Whole-House Generator Sizing Calculator
Size a permanent standby generator for your whole house from total connected load, demand factor, and fuel type.
Common conversions
| Input | Result |
|---|---|
| Small home, no central AC | 14 kW |
| Average home, one 2.5-3 ton AC | 17-20 kW |
| Home with AC + electric range | 22 kW |
| Larger home, two AC units | 24-26 kW |
| Large home with well pump + shop | 30-38 kW |
| Estate or large home, diesel standby | 48-60+ kW |
| Natural gas heat value | ≈1,000 BTU/cu ft |
| Propane (LP) heat value | ≈2,500 BTU/cu ft |
| Typical ATS transfer time | 10-20 seconds |
The math behind it
- Total connected load = 5 + 6 + 3 = 14 kW
- Apply the demand factor: 14 × 0.7 = 9.8 kW
- Round up to the nearest standard model above 9.8 kW
Everything you need to know
A whole-house standby generator is a permanently wired unit that starts itself within seconds of a utility outage and carries some or all of a home's circuits through an automatic transfer switch. Sizing one correctly means totaling the loads it needs to carry at once, applying a demand factor for how much of that load truly runs together, and then matching the result to a fuel type and a standard model size.
Matching your home to a standard kW size
Manufacturers such as Generac, Kohler, and Cummins build standby units in a handful of common sizes: 14, 17, 20, 22, 24, 26, and then larger diesel units in 30 kW steps. A smaller home without central air conditioning is often comfortable on a 14 kW unit covering the furnace blower, refrigerator, well pump, and lighting. Add a 2.5 to 3 ton central AC and most homes move to the 17-20 kW range. Stack an electric range or a second AC unit on top and 22-26 kW becomes the realistic floor. Large homes with a well pump, a workshop, and multiple AC zones often need 30 kW or more, which is where diesel standby units take over from gas and propane models.
Natural gas, propane, or diesel: picking a fuel
Natural gas is the simplest option where a utility line already reaches the house: fuel supply is unlimited for as long as the outage lasts, but a natural gas engine loses some output compared to the same unit running on propane, since natural gas has a lower BTU content per cubic foot. Propane stores on-site in a tank, runs cleaner than gasoline, and doesn't degrade in storage the way gasoline does, but a large standby unit can draw down a 500-gallon tank faster than homeowners expect during a multi-day outage. Diesel is reserved for the largest homes and estates; it needs no spark ignition, tolerates heavy loading well, and is rated with separate prime and standby figures under ISO 8528, so always size to the standby rating for backup duty.
Automatic transfer switches and load management
A whole-house ATS switches every circuit in the panel to generator power, which means the generator has to be sized for the worst-case moment when the AC compressor, the electric range, and the well pump all happen to be running at once. A managed or load-shedding transfer switch, like Generac's Power Management Module, instead lets a smaller generator serve the whole panel by briefly dropping a lower-priority circuit, such as the AC, whenever a higher-priority one, such as the range, turns on. That trade-off can shrink a 22 kW installation down to a 14 kW unit at the cost of the AC pausing for a few minutes at a time.
Common applications
A whole-house ATS pulls every circuit, so the generator must be sized for the worst case of HVAC, range, and well pump all running together, usually 17-26 kW for a typical home.
A smart load manager lets you install a smaller 14-17 kW unit by briefly shedding AC or the water heater whenever the range or dryer switches on.
Rural homes without a gas utility line convert standby units to run on an on-site propane tank; sizing the tank to the generator's full-load burn rate matters as much as sizing the generator itself.
Adding a hot tub, EV charger, or a home addition changes the connected load total. Recalculate before assuming an existing 14 kW unit still has headroom.
Common mistakes
Compressor inrush can be 3-6 times the AC's running amps. Without a surge allowance in the total, the genset sags in voltage or stalls on the first AC start.
Naturally aspirated engines lose roughly 3% per 1,000 ft above 500 ft. A 22 kW unit at 7,000 ft delivers closer to 18 kW, which can push a tight sizing into shortfall.
A 22 kW unit can burn 3 gallons of propane an hour at full load. A tank or gas line sized for a furnace and a range, not a generator, can starve the engine of fuel under full demand.
Wiring only today's circuits into the transfer switch means an EV charger or home addition later requires re-opening the panel. Leave spare capacity in the ATS during the original install.
Frequently asked questions
What size whole-house generator do I need for a 2,500 sq ft home?+
Most homes that size land between 17 and 22 kW, depending on whether central AC and an electric range run at the same time as everything else. Add up your actual essential circuits with the calculator above rather than sizing purely from square footage.
Do I need a 22 kW generator for my house?+
No, only if you plan to run central AC, an electric range, and a well pump simultaneously through a whole-house transfer switch. Many homes without an electric range or with a managed transfer switch do fine on 14 or 17 kW.
Is propane or natural gas better for a standby generator?+
Natural gas offers unlimited runtime from the utility line but slightly lower output per kW rating; propane stores on-site and burns cleaner but runs down a tank faster under full load. Choose natural gas if you have a reliable utility line and propane if you don't or want fuel independence.
Should I install a whole-house or a managed transfer switch?+
A managed (load-shedding) transfer switch lets you install a smaller generator, often one size down, by briefly shedding a lower-priority circuit like AC when a higher-priority one like the range turns on. A whole-house switch needs a bigger unit sized for every circuit running at once.
How long can a natural gas standby generator run continuously?+
Indefinitely, as long as the gas utility keeps supplying fuel, though manufacturers recommend a scheduled maintenance shutdown every 200 to 500 hours of continuous or cumulative run time for oil changes and inspection.
Does a whole-house generator installation need a permit?+
Yes, nearly every jurisdiction requires a permit for the electrical transfer switch and the gas or propane line, plus an inspection before the unit goes live. Skipping the permit can void the manufacturer's warranty and complicate a future home sale.
How much propane does a 22 kW generator use per hour?+
About 2.5 to 3 gallons per hour at full load, dropping to roughly 1.5 gallons per hour at half load, which is why a full 500-gallon tank rarely lasts more than a week of continuous full-load running.
Can solar panels or a battery replace a whole-house generator?+
No, not for extended outages on their own. A home battery can cover several hours of essential loads, but a multi-day outage in bad weather still needs a fuel-fed generator or a much larger battery bank than most homes install.
How often should I exercise a whole-house standby generator?+
Most units self-test weekly for 5 to 20 minutes under no load, which is the manufacturer default. It's worth confirming that self-test setting once a year and running the unit under an actual load, such as the AC, at least once a season.
Why does my 22 kW generator only put out 18 kW at my mountain cabin?+
Altitude derate. Naturally aspirated engines lose roughly 3% of output for every 1,000 feet above 500 feet elevation, so a cabin at 7,000 feet can see a 15-20% drop from the sea-level nameplate rating.