An important factor is how touchy this issue is. If power costs are a real bone of contention between the roommates then it might be safer to work off of the house’s power meter and pay for everything. Otherwise the roommate who is paying the power bill might find some pretext for disputing the payment made by mapman after mapman mysteriously consults with his tesla app. The house’s power meter is something the paying roommate can access and trust, and he can only be grateful to get more than adequate compensation according to his own measuring device.