Power and energy are different things
Most confusion over energy figures comes from one mix-up: power and energy are not the same quantity.
Power is a rate. It tells you how quickly energy is being used or produced at a given moment. The standard unit is the watt (W).
Energy is an amount. It tells you the total used or produced over a period of time. The standard scientific unit is the joule (J), and electricity is usually measured in kilowatt-hours (kWh).
A helpful comparison is a car. Speed is a rate, like power. Distance traveled is a total, like energy. Driving at 60 miles per hour tells you how fast you are going, not how far you have gone, until you know for how long.
The units one at a time
Watt (W). One watt is one joule per second. A small LED bulb may be rated at around 10 watts, and a kettle at around 2,000 watts, although ratings differ between products.
Kilowatt (kW). One kilowatt is 1,000 watts. Appliance ratings, home solar systems and heaters are often given in kilowatts.
Megawatt (MW) and gigawatt (GW). One megawatt is 1,000 kilowatts, and one gigawatt is 1,000 megawatts. Power plants and national grids are usually described with these. A plant described as having a capacity of 500 MW can deliver up to 500 megawatts at any moment.
Joule (J). The joule is the basic unit of energy in the metric system. It is small, so larger multiples are common: kilojoules (kJ), megajoules (MJ) and gigajoules (GJ). Food labels in many countries use kilojoules alongside or instead of calories.
Kilowatt-hour (kWh). One kilowatt-hour is the energy used by a 1-kilowatt device running for one hour. It is the unit on most household electricity bills. Larger totals are given in megawatt-hours (MWh), gigawatt-hours (GWh) and terawatt-hours (TWh), the last of which is often used for national annual figures.
Converting between them
One watt is one joule per second. An hour has 3,600 seconds. So one kilowatt-hour, which is 1,000 watts for 3,600 seconds, equals 1,000 x 3,600 = 3,600,000 joules, or 3.6 megajoules.
That single conversion lets you move between everyday electricity units and scientific ones.
Worked examples
These use round numbers for illustration, not measurements of particular products.
- A 100-watt device running for 10 hours uses 100 W x 10 h = 1,000 watt-hours, which is 1 kWh.
- A 2-kilowatt heater running for 3 hours uses 2 kW x 3 h = 6 kWh.
- A 10-watt LED bulb running for 5 hours uses 10 W x 5 h = 50 watt-hours, or 0.05 kWh.
- If a household uses 3,000 kWh in a year, its average power is about 3,000 kWh divided by 8,760 hours in a year, which is roughly 0.34 kW, or about 340 watts. Actual use rises and falls through the day, so this is only an average.
If electricity costs a certain price per kilowatt-hour, multiplying that price by the kWh used gives the cost. This is how the charge appears on a typical energy bill. For example, at a price of 0.20 per kWh, the 6 kWh from the heater above would cost 6 x 0.20 = 1.20 in whatever currency is used.
Prefixes at a glance
- kilo (k): one thousand
- mega (M): one million
- giga (G): one billion
- tera (T): one trillion
Case matters. A lowercase “m” in front of a unit means milli, one thousandth, while a capital “M” means mega, one million. The difference is a factor of a billion, so careful reading is worthwhile.
Common mistakes
Writing kW when kWh is meant. A battery described as “10 kW” has a power rating, which is how fast it can deliver energy. A battery with “10 kWh” holds a quantity of energy. Both figures matter and answer different questions, and the second is what people usually mean by battery capacity.
Dropping the “per hour” in solar claims. A solar array rated at 5 kW does not produce 5 kWh every hour. Solar power output depends on sunshine, season and weather, and the ratio of actual output to maximum is described by capacity factor.
Comparing capacity with generation. A 1 GW plant and a 1 GW wind farm have the same maximum power, but over a year they usually produce different amounts of energy.
Mixing primary and final energy. Some national statistics count the energy in fuels before conversion losses, while others count what is delivered. Comparisons across sources should check which is used.
Other units. Natural gas may be sold in cubic meters, therms or British thermal units (BTU), and oil in barrels. Conversion factors depend on the fuel’s energy content, which varies a little, so conversions are approximate unless a standard value is stated.
A quick checklist
When you read an energy figure, ask whether it describes a rate or a total, whether the prefix is thousand, million or billion, and whether the time period is stated. If a claim lacks one of these, the number may not be as informative as it looks.