How much do you pay per kilowatt? get your last power bill, and take the total and divide by KWH used - this will give you a flat rate, inclusive of any rate changes, delivery fees, etc etc

Then multiply the lamps watts by KWH cost by Hours of use - there ya go

eg.
I pay .05 per kilowatt. I want to use a 250W lamp 12 hours a day 12x250 = 3000 watts= 3 KWH
So thats .15c for each bloom day