Chuck Meister at Western Cichlids.com has a wonderful article and website that answers this question wonderfully.
http://westerncichlids.com.au/6/

(from Chuck)
Monthly Cost = (wattage) x (charge/kwh) x (hours/day) x 30 x .001

■(wattage) can usually found on each item
■(charge/kwh) can be found on your electric bill; residential and in Perth is about 13c per unit
■(hours/day) is how long the item is operating
For example, an aquarium pump might use 4 watts and runs 24 hours per day, so it’s monthly energy cost would be:

4 x .13 x 24 x 30 x .001 = .37 or about 37 cents per month
Adding up all the items for a typical 20 gallon setup might look like this:

Item Wattage Hours/Day Cost
Light 15 12 .70
Air Pump 4 24 .37
Power Filter 9 24 .84
Heater 100 4* 1.56
Total $3.47

* This number varies considerably depending on difference between room and aquarium temperatures. In cooler climates or seasons, the heater might run a total of 12 or more per day; in warmer situations, it may hardly run at all.
Larger aquariums will of course require somewhat larger amounts of energy, and it often pays to look into the wattage rating on items before purchasing them.
Chuck Meister

(I'm sure if Chuck has a desk it would look like this)
Source(s):
http://westerncichlids.com.au/6/


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