How can I figure out how much energy an appliance uses?

Multiply the wattage (w) of the appliance by the number of hours of operation (hr), to get the watt hours (whr): w x hr = whr 
If the appliance is on for less than an hour here are some quick conversions:
15 minutes = .25 hrs
30 minutes = .50 hrs
45 minutes = .75 hrs

Then divide the total by 1000 to get kilowatt hours or kWh: w X hrs = whrs/1000 = kWh

BELD’s rate is approximately 8.5 cents per kWh, so multiply the kWh total by $.085 to get the cost: kWh X $.085 = cost of running appliance

Finally, don’t forget to multiply your total by 30 to get a monthly total. Here’s the complete formula:

_______w X _______hrs =______whrs/1000 =______kWhr X .$085 = $______cost

And here are some examples: 
Running one 100w light bulb for 10 hours
100w x 10 hrs = 1000whrs/1000 = 1 x .085 = $.085 or 8.5 cents.

Running a 750w space heater for 3 hours a day for a month
750w x 3 hours = 2250whrs/1000 = 2.25 x .085 = .10125 x 30 = $3.04

Using two 1500w hair dryers for 15 minutes a day
1500 x .25 375/1000 =.375 x .085 = .032 (per dryer/per day) x 30 =.96 x 2 = $1.92

You can use this formula for anything with a known wattage except things that cycle on and off, including refrigerator/freezers, air-conditioners, de-humidifiers, dryers, etc.