How much energy is used if 500 watts of power is left on for 5 hours?

A kilowatt hour is equal to the amount of electrical energy consumed at a rate of one kilowatt. 1 kilowatt hour is the same as 1000 watt-hours.

So using that principle. A 500 watt microwave will consume 500 watt-hours in an hour, or 0.5 kilowatt-hour.

Comment

In practice, it's difficult to determine an answer to your question, because microwaves don't operate continuously but, rather, in short bursts. Without knowing the cycle rate of each burst of energy, it's impossible to come up with a definitive answer.