Battery Voltage, Ah and Wh Calculator
Calculate battery watt-hours, amp-hours, or voltage using any two known values, with kWh, mAh, and joule conversions.
Last updated: July 2026
Battery safety
- Do not use this energy calculation alone to select a charger. Use approved components matched to voltage, chemistry, connector, and current limits.
- Do not connect packs in series or parallel unless the equipment and battery design explicitly support it.
- Do not use or charge batteries that are damaged, swollen, wet, unusually hot, or produce an abnormal odor.
Calculated battery value
Formula and substitution
Reverse consistency check
This result is nominal energy based on the entered voltage and amp-hour rating. Actual usable energy may be lower because of battery condition, discharge limits, temperature, load, conversion losses, and battery-management-system cutoff.
What this voltage, Ah, and Wh calculator tells you
Use any two known battery ratings to calculate the third value in the relationship Wh = V × Ah. The results also show kilowatt-hours, milliamp-hours, joules, the formula used, and a reverse consistency check.
How to calculate battery watt-hours
Multiply nominal voltage by capacity in amp-hours: watt-hours = volts × amp-hours. For example, 48V × 21Ah gives 1,008Wh of nominal energy.
How to calculate amp-hours from Wh and voltage
Divide watt-hours by voltage: amp-hours = watt-hours / volts. A 1,008Wh battery at 48V therefore corresponds to 21Ah, or 21,000mAh.
How to calculate voltage from Wh and Ah
Divide watt-hours by amp-hours: volts = watt-hours / amp-hours. For example, 1,008Wh divided by 21Ah gives a nominal voltage of 48V.
Watt-hours versus watts
Watts describe a rate of power. Watt-hours describe an amount of energy. This calculator works with battery energy in Wh, not an appliance's momentary power draw in watts.
Nominal energy versus usable energy
Multiplying rated voltage by amp-hours gives nominal energy. Actual usable energy can be lower because rated voltage differs from the voltage at full or empty charge, and because temperature, load, battery condition, conversion losses, discharge limits, and battery-management-system cutoff affect real use.
Worked examples
Calculate watt-hours
A 48V 21Ah battery has 1,008Wh, or 1.008kWh, of nominal energy. Its capacity is 21,000mAh and its nominal energy is 3,628,800 joules. The formula is 48 × 21 = 1,008Wh.
Calculate amp-hours
For a 1,008Wh battery at 48V, divide 1,008 by 48. The result is 21Ah, or 21,000mAh.
Calculate voltage
For a 1,008Wh battery with a 21Ah capacity, divide 1,008 by 21. The result is 48V.
Frequently asked questions
How do I calculate battery watt-hours?
Multiply the battery's nominal voltage by its amp-hour rating: Wh = V × Ah.
How do I convert Ah to Wh?
Multiply Ah by the nominal battery voltage. Amp-hours alone cannot be converted to watt-hours without knowing voltage.
How do I calculate Ah from Wh?
Divide watt-hours by voltage: Ah = Wh / V.
What is the difference between watts and watt-hours?
Watts measure power at a moment; watt-hours measure energy delivered or stored over time.
What is the difference between amps and amp-hours?
Amps measure current. Amp-hours describe charge capacity over time and do not by themselves state battery energy.
Can I convert mAh to Ah?
Yes. Divide milliamp-hours by 1,000. One thousand mAh equals 1Ah.
How many Wh are in one kWh?
One kilowatt-hour equals 1,000 watt-hours.
How many joules are in one Wh?
One watt-hour equals 3,600 joules.
Is a 48V 20Ah battery exactly 960Wh?
The rating gives 960Wh of nominal energy. Real voltage changes during use, and usable energy may differ from that nominal figure.
Does the calculated Wh equal usable battery energy?
Not necessarily. Condition, temperature, load, conversion losses, discharge limits, and BMS cutoff can reduce usable energy.
Can this calculator check charger compatibility?
No. Compatibility also depends on voltage, chemistry, connector, current limits, and the battery-management system. Use only approved equipment.
Can I use this for e-bike, solar, RV, power-station, and backup batteries?
Yes, for a nominal V, Ah, and Wh relationship. It does not verify wiring, charger safety, battery health, state of charge, or usable capacity.