If you're thinking about starting a hydroponics garage farm and are curious how much you might potentially spend on farm lights (an important component in growing your own food - if you're set-up is inside), or are just interested in how much money you're spending on electricity for a given widget in your home, here's how you can figure it out: 

Watts * Hours Ran / 1000 * Kilowatt Hour Cost * Days In month = Cost per month

Example:

My electric company is run by thieves and so they charge me $0.10678 per kilowatt hour of usage.  Your own exact rate can be found on your electric bill somewhere.  If not, you can call the company and ask.

Let's say I want to know how much that 80 watt bulb is costing me if I leave it burning for 12 hours a day, every day of the month which has 30 days this time.  The substitution of values for the above formula would be as follows (just punch it into your calculator in the exact order it's written here):

80 * 12 / 1000 * 0.1678 * 30 = $4.83

To run a 80 watt bulb 12 hours a day every day for 30 days, using the daylight robbery electric rates in my city, will run me $4.83 cents per month.

Neat.  Not really, actually.  That really sucks.

Once you do this for a few of your appliances (for a real soul crushing experiment, try it with your Air Conditioner or your Washer), you will soon realize that supplementing your power consumption with renewable energy like solar and wind becomes much more appealing than you might have originally thought.