@Soohyun1 WoW ! Do you have anything in your listing or house rules stating not to change/move items in the rental ? Usually people are referring to furniture, but light bulbs qualify 🙂 You need to have a chat with them about changing out the light bulbs. Also, a space heater is an appliance. Renters should not be bringing in an appliance.
"tripped the main electric power multiple times already"~ Address this with them as whatever they are doing is obviously not safe. Reconnecting the power without reducing the load is not safe.
Space heaters are not meant to be used to increase the temperature of a whole unit, they are made to supplement just a small area for a short time. It is unsafe to run them constantly. Read the manual for full disclosure 🙂 I would also wonder if the tenants are running other appliances (stove, oven, drier) to generate heat.. and what are they creating the heat for ? Are they 'growing' something ?
Google: On average, it costs about 24 cents per hour to run a standard 1,500-watt space heater.
Doesn't sound expensive but that's just one heater for one hour.. One heater for 8 hrs a day= $50-60 in a month's time.
Is it snowing at the moment ? Does your place have adequate insulation, blankets and insulating curtains ? Has anyone else stayed there then complained it's constantly cold ? Everyone's different, but I've never known anyone who is so cold they have their temperature set that high 24/7. Not to pry, but is a guest also visiting the hospital for some sort of treatments (cancer, rehab); perhaps it has skewed their internal temperature regulator. Even so, changing out light bulbs is a pre-meditated move and I wouldn't allow that. Did they ask permission ? Have they explained their actions ? Are they acting normal in other respects ? Are they secretive ? Did they bring in lots of luggage/boxes, etc ?
You could let them know it's not acceptable, not safe, not allowed to continue. See how they react.
Tell them the electricity never turns off for you so it's got to be overloading due to the obvious heat they are generating. Explain it's not safe and they need to remove all the extra appliances they brought in and remove their light bulbs to bring the wattage down to safe levels for everybody 🙂
I feel for you, it's going to be an awkward conversation. But it's your house and they have created a problem for the house and it's safety. Good luck !