To figure out what size your whole house generator needs to be, you’ll need to decide which devices in your home you’ll want to power during an outage, and what their startup and running power requirements are. This is an important step, and there are two ways to do it.
One way is to have an electrician come out and measure all your appliances and circuits. This is the recommended way to do it, because if you under-measure, you risk overheating the whole house generator and doing damage to your electrical system, and if you over-measure you will waste money on the initial purchase cost of your whole house generator unit.
It is also possible to measure your wattage requirements yourself. With a little investigating you can find out the start-up and running power requirements of your appliances. One thing you might be tempted to do is use one of the many wattage calculators found on the internet. If you decide to go this route, proceed with caution. The wattage amounts for the listed appliances in these calculators are just estimates, and every appliance and every home is different. Wattage calculators also don’t take into account that not all appliances that you will be running off your whole house generator will run simultaneously; the usage for many appliances fluctuates. Also, when you are considering a whole house generator system, the wattage requirements of an individual light bulb is insignificant compared to the other, larger appliances and circuits.
Remember that the safest, most accurate way to determine what size whole house generator you need is to have an electrician do the measuring. But if you decide to take on the task yourself, use the online calculators just to get a rough estimate.
To read more about generator size, checkout our generator buyer's guide.