How to calculate watt hours
Watt-hours (Wh) measure energy. There are two common ways to calculate them, depending on the information you have: battery charge and voltage, or power and operating time.
From battery charge (mAh or Ah)
Wh = mAh × V ÷ 1,000 Wh = Ah × V A 1,200 mAh battery with a nominal voltage of 3.7V gives: 1,200 × 3.7 ÷ 1,000 = 4.44 Wh. Battery capacity is commonly listed in mAh or Ah, but that number alone does not tell you the battery's energy. You also need its voltage. For example, a 12V battery and a 3.7V battery with the same 1,200 mAh rating do not contain the same amount of energy.
Important: battery voltage can vary
When converting a battery's mAh or Ah rating to Wh, this calculator uses the voltage you enter as a representative or nominal voltage. A real battery's voltage can change as it charges and discharges, so the actual energy delivered may differ from the simple calculation.
For example, a lithium-ion battery commonly has a nominal voltage of about 3.7V, even though its voltage is higher when fully charged and falls as it discharges. Using 3.7V in the calculation gives a useful estimate of its energy capacity: 1,200 mAh × 3.7V ÷ 1,000 = 4.44 Wh.
For battery comparisons, use the manufacturer's rated or nominal voltage when available. For precise energy measurements, the actual voltage over the discharge period and the battery's real delivered capacity would need to be taken into account.
From power and time
Wh = W × t A 100W device running at a constant 100W for 5 hours uses 100 × 5 = 500 Wh, or 0.5 kWh. Kilowatt-hours are the unit commonly used on electricity bills. To estimate the running cost, multiply the energy used in kWh by your electricity rate per kWh.
Frequently asked questions
What's the difference between mAh and Wh?
mAh (milliamp-hours) measures electric charge, while Wh (watt-hours) measures energy. Watt-hours depend on both the charge and the voltage. Two batteries with the same mAh rating but different voltages can therefore have different energy capacities.
Why does battery voltage matter when converting mAh to Wh?
Because mAh measures electric charge, not energy. The same amount of charge represents more energy at a higher voltage. The relationship is Wh = Ah × V, so both the charge and voltage are needed to calculate the battery's energy.
Does battery voltage stay constant?
No. Battery voltage can change during charging and discharging. For a simple mAh-to-Wh calculation, a nominal or representative voltage is normally used. The resulting Wh value is therefore an estimate rather than an exact measurement of the energy actually delivered under every operating condition.
How do I convert watt-hours to kilowatt-hours?
Divide watt-hours by 1,000. For example, 500 Wh is 0.5 kWh. Kilowatt-hours are commonly used for electricity billing, so converting Wh to kWh is useful when estimating the cost of running a device.