What Size Generator for a House?

"What size generator for a house" has no single answer, because houses differ on exactly the variables that matter. The two that matter most are how the house heats and where the water comes from. Everything else is a rounding error by comparison.

Type 1: gas heat, city water, no air conditioning

The easiest house to back up, and more common than people expect. A gas furnace needs only its blower and controls powered, and city water arrives under municipal pressure whether or not your house has electricity.

2,100 running watts with 3,850 starting watts. A 3,500 running-watt quiet inverter generator covers this with large headroom — or, if your outages are short, a 2 kWh battery power station does it indoors and silently. This is the house where a generator is often the wrong purchase.

Type 2: gas heat, well pump, sump pump

Now the surge matters. A well pump and a sump pump both have motors, and losing either one has consequences — no water at all, or a flooded basement.

This is the classic American essential-circuit house and it wants a 5,000-6,500 running-watt dual fuel generator through a 30 amp transfer switch. Note that the fridge's surge still dominates — the well pump's 2,000 watt surge is second, so the fridge sets the starting requirement.

One caution: EWEB publishes figures for a 1/3 HP well pump. If yours is 1/2 HP or larger — and many deep wells are — the surge is substantially higher and you should read generator size for well pump before settling on a machine.

Type 3: add a window air conditioner

One window unit changes the picture considerably, and the two charts we cite disagree sharply about by how much — which is itself the lesson.

Taking EWEB's larger figure, the Type 2 house plus a 12,000 BTU window unit is 6,750 running watts, and the AC's 3,950 watt surge now exceeds the fridge's, so starting watts become 10,700. After derating: about 8,500 running watts.

That is an 8,000-10,000 running-watt portable generator, and at that level a 30 amp transfer switch becomes the bottleneck. Plan a 50 amp connection or an interlock kit.

Type 4: central air, electric heat, or both

Here we stop and tell you what we do not know. Neither published chart we cite gives central air conditioning figures, and we are not going to invent them for the load that most often decides which generator somebody buys.

What you do instead: read the nameplate on the outdoor condenser unit. It gives voltage and running amps — multiply for running watts — and often LRA, locked rotor amps, which is the start-up surge. A 3-ton residential system is commonly in the region of 3,500 running watts with a much larger surge, but your plate is the authority, not a generic figure.

Add that to the Type 2 or Type 3 total and you will generally land above 11,000 running watts, which means tri-fuel and a 50 amp connection. Before you commit to that, check whether a soft starter on the compressor brings you down a class — on the published claim of up to 70% startup reduction, it frequently does, and it costs far less than the next generator up.

Electric heat is a different and harder problem: resistive heating has no surge but draws its full rating continuously, and an electric furnace or water heater can exceed a portable generator's entire output on its own. The usual answer is to leave electric heat off the backup list and use a gas or propane space heater instead.

The short answer

One pick for each house type

#	Product	Best for	Score	Price
1	Generac 12,500W Starting Tri-Fuel The cheapest credible way to get a generator onto your home's natural gas line, which is the one fuel supply an outage usually does not interrupt.	Natural-gas hookup at the best price	9.1/10★★★★★	$920.00
2	WEN DF8000X 8,000W Dual Fuel The best-value correctly-sized home backup generator here: enough for every essential circuit, dual fuel, CO shutoff, a real transfer-switch outlet, and no money spent on capacity you will not use.	Essential circuits, dual fuel, lowest cost	8.5/10★★★★★	$623.17
3	Champion 4,500W RV Ready Inverter The best balance of quiet, clean power and real output for cord-based backup: enough for the essentials, under 3% THD, half the price of the Honda equivalent.	Quiet backup at a sane price	8.5/10★★★★★	$674.10
4	Westinghouse WGen9500DF 12,500W Dual Fuel The most broadly sensible home backup generator on the market: enough running watts for a whole essential-circuit panel, both transfer-switch outlets, and dual fuel — with the caveat that it has no CO shutoff.	The default whole-house pick	8.8/10★★★★★	$999.00

#ad Prices as of October 5, 2026; they may have changed since. Scores are our editorial judgment from published specifications, not test results — how we pick. How we are funded.