How this instrument works
Energy is what gets consumed; power is the rate at which it happens. A device rated at 100 W is drawing energy at 100 joules every second, and multiplying that rate by how many seconds it runs recovers the total — which is exactly what E = P × t does, just with the time counted in hours instead of seconds for a number worth reading. One watt-hour is a watt sustained for an hour, equal to 3,600 joules, chosen not because it is elegant in SI terms but because it lines up with how appliances are rated and how utilities bill.
This is the calculation behind a utility bill and behind a portable power station's spec sheet alike. A 2,000 W space heater and a 20 W LED lamp cost the same per hour of runtime only if you ignore the wattage difference; multiply each by the hours it actually runs and the gap becomes ten thousand watt-hours against two hundred. Off-grid and camping setups lean on the same arithmetic in reverse, sizing a battery's watt-hour capacity against the appliances it needs to carry through a night.
The formula assumes power stays constant for the whole interval, and real devices rarely oblige. A refrigerator's compressor cycles on and off rather than drawing its nameplate wattage continuously, and a motor's true electrical draw can differ from its rated watts once reactive power and power factor are accounted for. Multiplying a nameplate figure by hours run gives a ceiling, not a metered reading — for a load that truly holds steady, like a resistive heater or an incandescent bulb, the two converge.
- Enter the device's draw in the Power field — watts for electronics and small appliances, or switch the unit menu to kilowatts for heaters and motors.
- Enter how long it runs in the Duration, hours field; fractional values work, so 90 minutes of runtime is 1.5.
- Read the total in the Energy field. It fills in automatically as either value changes.
- Switch the Energy field's unit menu to kWh to match the units your utility bill actually uses.
Worked example — a 100 W device run for 5 hours
Take a 100 W device — a desktop computer with its monitor, or a small space heater on its lowest setting — and run it for 5 hours. The formula multiplies straight through: E = 100 W × 5 h = 500 Wh, which the Energy field also reads as 0.5 kWh once the unit menu is switched. Nothing about the arithmetic changes; only how the same total is expressed.
Scale the power up and the total scales with it: the same 5 hours at 2,000 W — a portable space heater running at full tilt — consumes 10,000 Wh, or 10 kWh, twenty times as much for twenty times the power. That linear relationship is the whole point of the formula: energy grows with power and with time, and with nothing else.
Questions
What's the difference between watts and watt-hours?
Watts measure a rate — how fast energy is being used right now, like speed. Watt-hours measure a quantity — the total energy used over some stretch of time, like distance. A 100 W bulb draws 100 W whether it has been on for one minute or ten hours; only the watt-hour total, and the electricity bill built from it, keeps growing the longer it stays lit.
How do I convert watt-hours into kilowatt-hours?
Divide by 1,000 — kilo always means a factor of a thousand. 500 Wh is 0.5 kWh, and 10,000 Wh is 10 kWh. Utility bills are priced per kilowatt-hour because a single watt-hour is too small a unit to make a sensible line item; a whole house typically uses tens of kilowatt-hours in a day.
Why doesn't this match the number on my actual electricity bill?
Because most devices don't hold their nameplate wattage for the whole time they're switched on. A refrigerator's compressor cycles rather than running continuously, and a laptop draws less at idle than under load, so its true average power sits below the printed rating. Multiplying the nameplate figure by hours run gives an upper bound, not a metered reading; only a plug-in energy meter captures the real average.
How do I get watt-hours from a battery's amp-hour rating?
Multiply amp-hours by the battery's voltage: Wh = Ah × V. A 12 V, 100 Ah deep-cycle battery holds about 1,200 Wh, or 1.2 kWh. This is the conversion camping and off-grid solar setups rely on, since batteries are usually labelled in amp-hours but the appliances they run are rated in watts.
Where does the 3,600 in 'one watt-hour equals 3,600 joules' come from?
From the definition of an hour in seconds. A watt is already one joule per second, so sustaining one watt for one hour means one joule per second for 3,600 seconds — 3,600 joules total. The watt-hour is not an SI unit; it survives because it matches how power ratings and billing periods are actually written down.