Whole-House Generator Sizing Guide

Whole-house generator sizing basics

Choosing the right size generator requires a few careful calculations and some information about how your home functions, how your family uses electricity and which appliances you’re willing to go without during a power outage.

Here’s some basic information you should plan to gather before sizing your generator:

You’ll usually need at least 200-amp service for a whole-home generator.

• Your appliances’ starting and running wattage: The most important thing to consider is how much electricity the items you want backup power for require during their startup phases and their running phases. Pay extra careful attention to refrigerators and AC units, which have high electricity requirements during startup due to their compressors, as well as electric-heavy items, like EV chargers and electric pool heaters.
• Your heating and cooling systems: According to the U.S. Energy Information Administration, about 70% of a home’s energy consumption goes toward heating and cooling. Make sure you understand the type of heating and cooling equipment you have. Be prepared for a significantly larger generator if you have a whole-home AC unit to back up or if you have electric baseboards, an electric furnace or an electric water heater.
• Your home’s size: You can use your home’s square footage to roughly estimate your ideal generator size and the cost of your whole-home generator. Generally, larger homes require bigger units to provide the same level of backup power.
• Your home’s energy efficiency: While you shouldn’t base your estimate on home size alone, your home’s energy efficiency can help you adjust a square-footage-based generator size estimate up or down.
• Your existing electrical system: Make sure you know if you have outdated 100-amp service or if your home has more modern 200-amp service. You’ll usually need 200-amp service to support a whole-home generator, and upgrading can add between $1,000 and $2,200 to your total.
• Your backup power needs: Finally, know what you want to back up with your generator. You’ll need a much smaller unit if you just want emergency power for your refrigerator and lights, but you’ll need to size up significantly if you want backup for your entire home, including heating, cooling, pool heaters and more.

» MORE: Best whole-house generators

Whole-house generator load calculation

When you hire a professional to install your generator, they’ll conduct a load calculation to ensure your unit has sufficient output to support your home through a power outage. You can take similar steps to get a better idea of the size of the standby generator you need, and then base your estimated costs on that figure.

You can follow the steps below to get started:

1. Figure out what you want backup power for. First, decide what you want your generator to power. Some homeowners want backup for their entire home so that they have no interruption to their routine. Others just want their standby generator to power lights and keep the refrigerator running to prevent food spoilage. Figure out where in this spectrum you fall, and make a list of everything you want or need to run during a blackout.
2. Consider simultaneous running needs. Consider which items you’ll run at the same time. Their combined power draw matters more than the draw of each item individually. Even a small, modern generator can power a refrigerator, but if you need to run your fridge, AC, lights, gaming computers and a pool pump at the same time, you’ll need a much larger generator. Figure out what you’d need to run simultaneously, and make a detailed list.
3. Determine the starting load for each appliance or device. Next, take everything on that list and find the starting watts, which can be two to six times higher than running watts. You can check the device or appliance manual, or look for the locked rotor amps (LRA) on the nameplate attached to the side or rear of the appliance. LRA tells you the maximum draw of the unit at startup, which can help avoid undersizing your generator.
4. Determine the running load. Next, check the running loads of all of the appliances and devices you plan to use simultaneously, which you can also find in the device manuals or on the nameplates.
5. Add the concurrent loads to get a baseline generator size. Your minimum generator size should have an output that’s higher than the highest startup draw and higher than all of the concurrent loads added up.
6. Add a 20% safety buffer. To avoid unexpected power issues, add 20% to 30% to that total when sizing your whole-home generator.

Add a safety buffer

Add 20% to 30% to your total wattage when sizing your whole-home generator.

Generator sizing for HVAC and pumps

Most homeowners installing a whole-house generator want backup for convenience and comfort in addition to safety, which usually means they want to be able to run their heating and cooling systems, plus some emergency equipment, like well pumps and sump pumps.

Not only is it useful to understand the average draw of these items and what size generator can support them, but it’s also crucial to include them in your calculation because many have higher starting wattages. Compressors kicking on from a standstill draw a surge of power and then ramp down once they’re running, so it’s important to consider the startup draw of these items.

Here’s a breakdown of average starting and running wattages for different types of heating systems, cooling equipment and water pumps:

You can use these estimates to get an idea of your generator size, but things like the number of HVAC zones you have, the energy efficiency of your home and the tonnage of your heating and cooling equipment can all affect power draw. It’s always best to have a professional size your generator for you, since they’ll take all of these factors into consideration.

Whole-home vs. essential load sizing

With all of this in mind, one of the most important things you’ll need to consider is whether you want backup power for your entire home or just for the essentials, like lighting, refrigeration and well-pump equipment.

The average homeowner would need just a 7- to 10-kilowatt (kW) generator for emergency backup, which costs $4,500 to $10,600. Whole-home coverage for an average-size property would push the installation cost to between $5,000 and $19,700, nearly doubling the price for a 12 kW to 20 kW generator.

Of course, you get value equivalent to what you pay for. An emergency-backup-only unit would only provide power for the essentials, whereas a larger unit could prevent interruptions to your daily routine by keeping your heating and cooling system running and providing more comfort and convenience.

Here’s how generator output, backup coverage and average cost compare:

Note that you can sometimes move down a pricing tier without sacrificing convenience and comfort by installing an automatic transfer switch with load management, which helps prioritize power when multiple appliances need it at once.

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