Monday, March 04, 2013

How do you calculate electricity costs using kWh?

This one has always been a puzzler because I didn't usually think about it much. Don't get me wrong, I always think about saving energy in terms of wattage. You'll catch me saying things like "Turn the damn light off, it's burning 100W per hour!", and "OMFG, the computer is burning 30W overnight when you leave it on doing nothing!".

In case you're not aware, electricity is usually billed by the kilowatt-hour (kWh). A watt (W) is a rate with the units Joules per second, a kilo means a thousand, and an hour is an hour. Notice that a watt is already a unit over time -- this always used to confuse me as you can see in the first paragraph about past ramblings.

A kilowatt-hour is then just the total of 1000 W used continuously over an hour. All that means is if you had a 1000 W light bulb (bright as the sun?) run for one hour, a kilowatt-hour is the total amount of energy used. This makes calculating electricity consumption easy!

How many kilowatt hours (kWh) does my device use?

Find out how much your electrical device uses in watts (usually labeled somewhere), divide by 1000, and then you'll have the energy consumption each hour.

For a 60 W light bulb: 

60 W x (1 kW / 1000 W) x 1 hour = 0.06 kWh

How much does running my device cost?

If you check your utility bills, your rate would more than likely be in dollars per kilowatt-hour ($/kWh) -- I don't know, maybe it's in joules to mess with you. To figure out the cost, take your consumption rate above and multiply it by your cost.

For the 60 W light bulb:  

0.06 kWh x ($0.10 / kWh) = $0.006

That amount make no sense... Let's assume we run the bulb for 8 hours a day over a year.

For the 60 W light bulb: 

$0.06 kWh x ($0.10 / kWh) x (8 hours / day) x (365 day / year) = $17.52 / year

Okay, looks more familiar. Unfortunately, calculating your bill is no longer that easy. We're in the electricity conservation and environmentally conscious age, so everything is metered. We have tiered rates, different consumption levels, etc. Your billing varies depending on what time of day you use things and how much you use over a period (e.g. one price for first 500 kWh, another for the next 300 kWh). Just divide your hours of use between those different tiers or take an average. Don't worry, your utility company won't mind charging you to do the math.