Most portable heaters use the same number of watts as a microwave, or are at least in the same range. But unlike a space heater, microwaves are only in use for short periods of time. So yes, a space heater will make your bills go up, but as long as you use it in the most efficient way — i.e. inside a small, enclosed space — you’ll be making the most of your unit. And if you use the calculator above, you’ll already have a solid picture of how much using the heater will cost you at the end of the month.