How this instrument works
The kilowatt-hour is a unit built for billing, not for physics. It multiplies a rate of use, one kilowatt, by a familiar span of time, one hour, and utilities across the world settled on it because household electricity use naturally falls into a range — a few to a few dozen kilowatt-hours a day — that reads cleanly on a monthly statement. Many countries' bills still call it simply 'one unit' of electricity, a shorthand that predates decimalised billing systems and has stuck around in everyday speech in India, Britain and elsewhere.
The kilojoule, by contrast, is a strict multiple of the SI's own coherent unit of energy, the joule — one newton of force acting over one metre, or equivalently one watt sustained for one second. Because a watt is already defined as one joule per second, and an hour is a fixed, non-negotiable 3600 seconds, converting a kilowatt-hour into kilojoules needs no international treaty and no measured constant: 1 kW × 3600 s = 3600 kJ, exactly, every time. That certainty is unusual among unit conversions — most owe their exactness to a historical agreement fixing an artefact, while this one falls out of arithmetic alone, given that an hour has always meant 3600 seconds.
That exactness is what makes the kilowatt-hour figure on a bill directly usable in a thermodynamics or engineering calculation the moment it is turned into kilojoules. Specific-heat problems, insulation-loss estimates, battery-capacity comparisons and appliance energy-use figures are almost always expressed in joules or kilojoules in textbooks and engineering references, even though the meter on the wall counts in kilowatt-hours — this conversion is the bridge between the two, and it never introduces uncertainty of its own.
- Enter the energy figure into the Kilowatt-hours (kWh) field — it opens at 1, the unit commonly called '1 unit' of electricity on many bills.
- Read Kilojoules (kJ) below it; the result updates on every keystroke and needs no rounding to be exact.
- For a running appliance, multiply its wattage by the hours used, divide by 1000 to get kWh, then enter that figure here.
- Working the other way? Divide your kilojoule figure by 3600 to return to kilowatt-hours.
- Feeding a specific-heat or thermodynamics formula? Keep the result in kJ, since most such formulas are built around joules, not kilowatt-hours.
Worked example — 1 kWh (one billing 'unit') turned into a water-heating estimate
A utility bill charges by the 'unit', where one unit equals exactly one kilowatt-hour. Type 1 into Kilowatt-hours (kWh) and Kilojoules (kJ) reads 3600.0 — the figure a physics or engineering formula, built around joules rather than kilowatt-hours, actually needs to work with.
Put that 3600 kJ to use: water's specific heat capacity is about 4.186 kJ per kilogram per degree Celsius, so raising water from a 20°C tap temperature to a near-boiling 100°C, an 80°C rise, needs roughly 4.186 × 80 ≈ 334.9 kJ per kilogram. Dividing 3600 kJ by that figure shows one kilowatt-hour carries enough energy, ignoring all real-world heat loss, to take about 10.75 litres of tap water from room temperature to near boiling — a rough but useful sense of scale for what 'one unit' of electricity actually represents physically.
Questions
Is 1 kWh exactly 3600 kJ, or is that rounded?
Exactly, with nothing rounded anywhere in the chain. A watt is defined as one joule per second, so one kilowatt is 1000 joules per second, and an hour is a fixed 3600 seconds; multiplying gives 1000 × 3600 = 3,600,000 joules, or 3600 kilojoules, with every figure in that calculation being a definition rather than a measurement. Unlike conversions built on a historical treaty, such as the inch or the pound, this one needs no international agreement at all — it falls straight out of what a watt, and an hour, already mean.
Why do electricity bills use kilowatt-hours instead of kilojoules or megajoules?
Scale and habit. A typical household uses somewhere between about 10 and 30 kilowatt-hours a day, numbers that fit comfortably on a monthly statement and are easy to compare month to month. The same usage expressed in kilojoules would run from roughly 36,000 to over 100,000 per day, and in joules alone would run into the tens of millions — technically correct, but far less legible to someone glancing at a bill once a month.
What's the difference between kilowatt-hours and kilowatts?
A kilowatt is a rate, how fast energy is being used or produced at a given instant; a kilowatt-hour is a total amount, that rate sustained for one hour. A 2 kW space heater draws energy at a constant rate of 2 kilowatts, but only accumulates 2 kilowatt-hours, and therefore 7200 kilojoules, if it runs for a full hour — run it for 30 minutes instead and it uses just 1 kWh, or 3600 kJ, half as much.
How does a kilowatt-hour compare to the kilojoules on a food label?
They are the same physical unit of energy measuring two very different processes — electrical work in one case, food's chemical energy in the other. A typical adult daily intake of about 2000 kilocalories works out to roughly 8368 kilojoules, which is close to 2.3 kilowatt-hours of energy — a useful comparison for intuition, though converting between food energy and electrical energy this way says nothing about the efficiency of turning one into the other.
How many kilojoules does a typical home appliance use per hour?
Multiply its wattage by the hours run, divide by 1000 for kilowatt-hours, then by this page's factor for kilojoules. A 1500-watt space heater run for one hour uses 1.5 kWh, which is 5400 kJ; a 100-watt light bulb left on for ten hours uses 1 kWh, the same 3600 kJ as this page's opening example, despite the very different wattage and duration that produced it.
Is this the same as converting to British thermal units?
No — kilojoules and British thermal units are both energy units, but they are not the same size, and this calculator only handles the kilowatt-hour-to-kilojoule step. One BTU is about 1.055 kilojoules, so a figure already converted to kilojoules here would need a further division by roughly 1.055 to reach BTU, a unit still common in North American heating and air-conditioning specifications.