Best Whole House Generator: How to Choose the Right Size and Type

Most whole house generator advice starts with the generator. That is the wrong end. The size you need is decided by your load list and by whether the system can shed loads — and the install is usually the bigger number on the invoice.

Size it from loads, not from square footage

Any sizing rule based on the square footage of your house is guessing. Two houses of identical size with different heating systems and different well arrangements need very different generators.

The method that works:

1. List the loads that must run at the same time. Heating system (blower or heat pump), refrigeration, well pump or sewage ejector, freezer, some lighting and outlets, internet, and — decide deliberately — air conditioning.
2. Sum the running watts. This is the continuous load the generator must carry.
3. Add the largest starting surge. Motor loads draw several times their running current briefly at startup. The generator has to survive the biggest single one, not the sum of all of them.
4. Leave headroom. Running a generator continuously at its ceiling is how you shorten its life.

The generator sizing calculator does this from a real appliance list. Do it before a contractor sizes it for you, so you can tell whether their number is reasonable.

Load management usually beats a bigger generator

This is the decision that most changes the price, and it is routinely skipped.

A load-management controller monitors what the house is drawing and sheds designated large loads when demand approaches the generator's capacity — pausing the air conditioner while the well pump starts, then letting it resume. Because your biggest loads are intermittent and rarely all start at once, managing them lets a smaller generator carry the same house.

The practical effect is often a full size class of generator, plus the smaller gas line and smaller pad that come with it. Ask every contractor quoting you to price the configuration with load management as well as without, and compare the totals rather than the generator sizes.

The related question is whole house vs partial generator — backing up a subpanel of essential circuits instead of the whole service. For many households that is the honest answer, and it costs far less.

Natural gas vs propane

Natural gas is the low-friction choice when you already have gas service: an unlimited supply that requires no refills and no storage. Its weakness is that it depends on the gas utility, which can be interrupted in the same events that take out power — rare, but not never, and worth thinking about after an earthquake or a major storm.

Propane gives you fuel independence and stores indefinitely without stabilizer. Its weakness is that the tank holds a finite amount, and refills during a regional outage compete with everyone else's.

The specification trap: the same generator produces less rated output on natural gas than on propane, because of the lower energy content per unit volume delivered. Dual-fuel units publish two ratings. If you will run on natural gas, size against the natural gas rating, not the headline propane number.

Whichever you choose, the gas piping has to be sized for the generator's demand in addition to everything else on the line. An existing line adequate for a furnace and a range often is not adequate once a generator is added, and upsizing that run is the install cost most often missing from an initial quote. More on this in generator fuel types.

The transfer switch is not an accessory

An automatic transfer switch (ATS) senses the outage, starts the generator, transfers the house across, and reverses the process when utility power returns — with nobody home.

It is also the code-required means of ensuring your generator can never backfeed onto the utility lines. Backfeed energizes conductors that a line worker has every reason to believe are dead. This is the reason improvised connections are not merely against code but genuinely lethal to third parties.

Two things to settle when quoting:

• Service-rated vs non-service-rated. A service-rated switch can serve as the service disconnect, which sometimes simplifies the installation and sometimes is required by the layout.
• Whole-service vs subpanel. A whole-service switch transfers everything and pairs with load management. A subpanel switch transfers only the circuits you moved onto an essential-loads panel.

See standby generator installation process for what the whole job involves.

The specs worth comparing

Rated output at the fuel you will use. As above — natural gas rating if that is your fuel.

Surge capability, separately from continuous rating. This is what starts your air conditioner.

Engine type and speed. Larger residential units generally run at a lower engine speed than small air-cooled ones, which affects noise and service life. Air-cooled units dominate the residential range; liquid-cooled units appear at the top end and are built for longer continuous runtime.

Published sound level in dBA, at a stated distance. Compare only figures quoted at the same distance and in the same mode — many manufacturers publish a quieter exercise-mode number alongside the running number.

Weekly exercise cycle. Standby generators self-test. Check the day, time and duration are adjustable, because this runs for the life of the machine and your neighbours will notice.

Warranty and local service. A standby generator needs oil changes and periodic service. Confirm who does that locally before you buy the brand, not after.

Clearances. Manufacturers publish minimum distances from the building, from openable windows and doors, and from combustibles. These decide where the unit can physically go, and they can rule out an otherwise obvious location. Local requirements may be stricter.

What the install actually includes

Ask for these as line items, because the gap between quotes is usually here rather than in the machine:

• The transfer switch and its installation
• The pad — concrete or composite
• Gas piping, sized for the added demand, and any upsizing of the existing run
• The electrical connection to the service, and any panel work
• Load management hardware, if used
• Permits and inspection
• Commissioning and the first service interval

Standby generator installation cost breaks down what drives each of these.

When a battery is the better answer

Standby generators win on unlimited runtime and on cold-weather multi-day outages. They lose on noise, exhaust, fuel logistics, permitting, and the fact that they do nothing for you the other 360 days of the year.

A large battery system with split-phase output can feed a transfer switch the same way, silently and with no combustion — and can be recharged from solar mid-outage. The honest comparison is in battery backup vs generator, and the current units are covered in best solar generators.

For many households the real answer is a smaller generator plus load management, or a battery plus a partial-load panel — not the biggest machine the driveway will hold.

Where to go next

• Size it: generator sizing calculator
• Whole house vs partial generator
• Standby generator installation cost
• Standby generator installation process
• Portable vs standby generator
• Battery backup vs generator
• Best solar generators

More in our generator and backup power guides.