Free watt hour calculator. Turn watts and hours into watt-hours, kilowatt-hours and joules, or solve instead for the power or the running time.
Power is volts times amps. Every row is that one multiplication on a different label format.
| If the label shows | Work it out | Watts |
|---|---|---|
| 1,500 W | Already watts | 1,500 W |
| 12.5 A at 120 V | 12.5 × 120 | 1,500 W |
| 6.5 A at 230 V | 6.5 × 230 | 1,495 W |
| 3 A at 20 V, USB-C | 3 × 20 | 60 W |
| 2.5 A at 12 V DC | 2.5 × 12 | 30 W |
Most nameplates give watts outright. If yours gives amps and volts, multiply them. That product is exact for DC and for resistive AC loads such as heaters and filament bulbs; a motor or a switching supply also has a power factor below one, so its real draw is lower than volts times amps.
A watt is one joule per second and an hour is 3,600 seconds, so a watt-hour is 3,600 joules. That is a definition, not a measurement.
| Unit | Joules |
|---|---|
| 1 kilojoule (kJ) | 1,000 J |
| 1 watt-hour (Wh) | 3,600 J |
| 1 megajoule (MJ) | 1,000,000 J |
| 1 kilowatt-hour (kWh) | 3,600,000 J |
| 1 megawatt-hour (MWh) | 3,600,000,000 J |
Watt-hours here are the energy a device would use at a steady draw. Real devices vary — a fridge cycles, a laptop idles, a heater follows its thermostat — so a plug-in energy meter is the only way to get an actual figure rather than an estimate.
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Energy is power multiplied by time, and that single line answers most of what people search for here. Give this calculator a wattage and a duration and it returns watt-hours, kilowatt-hours and joules. Give it an energy figure and a duration and it returns the average power. Give it an energy figure and a wattage and it returns how long that energy lasts.
A watt measures power — the rate energy moves — and NIST defines it as one joule per second. An hour is 3,600 seconds. Multiply them and a watt-hour is 3,600 joules, exactly. So a 1,500 W heater running for three hours uses 4,500 Wh, which is 4.5 kWh, which is 16.2 megajoules. All three numbers describe the same energy; only the unit changes. Your electricity bill uses the kilowatt-hour because a watt-hour is inconveniently small for a household: an average US home gets through about 10,500 kWh a year.
Watt-Hour Formula
1,500 W for 3 hours is 4,500 Wh, or 4.5 kWh. That is the figure your meter counts.
A charger drawing 5 W left plugged in for 30 days is 5 × 720 = 3,600 Wh, or 3.6 kWh.
1 kWh is 1,000 Wh. At a steady 60 W that is 16 hours 40 minutes, before conversion losses.
A 500 Wh station emptied over 8 hours averaged 62.5 W, whatever its peak output was.
Your meter charges per kWh and your appliance is stamped in W. Converting between them is the only way to connect what a device says about itself to what it costs you.
A portable station says 500 Wh or 1 kWh. Dividing by the wattage of what you plug in tells you how many hours you get before it is empty.
A 1,500 W heater used for ten minutes costs less than a 60 W bulb left on for a week. Only the time turns a power rating into an amount of energy.
You do not, because watts per hour is not a quantity. A watt is already a rate — one joule every second — so watts per hour is a rate of a rate, like miles per hour per hour. What is almost always meant is watt-hours: multiply the watts by the number of hours the device ran. A 1,500 W heater uses 1,500 watt-hours for each hour it is on.
Multiply the watts by the hours to get watt-hours, then divide by 1,000. A 100 W device for 5 hours is 500 Wh, which is 0.5 kWh. The division by 1,000 is the only step people usually skip.
3,600 joules, exactly. A watt is one joule per second and an hour is 3,600 seconds, so the two definitions multiply out with nothing left over. In practical terms, one watt-hour runs a 1 W LED for an hour, or a 10 W router for six minutes.
500 watt-hours, which is 0.5 kWh or 1.8 megajoules. The arithmetic is 500 × 1, and running the same 500 W device for six minutes would use a tenth of that, 50 Wh.
It depends on the voltage, which is why milliamp-hours alone cannot answer it. Charge in mAh multiplied by volts and divided by 1,000 gives watt-hours: 20,000 mAh at the usual 3.7 V cell voltage is 74 Wh. Our battery life calculator does that conversion and also checks the result against the airline carry-on limit.
No, deliberately. It reports the energy the arithmetic gives, so you can see the ideal figure and apply your own derate. Real inverters lose roughly 10 to 20 percent as heat, and charging a phone from a power bank loses more, so treat the runtime here as a ceiling rather than a promise.