Generator size corresponds to wattage, or the amount of power it can deliver. A simple way to determine how much wattage you need is to add up the total wattage of everything that will be plugged into the generator, factoring in the starting wattage of your tools. Keep in mind that a generator can’t run at its maximum capacity, also called surge power, for too long, so you may want to add 10% of the wattage calculation to ensure your generator will have enough power. The wattage of a generator is usually written out as kW (kilowatts) / kVA (kilovolts ampere), the capacity the generator has for power and its actual power output (for example, 20 kW / 25 kVA). There are 1000 watts in a kilowatt.