From Envirowiki

Something that is not often correctly understood when dealing with energy systems is the difference between energy and power.

In physics when energy is transferred by a force, work is done. The work is the amount of energy transferred. Power is the rate at which work is performed or energy is transmitted, or the amount of energy required or expended for a given unit of time.

Put simply, when work is done the energy required is equal to the amount of work that is done. For example, it might take X amount of energy to push a ball to the top of a hill, regardless of any other factors.

Power, on the other hand, is a rate of work. that is, X amount of energy over Y amount of time. so, while to get the same ball to the top of the same hill in 10 seconds or 5 minutes will take the same amount of energy, different rates of power will be required.

[edit] 1 Calculation

In the metric system, energy is most commonly measured in Joules (j), power most commonly measured in Watts (W). one watt for one second is the same amount of energy as on joule. a watt is a measurement of joules per second. in the opposite way, one joule can be thought of as one watt-second. a common measurement for electrical power is the Watt Hour (Wh)

If we say that it takes 500joules to get the ball to the top of the hill (small hill!), then to get the ball up the hill in 10 seconds, it requires 500/10 joules every second for ten seconds, or 50 watts for ten seconds. 50W * 10s = 500Ws, or 500 joules.

for the same ball to reach the top in 5 minutes, it requires 500/5 joules every minute for five minutes. to figure this out, you need to convert the minutes back to second. 5 minutes = 300 seconds. therefor it requires 500/300 joules every second for 300 seconds, or 1.666...W for 300 seconds. 1.666W * 300s = 500Ws = 500j.

both watts and joules use the standard metric prefix system:

SI prefix	multiple	Number	base 10
pico	0.000000000001	trillionth	x10-12
nano	0.000000001	billionth	x10-9
micro	0.000001	millionth	x10-6
milli	0.001	thousandth	x10-3
centi	0.01	hundredth	x10-2
deci	0.1	tenth	x10-1
n/a	1	one	x100
deca	10	ten	x101
hecta	100	hundred	x102
kilo	1,000	thousand	x103
mega	1,000,000	million	x106
giga	1,000,000,000	billion	x109
tera	1,000,000,000,000	trillion	x1012
peta	1,000,000,000,000,000	quadrillion	x1015

[edit] 2 =conversion

• 1 Watt second (Ws) = 1 joule
• 1 Watt hour (Wh) = 360 joules
• 1 Gigawatt hour (GWh) = 360,000,000,000 joules, 360 Gigajoules (Gj), or 0.36 Terajoules (Tj).

• 1 Petajoule (Pj) = 2.777... Terawatt hours (TWh)

[edit] 3 in the real world

in the real world, a common measurement for power is Megawatts: a largish modern wind generator might generate 3MW (although wind generators on average operate under their rated output, so let's say 2MW). an average sized Coal Power station might generate 1400-2000MW (1.4-2GW). this is the amount of power it generates constantly. if you want to find out how much energy such a generator might produce over a year, simply multiply it by the number of hours in a year.

there are 24 hours in a day, and 365.25 days a year (leap years included).

• 24 * 365.25 = 8766 hours/year.

so the coal power station, if it produces an average of 1.6GW all year, would produce

• 1.6GW * 8766h = 14025.6GWh = 14TWh
• 14025.6GWh * 60 minutes * 60 seconds = 5,049,216GWs = 5,049,216Gj = 5.05Pj

for the wind generator, which we assume generates 2MW year,

• 2MW * 8766 = 17,532MWh = 17.5GWh
• 17.532GWh * 360 = 6312.52Gj = 6.3Tj

any questions?