Energy is a property of physics that can be a little hard to describe completely. The most common definition is one that will work well for us. Energy is the ability to do work.

Power is just a rate of that work being done. Specifically, power is rate of energy per time (e.g. power generation, power consumption, etc.).

Energy = Power x Time[edit | edit source]

Energy equals power times time, i.e.

E = P * t


  • Energy is the ability to do work.
  • Power is the rate at which work is performed.
Symbol Description In Water In Electricity Base Units
Power P Rate at which work is performed Power=Current*Pressure (P=Q*H) Power=Current*Voltage (P=I*V) kg•m²/s³
Energy E The ability to do work Energy=Power*Time (E=P*t) Energy=Power*Time (E=P*t) kg•m²/s²

Common units of Energy and Power[edit | edit source]

Common Units of Energy and Power
Energy Power
joule joule/sec
calorie calorie/min
Btu Btu/hour
orange* orange/day
watt-hour watt
kilowatt-hour kilowatt

*Oranges[edit | edit source]

Okay so oranges are a made up unit of energy. But it could be a unit of energy. According to the USDA, one medium sized, 2-5/8" diameter orange, weighing 121g, has 59 Calories. And, because people in the USA do not want to think they are eating thousands of calories per serving, a Calorie is actually a kilocalorie, so one medium sized orange has 59,000 calories. So 1 orange = 59,000 calories and 1 orange/day = 40.97 calories/minute (take a moment to calculate this).

So why do people have a problem with watts[edit | edit source]

Watts are a rate. One watt is equal to one joule per second, i.e. 1 W = 1 J/s, but there is no unit of time in the name. Likewise one kilowatt-hour is a quantity with no explicit time. In fact, one kilowatt-hour equals 3,600,000 joules (take a moment to calculate this). One kWh could be an appliance drawing:

  • 1 kW for 1 hour
  • 2 kW for .5 hours
  • 1 W for 1000 hours (remember that the little k signifies kilo and means 1000x)


If you turn on 4 light bulbs, each rated at 25 W, how long can they be on before you reach 1 kWh?

[math]\displaystyle{ 4 bulbs \times \frac{25W}{bulb} = 100 W }[/math]

[math]\displaystyle{ E = P\times t \Rightarrow t = \frac{E}{P} \Rightarrow \frac{1 kWh}{100W} = \frac{1kWh}{.1kW} = 10 hours }[/math]

Kilowatt hour aka kWh[edit | edit source]

A kilowatt is just 1000 watts (kilo means 1000x). A kWh is 1000 watt hours.


How many kilowatt hours would be needed to run one 60W incandescent bulb for 100 hours?

[math]\displaystyle{ 60 W \times 100 hours = 6,000 watt hours }[/math]


[math]\displaystyle{ 6,000 watthours \times \frac{1 kilowatt}{1000 watts} = 6 kilowatthours = 6 kWh }[/math]

Electric energy consumption is usually metered and billed in kWh.

Analogies[edit | edit source]

Energy is a measurable quantity like distance. Power is a rate like speed.

Units Analog Units
Energy kWh Distance miles
Power W Speed MPH

Conversions[edit | edit source]

Joule Calorie Btu Watt-hour
1 joule 1 ~0.239 ~9.478 x 10^-4 ~2.778 x 10^-4
1 calorie ~4.184 1 ~3.966 x 10^-3 ~1.163 x 10^-3
1 Btu ~1,055.056 ~252.164 1 ~0.293
1 watt-hour 3,600 ~860.42 ~3.412 1

There is no such thing as kilowatts per hour

Hall of Shame[edit | edit source]

Please see the Hall of Shame for a short list of violators.

See also[edit | edit source]

Page data
Authors Lonny Grafman
Published 2006
License CC-BY-SA-4.0
Impact Number of views to this page and its redirects. Updated once a month. Views by admins and bots are not counted. Multiple views during the same session are counted as one. 25,438
Issues Automatically detected page issues. Click on them to find out more. They may take some minutes to disappear after you fix them. No main image
Cookies help us deliver our services. By using our services, you agree to our use of cookies.