If you want to substitute your own numbers, then you'll need two pieces of information

1)Computer power consumption in watts
2)Cost per KWh ( kilo – watt hour ) that your supplier is charging

My example uses a Windows XP computer in my home office which is 2 years old ( Newer hardware will consume less ). It doesn't have a power management scheme.

The power consumption varies depending on what it's doing. I decided on three categories; Active i.e doing work stuff, idling and hibernate. To all intents and purposes, the latter is the same as if the computer was switched on at the wall socket, but not powered on. Hibernate is going to be used to see what savings we could achieve if the computer was in this mode rather than idling.

The power consumption for each area is:-

Active: 195 watts
Idling: 130 watts
Hibernate: 6 watts

For electricity in the U.K, the cost per KWh works out at £0.15 GBP. If you're in the U.S, you can use figures from the U.S Government
here which equates to $ 0.0915.

I'll assume that the computer is switched on 24x7 all day with work activities taking up 8 hours daily over the course of a 5 day week.

Now that we have all of the variables, lets jump into the math.

Calculate the yearly running costs for our three areas.

Active = 52 weeks x Active hours per week x Cost per Kwh x Active power consumed / 1000
Active ( U. K ) = 52 x 40 x 0.15 x 195 / 1000 = £60.84
Active ( U.S ) = 52 x 40 x 0.0915 x 195 / 1000 = $37.11

Idling = 52 weeks x Idling hours per week x Cost per Kwh x Idle pwer consumed / 1000
Idling ( U.K ) = 52 x 128 x 0.15 x 130 / 1000 = £129.79
Idling ( U.S ) = 52 x 128 x 0.0915 x 130 / 1000 = $79.17

Hibernate = 52 weeks x Idling hours per week x Cost per Kwh x Hibernate power consumed / 1000
Hibernate ( U.K ) = 52 x 128 x 0.15 x 6 / 1000 = £ 5.99
Hibernate ( U.S ) = 52 x 128 x 0.0915 x 6 / 1000 = $ 3.65

We can conclude that if the computer was in hibernate, rather than idle, then we can save each year in the U.K £123.80 and in the U.S $75.52.

Just for fun, if you then use this information across various company sizes ( assuming they use identical computers ) , then you can create a table like this

Computers	U.K savings	U.S savings
100	£12,380	$7,551
1000	£123,801	$75,518
2500	£309,504	$188,797
5000	£619,008	$377,594
7500	£928,512	$566,392
10000	£1,238,016	$755,189

There might be something in this power management after all....