Standby Generator Buyer Guide: What Size Generator do I need?

Written By: MJ Logan

Generac Guardian Generator installed next to a blue two story home with white trim

Standby generators do exactly what they suggest, they stand by until the power goes out. Fully automatic, a standby generator engine starts within seconds after the outage hits and the generator begins producing power. The Automatic Transfer Switch (ATS) isolates the house from the electric utility and connects the generator to restore power. The whole process takes less than a minute and as little as ten seconds.

The first step in the purchase process determines which type of installation the homeowner needs and what size generator to buy. For residential homeowners and businesses, there are three general categories to consider.

  • Whole House Generator
  • Whole House Generator with Power Management
  • Generator for Essentials

Backup Generator Terms to Know

Whole House Generator System

The term "Whole House Generator" refers to the automatic transfer switch connection to the main panel. In this configuration, the transfer switch installs between the utility meter and the main circuit breaker panel with connections for the generator and for the utility lines. The switch isolates generator power from utility power and the two never exist on the same lines at the same time. Normally, the switch selects utility power and feeds the house from the utility. During an outage, after the generator starts and is ready to accept the load, the transfer switch disconnects the utility and only then connects the generator. When the utility restores power, the switch disconnects the generator first and then connects the utility.

By feeding the main circuit breaker panel from the generator, the whole house generator powers everything including major and large appliances like Electric Ranges, HVAC, and Water Heaters. It has the capacity to power everything at once—even multiple air conditioners running at the same time. For a large home, this could mean a liquid cooled generator with plenty of capacity. This is the most expensive category, but it offers exceptional reliability, especially in hot climates. Luxury homes, estates, and small businesses have greater power needs with generators up to 150kW.

Power Management allows a smaller whole house generator to power anything in the house, but it might not power everything at once. For example, in a home with two air conditioners, the second unit might wait for the first to shut off before it starts. Large appliances like air conditioners are run according to priority. Your electric dryer might have a low priority and the well pump a high priority. Some of the newer power management systems allow priorities to change as needed with a convenient smart phone app. A whole house generator with power management installation uses a smaller generator that costs less to purchase and install and uses less fuel than the larger liquid-cooled models.

Essential Circuits Generator

Generators for Essential Circuits only power the most important circuits. Before installation, the homeowner chooses which circuits are most important. The transfer switch has a limited number of circuit breakers. In an outage, only the circuits chosen by the homeowner have power. An essential power generator will cost less than a whole house generator with power management for the same size house.

Common Circuits for Essentials

  • Air Conditioner
  • Furnace
  • Refrigerator
  • Freezer
  • Sump Pump
  • Garage Door
  • Well Pump
  • Kitchen Lighting and Appliance Outlets
  • Circuits with Medical Equipment.

The Air Conditioner could be essential for someone especially sensitive to heat. In the winter, the furnace keeps the home comfortable. Refrigerator and Freezer stay cold and preserve food. Sumps Pumps keep basements from flooding. Homes with a garage door opener should maintain access to their vehicle. It's essential to drink water or use plumbing fixtures, cook food and have a few lights. Medical equipment is essential.

Generator Size

Smaller homes have lower power needs than larger homes. A home in the Desert Southwest probably uses more power in the summer than a house near the northern border. An important part of sizing includes local building codes which may include requirements not listed here. A whole house generator for a small home might only have enough capacity for essentials at a large home. A whole house generator with power management for a large home is probably more power than a small home needs.

It all depends on your home and family's power requirements. It's also important to choose a generator with a little more capacity than you need. Air cooled generators in general should run at at no more than 80 percent rated capacity most of the time. Short periods at close to maximum capacity won't hurt the generator. Just like a car, you don't drive it at full speed for very long.

What Size Generator Do I Need for My House Calculator

What Size Standby Generator do I Need?

Whole House Generator—All the Power You Can Use

The Best Standby Generator for Large and Luxury Homes

Whole house generators produce enough power to handle everything in your house at the same time. Buy a unit large enough and you can power all the air conditioners, your large electric appliances, and still have plenty of power left over to run the pool, the backyard party lights, and the rec room arcade.

These liquid cooled standby generators supply from 20 kilowatts all way to 100 kilowatts or more. They can provide backup power to midsize houses or the largest luxury homes and estates. They may make less noise than smaller air-cooled units.

Consider a whole house generator if you run multiple (more than two) central air conditioners and have large power requirements that must be met. If it's not important to run everything at once, consider a generator with power management.

Liquid Cooled Generators

Whole House Generator with Power Management

The best standby generator option for most mid-sized to large homes.

Most homes don’t use all the power their electrical panels suggest. Even if every appliance ran at the same time, it’s rare for the total power consumption to equal the main panel capacity. Most homeowners will find that an air cooled standby generator with the right Automatic Transfer Switch is all they need.

These generators offer 10 to 26 kilowatts of power

An automatic transfer switch with power management capability can prevent overloading the generator by limiting when certain appliances receive power. A home with two central air conditioners is a good example. The automatic transfer switch can prevent both air conditioners from starting at the same time. When one starts, the second waits for the first to stop before it will run. In many cases, the two units will alternate as necessary to keep the house cool.

Air Cooled Whole House Generator System with Power Management

Home Standby Generator for Essential Power

Homeowners may opt for less power and expense with additional fuel savings—especially important in rural areas where generators run on propane instead of natural gas. A generator for essential power might be the same as a whole house house generator for a small home, but only powers a limited number of circuits. Most standby generators for essentials range from 7kW to 12kW, but larger homes with greater power needs may require a larger generator, even just to power the most critical circuits. A whole house transfer switch and generator with power management could manage the entire home and lockout certain loads the homeowner chooses not to run in a power outage.

An essential power standby generator uses less fuel than one powering the entire home. As electrical uses goes up, fuel consumption increases with the load. Only powering essentials lowers the electrical load—important for installations that rely on a propane tank for fuel. In the middle of winter, the generator may have to share the propane supply with the furnace.

Most essential power generators are the same models that power large to small homes. The main difference is the automatic transfer switch and the circuits powered. Because the generator is smaller, it costs less to purchase and run. They keep the sump pump operating, run smaller air conditioners, and of course, the food in your refrigerator or freezer stays cold or frozen while you can have some lights and the television.

8kW Generator - 11kW Generator — 12kW Generator - 13kW Generator


menu Article Content

Back to Top

menu Article Content

Back to Top