fridge and lights run for about 4.6 hours.
- A 10 A charger refills a 100 Ah lead-acid battery from 20% in about 9.4 hours.
Typical battery characteristics
| Battery type | Nominal voltage | Usable depth of discharge | Charging efficiency |
|---|---|---|---|
| Flooded lead-acid | 12 V | about 50% | about 80 – 85% |
| AGM / gel | 12 V | about 50 – 60% | about 85 – 90% |
| Lithium iron phosphate (LiFePO₄) | 12.8 V | about 80 – 90% | about 95 – 98% |
| EV traction battery (NMC) | 350 – 800 V pack | buffer managed by the car | about 90 – 95% |
Related
About these results
Results are theoretical estimates from standard engineering formulas. Real-world figures depend on conditions these formulas do not capture. Always follow the manufacturer's specifications for maintenance, loading and safety decisions.
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