Energy is power multiplied by time, and your supplier sells it by the kilowatt-hour. Set the wattage, the hours it runs each day and the rate you pay, then fast-forward a day and watch the meter count it up. A small load left on all day can easily beat a big one used in short bursts.
Energy per day E = P × t ÷ 1000
3.00 kWh
1500 W running 2.00 h a day
Cost per day = E × r
0.60
3.00 kWh at 0.20 per kWh, in whatever currency your rate is in.
Cost per month 30 × daily
18.00
30 days at 0.60 a day
Cost per year 365 × daily
219.00
365 days at 0.60 a day
Hours to use 1 kWh = 1000 ÷ P
0.667 h
A 1500 W load gets through one kilowatt-hour in 40 min.
Power P1500 W
Run time t2.00 h/day
Rate r0.20 /kWh
A month here is 30 days and a year 365 days, so both long-run figures are the daily cost scaled up — not a real billing period, and with no standing charge added. The rate and every cost share one unnamed currency: put your own rate in and read your own currency out.
Try this: 3500 W for half an hour, then 100 W left on around the clock. The little one costs more.