Why is the whole-home unit so large for a house that never draws that much?

Because the code sizes it to the calculated load, which is a design figure with demand factors, not a measurement. The generator must be able to start and carry that load; it will spend most of an outage at a fraction of it.

Does the heat pump have to be on the generator?

No. Heating is the loss people feel first in a winter outage, but the compressor alone is 6 kW here while the strip heat is 15 kW. Backing up the compressor and shedding the strips is the common compromise, and load management is what makes it legal on a smaller unit.

Is the fuel cost realistic?

The figures use the generator cost calculator’s model: propane at about 0.15 gallons per kWh, $3.00 a gallon, 72 hours at 50% of the rated size. Your generator’s specification sheet gives the actual consumption curve; the calculator lets you change the size, load and price.

Portable generator through an interlock?

An essentials setup of 16.1 kW is beyond most portable units, which top out near 9 to 12 kW on gasoline. Dropping the well pump or running one pump at a time brings it into portable range; the estimator shows the figure without the pumps when you clear them.

GeneratorsCalculated per NEC 2023

Standby generator sizing: essentials only or the whole home?

The whole-home answer for this 2,800 ft² all-electric house is a 48 kW liquid-cooled unit: the NEC 220.82 calculation is 38,083 VA (38.1 kW), and NEC 702.4(B)(2)(a) requires a generator on a whole-house transfer switch to carry the full calculated load. The essentials answer is the 18 kW air-cooled class: lights, receptacles, refrigerator, furnace blower and the well and sump pumps come to 16.1 kW. Running at half load through a 3-day outage on propane at $3.00 a gallon, the two burn about 259 and 97 gallons, $778 against $292. The heat pump’s 15 kW of strip heat is most of the gap.

Updated September 23, 2026. Numbers on this page are computed by the same engine as the calculators, under NEC 2023.

Worked example: 2,800 ft² all-electric house with a heat pump, well and sump pump

Calculated per NEC 2023

Optional method (220.82)

Inputs

NEC edition	NEC 2023
Floor area	2,800 ft²
Small-appliance circuits	3
Laundry circuits	1
Cooking	Electric range 12 kW
Clothes dryer	5,000 VA
Appliances fastened in place	Electric water heater 4,500 VA; Dishwasher 1,200 VA; Garbage disposal 900 VA
Heating	Heat pump compressor 6,000 VA with 15,000 VA supplemental strip heat
Air conditioning	None
Motors	Well pump 1,656 VA; Sump pump 1,176 VA

Result

Calculated load	38,083 VAfrom 61,832 VA connected
Calculated current	158.7 Aat 240 V
Minimum service rating	175 A240.6(A) standard rating, not less than 100 A
Whole-home generator	48 kW classFull calculated load 38,083 VA; 702.4(B)(2)(a)
Essentials generator	18 kW classLighting and receptacle circuits 14,400 VA at the optional-method demand, plus 1,500 VA refrigerator and furnace allowance and 2,832 VA of pumps
3-day outage fuel, whole-home	$77848 kW at 50% load, propane at $3.00/gallon, 259 gallons
3-day outage fuel, essentials	$29218 kW at 50% load, 97 gallons

Line item	NEC	Connected	Demand	Basis
General lighting and receptacles	220.82(B)(1)	8,400 VA	(in subtotal)	2,800 ft² × 3 VA/ft²
Small-appliance and laundry circuits	220.82(B)(2)	6,000 VA	(in subtotal)	4 circuits × 1500 VA
Appliances fastened in place	220.82(B)(3)	6,600 VA	(in subtotal)	3 at nameplate
Ranges, ovens and cooking units	220.82(B)(3)	12,000 VA	(in subtotal)	1 at nameplate
Clothes dryers	220.82(B)(3)	5,000 VA	(in subtotal)	1 at nameplate
Permanently connected motors	220.82(B)(4)	2,832 VA	(in subtotal)	2 at nameplate
Other fixed loads	220.82(B)(3)	—	(in subtotal)	None
EV supply equipment	220.82(B)(3)a	—	(in subtotal)	None
General load after demand factors	220.82(B)	40,832 VA	22,333 VA	First 10,000 VA at 100%; remaining 30,832 VA at 40% = 12,333 VA
Heat pump compressor at 100% plus supplemental heat at 65%	220.82(C)(3)	21,000 VA	15,750 VA	100% of 6,000 VA + 65% of 15,000 VA = 15,750 VA; largest selection
Air conditioning and cooling	220.82(C)(1)	—	0 VA	None

Whole-home backup must cover 38.1 kW, so the next class is 48 kW. The essentials framing keeps 2,832 VA of pumps plus lighting, receptacles and a refrigerator/furnace allowance, 16.1 kW, and fits an 18 kW unit.

Generator classes are typical residential standby ratings; confirm with the manufacturer’s specification sheet.
A whole-home transfer switch without load management must carry the full calculated load (NEC 702.4(B)(2)(a)).
The essentials figure assumes a limited transfer switch or load-managed circuits for lights, receptacles, refrigerator, furnace blower and pumps only.

The rule that decides which number applies

NEC 702.4(B)(2) covers optional standby systems with automatic transfer. If the transfer equipment can connect the generator to the whole house and there is no load management, the generator must have capacity for the full calculated load under Article 220, exactly as the service was sized. If a load-management system (Article 750 energy management, or a listed generator load-shedding module) keeps the demand within the generator’s rating, the generator may be sized to the managed load instead. A manual transfer switch feeding selected circuits only needs to carry those circuits.

That is the whole difference between 48 and 18 kW on this house. Nothing about the house changed; only which loads the generator is allowed to see.

What "essentials" means here

The essentials figure takes the general lighting and receptacle circuits at the optional-method demand, adds a 1,500 VA allowance for a refrigerator and furnace blower, and adds permanently connected pumps at nameplate because a well or sump pump is exactly what an outage threatens. It leaves out cooking, the dryer, the water heater, the heat pump and its strips, and the air conditioner.

In practice most essentials installations add one or two comfort loads back: a gas furnace blower is already in, a heat pump compressor without strips is another 6 kW here, and a single 240 V well pump is often the reason a homeowner wants backup at all. Each addition moves the class; the estimator recomputes it as you toggle appliances.

Load management as the middle path

Most standby manufacturers sell load-shedding modules that let a smaller generator feed the whole panel by dropping the heat strips, water heater or dryer when the generator nears its limit. Under 702.4(B)(2)(b) the generator is then sized to the managed load. On a house like this one, a 26 kW unit with the strips and water heater on shed relays backs up everything the family notices at a third of the fuel burn of a 48 kW machine.

Frequently asked questions

Why is the whole-home unit so large for a house that never draws that much?: Because the code sizes it to the calculated load, which is a design figure with demand factors, not a measurement. The generator must be able to start and carry that load; it will spend most of an outage at a fraction of it.
Does the heat pump have to be on the generator?: No. Heating is the loss people feel first in a winter outage, but the compressor alone is 6 kW here while the strip heat is 15 kW. Backing up the compressor and shedding the strips is the common compromise, and load management is what makes it legal on a smaller unit.
Is the fuel cost realistic?: The figures use the generator cost calculator’s model: propane at about 0.15 gallons per kWh, $3.00 a gallon, 72 hours at 50% of the rated size. Your generator’s specification sheet gives the actual consumption curve; the calculator lets you change the size, load and price.
Portable generator through an interlock?: An essentials setup of 16.1 kW is beyond most portable units, which top out near 9 to 12 kW on gasoline. Dropping the well pump or running one pump at a time brings it into portable range; the estimator shows the figure without the pumps when you clear them.