Generator Sizing Guide: Three Real Houses, Whole-Home and Essentials, Sized by the NEC Calculation

Most generator sizing advice starts from square footage and ends with a shrug. The honest way to size a standby generator is the same way a service is sized: run the NEC load calculation for the house, then apply the one rule that decides which loads the generator has to carry. This guide does exactly that for three houses. Every number comes from the load engine behind our calculators.

The rule that decides everything

NEC 702.4(B) covers optional standby systems. If the transfer switch connects the generator to the whole house and there is no load management, the generator must be able to carry the full calculated load from the Article 220 calculation. If the generator feeds only selected circuits, or an energy-management system sheds loads to keep demand inside the generator's rating, it only has to carry those loads.

So every house has two answers: the whole-home size and the essentials size. The gap between them is usually the cooking, laundry, water heating and space conditioning loads.

Three houses

The standby classes used here are the common residential ratings: 7.5, 10, 14, 18, 22, 26, 30, 38, 48, 60 kW. Air-cooled units stop around 26 kW; above that you are into liquid-cooled equipment.

The gas house

A 1,500 ft² home with gas heat, gas water and gas cooking calculates to 13.7 kW under the optional method. Whole-home backup is a 14 kW unit. The essentials load, 10.5 kW, lands in the same class, which is the tell: on a gas house there is little reason to buy a limited transfer switch, because the whole-home unit is already small.

The mixed house

A 2,200 ft² home with an electric range and dryer but gas heat and water calculates to 22.3 kW. Whole-home is a 26 kW unit; essentials is 14 kW. This is the house where the decision matters: the range and dryer are big loads that almost nobody needs during an outage, and leaving them off the backed-up circuits cuts the generator nearly in half.

The all-electric house

A 3,000 ft² home with a heat pump and 10 kW of strip heat, an electric water heater, two cooking appliances and a well pump calculates to 36.2 kW. Whole-home is a 38 kW liquid-cooled unit. Essentials, with the well pump included because a well is the load that matters most in an outage, is 15.2 kW, an 18 kW unit. Most owners of a house like this end up in the middle: a mid-size generator with a load-shedding module on the strip heat and water heater.

For a house-by-house explanation of the whole-home and essentials figures, see what size generator a 2,000 sq ft house needs and essentials versus whole-home standby sizing.

What "essentials" includes

The essentials figure in the table takes the general lighting and receptacle circuits at the optional-method demand, adds a 1,500 VA allowance for a refrigerator and a furnace blower, and adds any permanently connected pumps at nameplate. It leaves out cooking, dryers, water heaters, electric heat and central air. If you want the air conditioner on the essentials circuit, add its nameplate to the essentials figure and pick the next class up; a 3-ton unit is roughly 3,600 VA.

What each size burns

Runtime cost is where oversizing hurts. Fuel consumption tracks the generator's rated size and its load fraction, so a 38 kW unit idling along at 30% burns more than an 18 kW unit working at 60%.

These figures use the generator cost calculator's consumption model at 50% of rated output for 72 hours, propane at $3.00 per gallon. Natural-gas units burn roughly 10 cubic feet per kilowatt-hour; check your utility's rate and the specification sheet, because consumption curves differ between manufacturers. The generator cost calculator lets you change the size, load, fuel and price.

Load management: the middle path

Manufacturers sell load-shedding modules, and NEC 750 recognizes energy-management systems, that let a smaller generator feed the whole panel by dropping the biggest loads when demand nears the rating. Under 702.4(B)(2)(b) the generator is then sized to the managed load rather than the full calculation. On the all-electric house above, a 26 kW unit with the strip heat and water heater on shed relays backs up everything the household notices at a fraction of the fuel burn of the 38 kW machine.

Transfer switches and interlocks

A whole-home automatic transfer switch is what makes the full-calculation rule apply. A manual transfer switch that feeds a handful of circuits, or a generator interlock kit that lets the main panel be back-fed from a portable generator through a dedicated breaker, is the essentials framing. Interlock kits must be listed for the specific panel, and the back-feed breaker must be sized to the generator's output.

How to get your number

1. Run the Home Load Estimator with your square footage, appliances, heating and cooling. It returns the optional-method calculation and both generator figures.
2. Decide which loads you need in an outage, and whether load management is on the table.
3. Take the calculated load, not the square footage, to the generator dealer. Ask for the specification sheet's continuous rating on your fuel and altitude.
4. Have the installer run the same calculation on your actual nameplates; the permit will require it.

The estimator is free and runs the same engine as the contractor tool, so the number you bring is the one the installer would start from.