Use the Electricity cost calculator:
You need to know:
1. Wattage of Appliance?
2. How long is it used each day? Hours
3. Cents per KWh?
You can get the answer for Q3 by looking at your electricity bill and calculating it from there.
For example, Hydrofarm JSV4 4-Foot Jump Start T5 Grow Light System (http://www.amazon.com/Hydrofarm-JSV4-4-Foot-Start-System/dp/B0001XLSGQ) uses 54W:
1. Wattage of Appliance? 54W
2. How long is it used each day? Hours = 12
3. Cents per KWh? 12 cents in Florida