Sizing Generators Correctly

How to Size a Generator

There are lots of opinions about how to size a generator correctly.
But for residential generators, it’s not as hard as people make it out to be.

Keep it simple. A common mistake in sizing generators is making it complicated. You can spend hours analyzing appliances, adding up loads and accounting for surges; but you still won’t have an accurate picture of what actually happens during a power outage. This is because power demand changes in real time.

Some of the things which change power demand are:

Changes in HVAC usage due to weather and outdoor temperature.
Large appliances like electric water heaters cycling on and off.
Winter time auxiliary heat kicking in during severe cold.
Changes in appliance usage due to emergencies.

Making a list of appliances and their individual power usage doesn’t account for the fact that you never know what combination of appliances will be in use at the same time, which drastically affects total load. 

If only one major appliance is used at a time, the electrical demand on the generator is low. But when multiple appliances operate, the totals escalate quickly. Sometimes the homeowner doesn’t have easy control over what runs together. Water heaters don’t have a control knob, and auxiliary heat kicks in several times a day unknown to the homeowner.

In an emergency, homeowners often use more electricity than they plan for. Many homeowners also don’t realize how much electricity their appliances use, especially in an all-electric home without any gas appliances.

As you can see, there is a huge difference in the amount of power consumed by a home with all-electric major appliances vs. the gas appliance home. The electric home will require a larger generator to handle peak demand, or a system of load control modules that ensure some appliances don’t run together when the power is out.

Gas appliances are a benefit in every way. They relieve load from a generator and enable you to install a generator system without load control modules. Still, a minimum generator size of 20kW is recommended to allow enough spare capacity for motor starting of the HVAC system. In addition, generators smaller than 20kW in size are about the same cost as a 20kW unit, and are special order – requiring additional shipping costs. Heritage Generator does not install units smaller than 20kW.

I don’t need a big generator. I only want to run a light and my refrigerator.

-common homeowner sentiment

The internet says I need a 22kW.

– homeowner

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