How to use the Watt-hour Calculator
A watt-hour (Wh) is a unit of energy, not power. Power (watts) tells you the rate at which energy is used at a given instant; energy (watt-hours) tells you the total amount used over a stretch of time. The relationship is simple: E = P × t, where P is power in watts and t is the duration in hours. Multiply a 1500 W space heater by 3 hours of use and you get 4,500 Wh — that total, not the 1500 W rating, is what determines your electric bill and how long a battery will last.
Deriving watt-hours from power and time
Power is defined as energy per unit time: P = E / t. Rearranging for energy gives E = P × t. Because the watt-hour is a hybrid unit (watts × hours), the time value must be converted to hours before multiplying — 90 minutes becomes 1.5 h, and 2 days becomes 48 h. This calculator handles that conversion automatically: enter power in watts, kilowatts, or milliwatts, and time in minutes, hours, or days, and it normalizes both to W and h internally before computing E = P × t.
Watt-hours, kilowatt-hours, joules, and amp-hours
Electric utilities bill in kilowatt-hours (kWh = Wh ÷ 1000) because raw watt-hours are a small unit for household consumption. In the SI system, energy is measured in joules, and since 1 watt = 1 joule/second and 1 hour = 3,600 seconds, 1 Wh = 3,600 J exactly. For batteries, capacity is often labeled in amp-hours (Ah) rather than watt-hours; the two are related by voltage, Wh = Ah × V, so the same 100 Ah rating stores twice the energy at 24 V as it does at 12 V. Always compare battery capacity in Wh, not Ah, unless the voltages match.