Amp Hours to Watt Hours Calculator and Formula

This amp-hours-to-watt-hours calculator converts a battery’s capacity in amp-hours (Ah) and its voltage (V) into energy in watt-hours (Wh). Enter the amp-hours and voltage to get the estimated watt-hours; the result applies to the voltage you provide.

Formula

Watt-hours = amp-hours × volts, or Wh = Ah × V.

• Ah is the battery capacity in amp-hours.
• V is the voltage used for that capacity rating, in volts.
• Wh is energy in watt-hours. Divide watt-hours by 1,000 to express the same energy in kilowatt-hours (kWh).

The calculator’s first result is watt-hours, the requested conversion; its second result expresses that energy in kilowatt-hours. The calculation assumes the amp-hour rating and voltage refer to the same battery or battery system. It gives nominal energy, not a guarantee of how much energy a device can use.

Worked Example

Using the calculator’s default values, capacity is 10 Ah and voltage is 12 V.

1. Multiply capacity by voltage: 10 Ah × 12 V = 120 Wh.
2. Convert to kilowatt-hours: 120 Wh ÷ 1,000 = 0.12 kWh.

The calculator returns 120 Wh and 0.12 kWh.

How the Conversion Works

Amp-hours describe electric charge delivered over time; watt-hours describe energy. Voltage connects the two quantities. In basic electrical terms, watts equal volts multiplied by amps. Since an amp-hour represents one amp flowing for one hour, multiplying Ah by V gives watt-hours: (A × h) × V = W × h.

For example, a 5 Ah battery rated at 3.7 V has a nominal energy of 18.5 Wh. A 20 Ah battery at 24 V has 480 Wh, or 0.48 kWh. These examples show why amp-hours alone cannot tell you how much energy a battery stores: the voltage matters too.

For a battery pack made of cells, use the pack’s overall capacity and nominal pack voltage together. Do not multiply a pack’s Ah rating by the voltage of one cell unless those values describe the same configuration. A series connection raises voltage while keeping Ah capacity the same; a parallel connection raises Ah capacity while keeping voltage the same, assuming identical cells and a suitable design.

When to Use the Result

Watt-hours make it easier to compare batteries with different voltages and to estimate energy needs for portable electronics, backup systems, and other equipment. They can also provide a first-pass runtime estimate when you know a device’s power use:

• Approximate runtime in hours = usable Wh ÷ device power in W. A 120 Wh battery supplying a 30 W load would suggest 4 hours under ideal conditions.
• Energy in kWh = Wh ÷ 1,000. This is useful when comparing a small battery’s stored energy with larger energy quantities commonly expressed in kWh.
• For an estimate in the other direction, Ah = Wh ÷ V. The voltage must still be specified.

Runtime estimates are not exact. Conversion losses, the battery’s condition, temperature, discharge rate, the device’s changing power demand, and the battery’s protection or cutoff settings can all reduce usable energy. In particular, do not assume that all nominal battery energy can be drawn safely or effectively.

Common Mistakes and Limits

• Using Ah without voltage: There is no unique Wh answer without a voltage. A 10 Ah battery at 12 V is 120 Wh; at 24 V it is 240 Wh.
• Mixing units: Use amp-hours, not milliamp-hours, unless you convert first. Divide mAh by 1,000 to get Ah; for instance, 5,000 mAh is 5 Ah.
• Confusing power with energy: Watts (W) measure a rate of energy use; watt-hours (Wh) measure energy. A 100 W device running for two hours uses about 200 Wh before losses.
• Treating nominal voltage as constant: A battery’s actual voltage can change during charging and discharging. The calculator uses the voltage entered, not a detailed voltage curve.
• Ignoring usable capacity: The rated energy may differ from energy available at the output because of discharge limits, conversion equipment, and operating conditions.
• Rounding too early: Keep the input values as stated on the battery or specification sheet, then round the final estimate to a sensible precision. The result should not imply greater measurement accuracy than the inputs support.

For a more realistic runtime calculation, use usable battery energy and the load’s average power, and account for inverter or converter efficiency if applicable. A label’s capacity rating may have been measured under stated test conditions, so consult the product’s specification for those conditions when accuracy matters.

Frequently Asked Questions

How many watt-hours are in one amp-hour?
There is no single answer without voltage. At 1 V, 1 Ah equals 1 Wh; at 12 V, 1 Ah equals 12 Wh. Use Wh = Ah × V.

How do I convert mAh to Wh?
First divide milliamp-hours by 1,000 to get amp-hours, then multiply by voltage. For example, 5,000 mAh at 3.7 V is 5 Ah × 3.7 V = 18.5 Wh.

Can I calculate watt-hours from amp-hours alone?
No. You need the battery’s voltage as well. Two batteries with the same Ah rating can store different amounts of energy if their voltages differ.

How do I convert watt-hours back to amp-hours?
Divide watt-hours by voltage: Ah = Wh ÷ V. For example, 120 Wh ÷ 12 V = 10 Ah, assuming the figures refer to the same nominal battery system.

Does a higher Wh rating mean a battery lasts longer?
It generally indicates more nominal stored energy, but runtime also depends on the device’s power demand, usable capacity, and conversion losses. Compare batteries using the same kind of rating and consider the actual load.

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in ah | Capacity | Ah | 10 | 0
in v | Voltage | V | 12 | 0
out wh | Energy | Wh | ah * v | 0
out kwh | Energy | kWh | ah * v / 1000 | 2