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Power and Energy explained
The difference between "Power" and "Energy"

You may be interested to know that energy and power are different, and there's a scientific fact which is easy to explain.

Although you sometimes find people using the terms "power" and "energy" almost interchangeably, that's only because they don't understand the facts about what these things really are.

ENERGY is a type of stuff like money in the bank. It's something which has a quantity, almost as if you could have bags of it. Batteries can be filled up with energy, like cups can be filled up with fluid. Energy can be considered to have amounts. Energy can be measured in Joules *, or in Kilowatt-hours (KWh), and in various other energy units, like the way amounts of money can be measured in pounds, dollars, euros, etc. The type of concept of a battery full of energy is like a bucket full of water, or like a bag full of cash.

POWER is a rate of flow of energy. It is energy per second, so its standard unit is the Joule per Second (which is the same as Watts). It can also be measured in kilowatts, megawatts, etc. Using the money analogy, power is like income or spending power, a rate at which you can transfer money. Using the water analogy, power is like the flow of water through a tap (litres per second, gallons per hour, etc).

As power and energy are different things, they can't be compared directly. Although it wouldn't be quite like comparing chalk and cheese, it would be more like comparing chalk with blackboard-writing rate, or comparing cheese with lunch-munching rate.

Energy Units: Energy is measured in Joules. There are 3,600,000 joules in a kilowatt-hour. Here's a table of Energy Units:xxx


Joules equivalent





BTU *2







4.184 x 1015
(exponential notation)

The megaton is included as it's an energy unit. Although originally devised as a yardstick for atomic bomb explosions, it's handy for quantifying asteroid impacts and other natural catastrophic events.


Power Units: Power is measured in Watts (which are joules-per-second). Here are a few more Power Units:


Watts equivalent











There is an odd unit of power which is sometimes used, the "kilowatt-hour per year" (KWh per year). This is usually seen on boasts about the "energy efficient" or "environmentally friendly" nature of some electrical device. It makes about as much sense as describing the quantity of water in a swimming pool as so-many "gallons-per-hour DAYS", ie how many days it would take to fill it at however many gallons per hour the water supply could manage. It would be much better to just say how many gallons or litres there were in the pool. Similarly, the "KWh per year" has a watt conversion-rate such that a KWh per year is about 0.11 watts, or... 9 KWh/year = 1 watt.

If you're wondering about the mixing up of terminology and how many people get it wrong, you're in good company. Even electricity companies such as E.ON have got this mixed-up!

An Example of Energy and Power in practice:

If you have a microwave oven that's rated at 650 watts, and you heat a litre of water for 60 seconds, the microwave oven will transfer 650 joules of energy per second into the water (minus whatever inefficiencies there are in the system). 650 watts of power represents 650 joules of energy per second. So, after the 60 seconds is up, the water will contain 650 x 60 = 39,000 joules more energy than it started with. (39 thousand joules of heat in a litre of water means it will be about 9 degrees Celsius warmer. Why 9 degrees? See Specific Heat of Water).

When you pay for electricity, you are paying for energy. Each kilowatt-hour is a "unit" on the electricity bill, and a kilowatt-hour is 3,600,000 joules. How much you pay for each "unit" is a matter of agreement between you and your electricity supplier. Often there is fierce competition between electricity companies to attract customers by offering the best deal, and you can see a variety of electricity companies to choose from here. Also, there are places such as uSwitch, (energy comparison sites), where they'll compare the prices for you.

Gas is cheaper per joule than electricity, but you can't run your computer off gas unless it's got a special gas-fired computer power supply, and they are uncommon.

* JOULES: A measurement of energy, not to be confused with Joules Fashion Shop.

*2: BTU (British Thermal Unit) is a unit of energy (not power). It's mentioned in The Matrix that humans produce 25000 BTU. Is this true? Presumably it's supposed to be 25000 BTU PER DAY rather than 25000 BTU. ie 305 watts.

The BTU/hour is an old-style heat power unit. Airconditioners and boilers are sometimes rated in BTU/h. To convert from BTU/h to watts, multiply by 0.293 . Occasionally an aircon can have its power usage in watts and its effective refrigeration stated in BTU/h. When you do the calculations you may be amazed to see the power out is more than the power in. So, more than 100% efficient? The reason is because it's a heat pump. More about this on the page of How airconditioners and fridges work