@Emily1419 It is a big deal for a guest to bring their own appliances to plug in to the host's electric without permission- it's extremely presumptuous.
How much electricity it costs to run is entirely dependent upon how many kwh the appliance consumes and how much the electricity costs. To say it costs 11 cents or 20 cents an hour is only your guess. Electricity is really expensive in some areas.
Where I live, electric is charged at a base rate for 150 kwhs, which is quite inexpensive. But if you go over the base number of kwhs, the rate goes up, and if you go over the second rate tier, the cost of electricity skyrockets to quadruple the base rate on the entire bill. And if you have one month at that rate, you are locked into that rate for the following 6 months, even if you never get near that many kwhs on subsequent bills.
So if a guest were to do something like that at my place, it could end up costing me hundreds of dollars.