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Generator Sizing Buying Guide

Lauren Bongers

By Lauren Bongers
Published August 16, 2023

Wondering how to figure out what size generator you need? With this sizing buying guide, including a printable worksheet to calculate wattage of your devices, you'll learn how to choose the right size generator for your needs.

A Generac whole-home generator outside of a home at night.

Types of Generators

A Westinghouse portable generator supplying power to an RV.

Generators are extremely helpful for providing a dependable power supply when you need it, whether that’s during a power outage, on a camping trip or otherwise. There are several types of generators to choose from. Below are the most commonly used types.

Portable generators can be used to power devices ranging from TVs and small appliances to power tools and lights. They’re ideal for a variety of applications including camping, for work on jobsites, and at home during power outages.

Inverter generators are lightweight, portable, and quiet. They’re ideal for activities like tailgating and RV camping. Their output is more limited than conventional portable generators.

Whole-home generators, also called standby generators or standby home generators, offer a more permanent solution for power protection. They can run for an extended amount of time on natural gas or liquid propane. Fully automatic models can sense power outages and produce power immediately.

What Size Generator Do I Need?

When choosing a generator, this is one of the most commonly asked questions. The answer really depends on your needs and how many devices you want to power at the same time. To figure this out, put together a list of essential devices you want to be able to run as well as the wattage information for each one. The more watts a portable generator produces, the more appliances and tools it can power at the same time.

Wondering how to calculate watts for a generator? Follow these four simple steps to effectively size a portable generator.

  1. Choose and list essential devices that you want to be able to power concurrently.
  2. Check the manual or data plates of each device to find the running watts of each device listed, add them and record that number.
  3. Find the starting watts of each device listed and record that number.
  4. Choose the device with the highest starting watts. Add that number to the total running watts. This will give you the minimum starting watts of a generator to fit your needs.

How Many Watts to Run a House?

A Generac Guardian home standby generator next to a privacy fence of a home.

If you want to run a whole house on a generator, instead of just a few essential devices, you’ll want to invest in a whole-home generator as opposed to a portable generator. Whole home generators, also known as standby generators, are the most powerful generators on the market. Choosing the right whole-home generator size enables you to power your entire home during a power outage

Because of the power they offer, whole-home generators are one of the more costly options. They also require professional installation and maintenance. Whole-home generators run on either liquid propane or natural gas and just like a home air conditioning unit, they automatically turn on when needed and turn off when they’re no longer needed. To learn more about home generators, check out our buying guide.

General Wattage Requirements

A Generac 6,500-watt gasoline portable generator.

Answering the question “How big of a generator do I need?” isn’t cut and dry. However, the most accurate way to determine what size generator you need is to calculate wattage using a wattage worksheet. Below you’ll find general wattage needed for common uses.

500 to 3,000 Watts: Recreation, power tools, small appliances, lights.

3,100 to 6,000 Watts: Recreation, heating and cooling, mid-size appliances.

6,100 to 10,000 Watts: Emergency power, heating and cooling, large appliances, water heaters.

10,100 Watts and Above: Emergency power and whole home.

Generator Safety Tips

A person setting up a Westinghouse portable generator outdoors.

Before using a generator, it’s important to familiarize yourself with critical safety requirements. For example, you should never set up a generator inside a home or garage as gas-powered generators create carbon monoxide. A good rule of thumb is to keep a generator at least 20 feet away from your home. For more detailed safety information, closely review your generator's safety manual and check out our video on how to safely use an emergency generator.