How to Size a Whole House Generator

Choosing the right generator with enough capacity provides backup power for an entire home during an outage.

Whole House Generator Size

When the power goes out, a whole house generator must do the job of the electric utility and supply the entire house with power. By managing power, a generator with less capacity than the electric utility connection can power the house. A whole house generator system manages power by giving priority to the most essential circuits and a lower priority to loads the homeowner decides are less important.

In one example, the homeowner might give higher priority to the central air conditioner and lower priority to the electric dryer. If the air conditioner is not running, priority shifts to the dryer and it is allowed to run until the central air conditioner takes priority.

Some heavy loads could overload or come close to overloading the generator. In a home with 15kW of supplemental baseboard heating, the heat strips would use three-fourths of a 20kW generator’s power capacity and not leave enough power for the rest of the home’s circuits. In this case, because the heating strips are supplemental, the homeowner might choose to prevent baseboard heating from using generator power by locking it out with power management.

Not every homeowner chooses to allow the generator to manage power. Every circuit must be available, even during a power outage. The generator must have enough capacity to supply all the household loads including those most homeowners would label unnecessary.

Whole House Generator Sizing

Whole House Generator Sizing starts with a load calculation for the entire home. The National Electrical Code spells out the load calculation for determining utility service capacity in NEC article 220 and the tables (T-220-x) included within the article.

In this article, we use a table to list all the calculations required for each type of load. If a calculation does not apply, don’t enter it on the table. For example, don’t include an electric dryer for a home with a gas dryer.

However, some loads must be included in the calculation even if the home does not have a circuit dedicated to that load. For example, the minimum number of small kitchen appliance circuits is two because even if circuits were not dedicated to kitchen appliances, the homeowner will still use small kitchen appliances like a coffee maker, toaster, etc. This is common in older homes with a single circuit for the kitchen.

Other loads are fixed and cover the minimum for that type of appliance. For example, a homeowner might purchase a 1200-watt microwave, or a 1000-watt microwave. It doesn’t matter because the microwave uses a circuit designed to handle a 1500-watt appliance. Therefore, the load calculation includes a 1500 watt microwave circuit and is not sized according to the easily replaceable microwave.

Most appliances have nameplates that include the power requirements. Some calculations specify a minimum load, but if the nameplate lists a higher power requirement, use the nameplate power if specified in the table.

Permanently installed appliances like electric heating, and electric water heaters use the power requirements listed on the nameplate.

Some loads like Air Conditioners have large motors that require more power to start, and they may run for long periods. The calculation requires multiplying the nameplate rating by a percentage greater than 100 percent.

The Whole House Generator Load Calculation Table

Look to the next section for a complete explanation of the load calculation table and how to size a whole house generator using the table.

Note: Some appliance nameplates may specify Watts, others might use Volt-Amps or VA. Some nameplates might provide kilowatts or kW.

VA = Volts x Amps = Watts.

kW x 1000 = Watts.

Whole House Generator Load Calculation Table

* = Required Entry in Watts

Whole House Generator Load Calculations

Lighting Load (1a-8a)

1. For General Lighting, multiply the home’s living space in square feet by 3 watts per square foot. For a 2000 square foot home, this equals 6000 Watts (3 watts per square foot x 2000 square feet).
2. Required Small Appliance circuits filled in for 3000 Watts.
3. We can leave Additional Small Appliance circuits blank if there are none or add 1500 Watts for each additional small appliance circuit.
4. The required Laundry Circuit filled in for 1500 Watts.
5. Lighting Load Max— Add up the General Lighting load, Small Appliance Load, Laundry load, and any additional small appliance circuits (1500 Watts each). 6000 Watts + 3000 Watts + 1500 Watts = 10,500 Watts.
6. Of the 10500 Watts, count the first 3000 Watts of Lighting Load at 100 percent (Already filled in).
7. Add the remainder of the Lighting Loads up to 120,000 Watts at 35 percent. (10,500 Watts – 3000 Watts) x .35 = 7500 Watts x .35 = 2625 Watts.
8. Lighting Load does not exceed 120,000 Watts—leave the last lighting load entry blank.

Appliance Load (1b-9b)

Major Appliances (1c-3c)

1. Electric Dryers require minimum of 5000 Watts per dryer (number of dryers x 5000 Watts.)
2. Electric Water Heaters use the nameplate watts per dryer (add up the nameplate Watts of each water water.)

3. Electric Cooking. Count the number of appliances and use the required watts from the table. For example, a built-in oven plus a countertop range is two appliances for 11,000 Watts.

Heating and Air Conditioning (1d-5d)

1. Multiply the nameplate power of the largest central air conditioner by 125 percent. (nameplate power in watts x 1.25).
2. Multiply the nameplate power of the largest heat pump by 125 percent. (nameplate power in watts x 1.25).
3. Supplemental Electric Heating (Baseboard, strip, etc.) Add total watts of electric heat.
4. Enter the total of any additional heat pump or air conditioner nameplate ratings.
5. Total the entries from 1-4 and enter it here.

Optional Equipment (1e-5e)

1. Pool or Spa Motor. Largest Nameplate rating x 125 percent.
2. Remaining motors. Total of Nameplate Ratings at 100 percent.
3. Lights and Miscellaneous lighting.
4. Total 1e-3e and enter it hear.
5. Total all other miscellaneous loads (welders, kilns, etc) and enter it here.

Sum of All Totals (1T-9T)

1. Lighting: Total of lines 6a+7a+8a.
2. Appliance: Total from line 9b.
3. Dryer: Total from 1c.
4. Water Heater: Total from 2c.
5. Cooking: Total from 3c
6. Space Heating / Air Conditioning: Total from 5d
7. Pool / Spa: 4e
8. Optional Equipment: 5e
9. Add up the watts from lines 1T-8T.

Whole House Generator Size

From the total load calculation at the end of the table, we can determine the generator capacity required for unmanaged whole house power.

Multiply the Total Load (Line 9T) x 1.25 to provide a 25 percent margin for growing power needs. Operating the generator below maximum capacity extends generator life. Many manufacturers suggest no more than 75 percent power for 80 percent of the generator run time.

If some loads should never run on generator power, add a managed power option and lock those loads out whenever the generator supplies power to the house. That way, you can deduct the power from the whole house generator capacity required. For example, if a 3000-square-foot home has 20,000 Watts of supplemental electric baseboard heating, you could lock out the supplemental heating and reduce the generator capacity requirement by 20,000 Watts.

Another option manages power to heavy loads and only allows them to run when the generator has enough power. A house with two central air conditioners, for example, could run the most important air conditioner first. When the thermostat shuts it off, the second generator (with a lower priority) runs. Power management allows a smaller generator to run a home’s heavy loads, but not all at the same time. Select a generator size that allows the heaviest load to run, then assign load priorities to allow each to run in turn.

The downside to power management—some loads might never run unless you change priority, which is possible with some of the newer systems. On a hot day, two air conditioners might take turns, but the electric dryer or water heater never get a turn because the a/c units run all day.