Once you have the total number of running watts needed, add the highest number in the surge column to figure out what size generator rental you need. For example, if you are renting a small bounce house and a cotton candy machine, you need a total of 1455 running watts. The bounce house is going to need a surge of power to blow up; so, add 1955. The total watts needed is 3410. I'd recommend the 3500 watt generator. Below are common generator rental sizes with the average cost to rent.