Bathroom Heater Fan Costs Too Much for Rental Apt?

[Peace Lily]

Dear BP Members,

We installed a ceiling heater and exhaust fans for a small bathroom for our rental property. There is no heat source in the bathroom. That is why we took the suggestion from our contractor. However, we feel that the tenants (students) can keep the heat switch on for a while or forget to turn it off etc. Since we are paying the electricity, we are concerned that it may end up costly. We thought about setting the timer but the duration can still be as long as 30 min. It is a small bathroom and the heat can come in from the living room. All it needs is just a bit heat to take the chill out at times. Is there any other option to heat up a small bathroom without costing much on electricity?  Thank you!