How Big of a Home Generator Do You Need?

Power outages are an increasingly common part of American life.

Whether an outage lasts for a few minutes or a few hours, most people would prefer to keep the lights on and the fridge running during a blackout. And you can -- with the help of a portable or home generator. These generators use gasoline, propane or natural gas (all fossil fuels) to keep the electricity on.

But even if you've already decided you want a home generator, there are more decisions to make. Chief among them is choosing the right generator for your home. There are disadvantages to going too big or too small, so you want to try and get it right.

Here's how to figure out what the right size is for your home generator.

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Calculating the right generator size

Ultimately, choosing the right size for your generator is about making sure it can do what you want it to do, according to Kyle Raabe, president of consumer power at Generac.

Some people want their entire home to remain powered without interruption, while others just want to charge their phone and keep their food from spoiling. "What's important to people is different," Raabe said.

A generator that's too small will be an obvious disappointment, because it will leave some of your essential appliances in the dark. And if you go too big, you'll have extra power, but you'll probably pay more than you need to, Raabe said.

What's the deal with wattage?

Wattage is a measurement of the amount of electricity your home is using at any given time with your appliances running. Raabe said the average "running wattage" for a home is 2 to 4 kilowatts. But the important number for generators is "starting wattage," which is the measurement of how much power is being used during a surge of demand when all of your appliances kick on; that can be anywhere between 18 to 26 kW, Raabe said.

The electrical code in the US generally requires that home generators be sized to handle that starting wattage, which would accommodate all of your appliances turning on at once after the generator starts running.

Using wattage to find the right sized generator for you

Once you understand how much starting wattage your home needs -- let's say 22 kW -- you'd want a generator with the same wattage -- 22 kW -- or more.

Let's say you want a generator that can keep your fridge, A/C and water heater running, but you don't need your entire home backed up. The combined starting wattage of those three appliances is likely between 10 and 15 kW, which is the size of the generator you'd want. 

There are some online calculators that allow you to plug in your appliances and home details and get an estimated starting wattage and generator size.

You can also go a bit smaller than your total starting wattage if you stagger your appliances when they turn back on. Because appliances draw a big surge of energy right when they turn on, and then return to a much lower running wattage, you can reduce your max starting wattage by staggering when your appliances turn on after a blackout -- a feature that some generators offer as a default, Raabe said.

Read more: How to Choose Between Home Batteries and Generators

Generator size rules of thumb

It's hard to generalize the rules about sizing your generator, Raabe said. Each home and each person's needs are different.

That said, if you don't want to do any math, you can get a rough idea using an online calculator, and then bring in an expert to inspect your home and give you a more precise estimate, according to Raabe.

"A lot of people will just go bigger," Raabe said, because it gives them peace of mind. Or they'll use the staggering technology to reduce the size of the generator they need. "That's by far the preferred way to go," he said.

Should you get a generator?

To answer this question, Raabe says you should start by asking yourself: "What's at risk?" and "What's the problem you're trying to solve?"

For some people, blackouts are a minor inconvenience, or they can easily travel to a place that does have power until their lights come back on.

For others, blackouts might mean losing access to water from a well pump or refrigeration of critical medications. Especially for older folks who might not be able to travel to safety, a blackout is a bigger risk. "Being without power, there's a level of seriousness there," Raabe said.

If you fall into the latter category, you should probably think about getting a generator if you can afford it. But if you don't mind the occasional candlelit evening, you can probably get by without one.

Generator alternatives

If you're looking for a fossil fuel-free generator alternative, you have options.

If you're looking for partial (and portable) backup, you can look into portable power stations, batteries you can charge up and move around to wherever you need power. A solar generator (which adds portable solar panels to a power station) would let you charge up using the sun, instead of grid power. 

Home batteries can back up your whole home if you get enough of them, but more typically back up essential appliances. Paired with solar panels, batteries can let you use solar power through the night. 

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