What Size Generator Does My Home Need?
The short answer: it depends far more on what has to keep running than on how big the house is. This page explains what actually drives the number, so you can walk into the conversation knowing roughly where you stand.
The Short Answer
Generator size is decided by the electrical load you want to carry during an outage, not by the floor area of the house. Two 2,200 square foot homes on the same street can need very different units: one with gas heat, a gas water heater and a single HVAC system sits comfortably in the lower half of the residential range, while the identical house next door with electric heat, an electric water heater, an electric range and two HVAC systems can need close to double the capacity.
That is why nobody honest will quote you a size over the phone. What we can do, and what the rest of this page is for, is show you which loads move the number and roughly where a home like yours tends to land.
Want the range for your specific home?
Our website assistant will walk through the same questions an estimator asks and give you a preliminary range in about a minute. Open it from the button in the bottom right of any page, or request the free on-site calculation.
Whole-Home or Essential Circuits?
This is the first question because it changes everything after it. It is a decision about the transfer switch as much as about the generator.
Essential circuits
A transfer switch backs up only the circuits chosen at installation. Typically that means heating and cooling, the refrigerator and freezer, some lighting and outlets, a well or sump pump, and the internet.
- Smaller generator, lower installed cost
- Circuits are fixed at installation
- Some of the house stays dark
- Works well where heat and hot water are gas
The trap: finding out during a January outage that the heat pump was not on the backed-up panel. Decide what is essential in winter, not in June.
Whole-home
A transfer switch covers the entire electrical panel. Everything works during an outage exactly as it does normally, and nobody has to remember which outlets are live.
- Nothing to think about during an outage
- Larger generator, higher installed cost
- Load management can sometimes reduce the size needed
- Usually necessary for all-electric homes
Worth knowing: whole-home does not always mean the largest unit. Smart load management can stage big loads so they do not all start at once.
kW, Watts, Amps and Starting Power
Generators are sold by kilowatt rating and houses are wired in amps, which is why the two sets of numbers never quite line up in conversation. The short version:
- Watt
- The unit of electrical power. A 100-watt bulb draws 100 watts while it is on.
- Kilowatt (kW)
- One thousand watts. A generator described as 22 kW is rated to supply 22,000 watts continuously. This is the number on the model name.
- Amp
- The unit of electrical current. Your home's service is rated in amps — commonly 100, 150, 200 or 400. At a fixed voltage, more watts means more amps. The service rating decides which transfer switch is appropriate; it does not by itself decide the generator size.
- Running watts
- What a device draws steadily once it is going. Easy to add up, and the number most online calculators use.
- Starting watts
- The brief surge a motor pulls as it starts. An air conditioner, a well pump, a compressor or a refrigerator can draw several times its running figure for a moment. This is usually what decides the generator size, and it is the part a chat window or a spreadsheet cannot establish reliably.
That last one is the reason a load calculation happens on site. Adding up nameplate ratings tells you the steady demand; it does not tell you what happens at the instant two motors try to start together on a hot afternoon. An electrician measures and plans for that.
The Common Residential Ranges
These are the coverage bands Gen-Man works to. They are a guide to where a home like yours usually falls, not a recommendation for your home specifically.
- Lighting and outlets
- Refrigerator and freezer
- One central HVAC system, up to about three tons
- Well pump or sump pump
- Key appliances
Often suits homes of roughly 1,500–2,500 sq ft with gas heat and hot water.
In our catalog: 10 kW, 14 kW
- Everything in the essential band
- Electric range or oven
- Larger HVAC systems
- Home office and electronics
- Full comfort through an outage
Often suits most North Alabama homes of roughly 2,000–4,000 sq ft.
In our catalog: 18 kW, 20 kW, 22 kW
- Larger and custom homes
- Two or more HVAC systems
- Electric heat and electric water heating
- Shops and outbuildings
- Medical equipment, EV charging
Above the air-cooled range this moves to liquid-cooled equipment, and sometimes to light commercial units.
In our catalog: 24 kW, 26 kW, 27 kW, 28 kW, 30 kW
What Actually Moves the Number
Roughly in order of how much difference each one makes.
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How many central HVAC systems
The single biggest factor in most homes. One system is routine. Two changes the calculation significantly, because both could call for cooling at the same moment and the startup surge stacks. Three or more usually pushes a home out of the air-cooled range.
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Whether the heat is gas, electric resistance or a heat pump
Gas heat barely touches the generator — it needs power for the blower and the controls, and that is all. Electric resistance heat is one of the heaviest loads in any house. A heat pump sits between the two, but its auxiliary strips can behave like resistance heat on the coldest mornings, which is exactly when you need it.
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Whether the water heater is gas or electric
An electric water heater is a substantial and often overlooked load. A gas one is close to free from the generator's point of view.
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Electric range, oven and clothes dryer
Individually significant, and they are often all electric in the same house. Some households are happy to leave the dryer off during an outage; that is a legitimate way to keep the unit smaller.
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Well pump, sump pump or septic pump
Not large loads while running, but heavy starters. On a well, losing the pump means losing running water entirely, so it is rarely optional.
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Shops, outbuildings, pools and EV chargers
Easy to forget when listing what needs to run, and each capable of moving the answer a whole band. Mention them at the site visit rather than after installation.
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Square footage
Last, deliberately. It correlates loosely with load because bigger houses tend to have more of everything, but it is a symptom rather than a cause. An all-electric 1,800 sq ft house can need more capacity than a 3,000 sq ft house with gas appliances.
What This Page Cannot Do
This guide will get you close enough to have a sensible conversation and to know whether a quote you have been given is in the right area. It is not a load calculation, and nothing here should be treated as one.
Four things still have to be established at the property before a unit can be chosen:
- Motor starting demand. What your specific equipment actually draws at startup, measured rather than estimated.
- The electrical service and panel. Its rating, its condition, and whether the transfer switch can be fitted without an upgrade.
- The fuel supply. Whether the gas meter and line can carry the additional load, or what size propane tank the site needs.
- Placement. Clearances, setbacks, exhaust direction and service access, all of which are governed by manufacturer requirements and local rules.
Gen-Man does all four as part of a free site evaluation, and gives you a written estimate covering the generator, the transfer switch, the pad, the wiring, permits and labor before you commit to anything.
Generator Sizing FAQ
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We come to the property, measure the actual load, check the panel and the fuel supply, and give you a written estimate. No charge, no obligation, and no pressure to decide on the spot.