What Size Generator Do I Need? Whole House Generator Sizing Made Easy

You’ve decided that you’re ready to invest in a standby generator, or a permanent unit capable of powering your essentials during an outage. 

You considered a less expensive portable unit or another low wattage standby, but you fear the smaller generator may not have the power you need to support your wattage. 

But how much wattage do you actually need— and what size home generator do you need to fuel your house-full of electronics?

We’re here to help you choose the right size generator for your electrical needs.

How Many Watts Do You Need?

The first step in choosing your ideal home generator size is to make a list of all the electronics you’d like to power during the outage. 

Jot down some bullets with the essentials first, such as keeping your fridge running, a few lights on and your AC pumping (here in Florida, this is a must!). Next to that, create a column of “nice-to-haves,” like powering your hair dryer or microwave, etc. If you’d like the luxury of having everything run as normal, sharpen your pencil and mark down all the items you’d use in the event of a few-day outage.

Next to each item you wrote down, jot down two numbers: the starting wattage and the running wattage of each item. These calculations are the amount of juice you need to get the device on as well as how much energy you’ll need to keep them powered. 

Not sure how many watts your hot water heater or TV requires? Honda compiled a list of common household electronics to help you determine your generator wattage. Or, type in your appliance type on Energy.gov to discover each device’s wattage rating manually. 

Don’t Forget Your Automatic Transfer Switch

Now that you have a list of all the electronics in your house and their wattage requirements, you can start doing some math. But first, let’s not forget to factor in your automatic transfer switch (ATS), if you have one. 

While some generators need to be manually turned on in the event of an outage, many standby units can detect a disruption in current all on their own, and kick on automatically with the help of an ATS.

This switch needs juice to operate— usually the equivalent of your breaker panel’s amperage. For instance, if you have a 200-amp breaker panel, then you’ll likely have a 200-amp switch to match. 

Since amps are not the same as watts, you’ll need to do a little conversion: watts = amps × volts.

Once you’ve determined the wattage of your ATS, tack that onto your list and do some simple addition to discover your necessary wattage load. 

A Generator Sizing Scenario 

Let’s walk through a fictitious situation to help you calculate your home generator’s wattage requirements.

Scenario 1:

The Smiths have very different ideas about what they would like powered during an outage. 

Mr. Smith only wants to power the “essentials” and he made the following wattage list to keep his Fort Myers home running in the event of a few-day outage:



2 light fixtures 

175W (far less if they are using LED bulbs)

Electric water heater


Central A/C




Phone and tablet charger





Scenario 2:

Mrs. Smith, however, wants to enjoy more luxuries if the power were to be out for many days in a row! She doesn’t want to be without entertainment, her kitchen or basic home care tech like her washer and dryer. 

Here’s Mrs. Smith’s list:



15 light fixtures (all the lights in their home)


Electric water heater


Central A/C




Phone and tablet charger


2 laptop computers








Clothes washer and dryer

3,000W (collectively)



WiFi and cable modem


Garage door opener




Now that they’ve determined their wattage in “watts,” the Smiths must convert their power requirements into kilowatts.


Watts vs. Kilowatts

Home generators are sold in measurements of kilowatts, which is why the Smiths have to do one final conversion to find the right home generator size. 

Google provides a nice tool for converting W or kW when you search watts vs. kilowatts. Using Mrs. Smith’s example above, we can see that 22,715W would require 22.715 kilowatts. So all you have to do it swap your comma for a period, as it’s a 1,000 to 1 ratio.

If Mr. Smith had his way, he’d only need a 11 or 12 kW generator to power his 10,225W load (it’s always safer to add an extra kilowatt or two just in case!). If Mrs. Smith got to choose, they’d need a 25 kW home generator.

And that’s it! Now you know how to calculate your home generator wattage requirements and choose the right sized generator for your electrical needs.

More Decisions: Fuel Type, Brand & More

Even though you’ve figured out how to determine your home’s generator wattage requirements, that’s not all you have to consider before purchasing a new power unit.

Do you know if you’ll get a propane vs. gas generator? How about much your generator cost will increase as your wattage requirements also increases? Then there’s another big decision: which brand do you choose? 

We discuss all of these things and more on our Choosing a Home Generator page. Simply click the image below  for free advice from your friendly Southwest Florida electricians at SWFL Electric. 

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