How do you go about asking a roommate to pay more for the utility?
Try to get a fair and accurate estimate of how much it costs to run the heater. If you can look at one months bill when it wasnt used for comparison with a bill that includes the heater. Sometimes newer item will tell you the electrical output of your appliance and with some mathematical calculations you can get a close amount. You can also call the electric service to find a monthly charge for either with heater use or not. Then explain to roommate that the amount is enough that you must ask for their reimbursement if the inflated bill.