how much does and electric heater cost per month (room size)?

Answer 1

most of the time it will tell you on the box

Answer 2

The cost to operate an electric heater can be calculated by the operating time, times the Watts used, times the kilowatt cost per hour .
Most electric heaters have a max setting of 1500 watts.
Assuming that your heater is 1500 watts and you run it 10 hours per day and the cost of your power is $0.08 per kilowatt hour the cost would be calculate as per the example below.

1.5 X 10 X $0.08 =$1.20 or $0.12 per hour of use.