What Size Generator Do I Need for My House?

What Size Generator Do You Need for Your House?

Most San Angelo homes are fully backed up by a 22kW to 24kW air-cooled standby generator, central air conditioning included. Homes under 2,000 square feet often run fine on 14kW to 18kW. If you only want essentials like the fridge, some lights, and internet, a 10kW standby or a 5,000-watt portable covers it. The exact number comes from a wattage load calculation, not square footage.

Two homes the same size can need very different generators depending on how they cool, heat, and pump water.

Get it wrong and you either sit in the dark with a stalled generator or overpay by thousands. The answer comes down to two figures: what your equipment draws while it runs, and the much bigger spike it pulls the instant a motor starts.

Running Watts vs. Starting Watts: The Number That Sizes Your Generator

Starting watts sizes a generator, not running watts. Running watts is the steady power an appliance uses once it is on, like a fridge humming along. Starting watts, also called surge or locked-rotor watts, is the brief spike a motor pulls the moment it kicks on, usually two to three times its running load for two to three seconds.

Anything with a motor or compressor has this surge: your AC, well pump, refrigerator, furnace blower. Lights, a TV, or a phone charger have almost none, so their running and starting watts match.

A generator has to survive the largest single surge on top of everything already running, or it stalls. That is why a unit that looks big enough on paper trips the first time the AC compressor fires on a July afternoon.

How Do You Actually Calculate Generator Size?

You add up running watts, then add the single largest starting surge on top. The steps:

1. List every load you want backed up. Be honest about what “essential” means in your house.
2. Write down running and starting watts for each. Nameplate labels, spec sheets, and owner’s manuals have them. Motor loads list locked-rotor amps (LRA); multiply LRA by voltage to estimate the surge.
3. Total the running watts of everything that could run at once.
4. Find the one appliance with the highest starting watts (almost always the central AC or the well pump) and add that single surge to your running total.
5. Add 10 to 20 percent headroom. Past about 30 percent just wastes fuel and runs the engine under light load.

For a whole-home unit, licensed electricians skip the appliance list and run a formal load calculation under the National Electrical Code. The NEC Article 220 optional method (220.82 for a new one-family dwelling, 220.83 for adding a generator to an existing home) applies demand factors to your square footage and fixed appliances, and lands most 2,000-square-foot homes with central air in the 22kW to 26kW range. That is also what the permit and inspection expect to see.

What Size Standby Generator Fits a Whole Home?

A standby generator either powers everything or just the circuits you pick. Whole-home means every outlet works and both AC units run. Essential-circuits, also called managed or critical-load, powers only the circuits you choose and costs less to install.

Roughly how the sizes land by home:

• Small, under 1,500 sq ft: about 7.5 to 10 kW for essentials, 14 to 18 kW whole house with AC. A Generac 14kW Guardian covers many.
• Mid, 1,500 to 2,500 sq ft: 10 to 14 kW essentials, 18 to 24 kW whole home. The popular Generac 22kW Guardian lands here.
• Large, 2,500 to 3,500 sq ft: 14 to 18 kW essentials, 24 to 26 kW whole home, often a Generac 26kW Guardian.
• Very large, 3,500+ sq ft with two AC units: 18 to 22 kW essentials, 26 kW or a liquid-cooled unit for the whole house.

Generac’s air-cooled Guardian line runs 14, 18, 22, 24, and 26 kW, and the 26kW is the most powerful air-cooled home standby made. Above that, or for two large compressors that start together, you move to liquid-cooled generators.

The 22kW stays the most popular size because it covers most single-family homes with room to spare. For what one costs installed, see how much a Generac 22kW generator costs to install.

How Much Do AC and Well Pump Surge Loads Change the Math?

In West Texas, the air conditioner and the well pump are the two loads that decide your size. A 3-ton central AC draws roughly 2,400 to 3,000 running watts, but its compressor surges to 6,000 to 9,000 watts for a couple of seconds every start. Through a San Angelo summer that compressor cycles constantly, so the generator absorbs that surge over and over.

Well pumps behave the same way and matter in rural Tom Green County, out toward Grape Creek, Christoval, and Water Valley, where many homes pull their own water. A 1 to 1.5 HP submersible pump can surge to several thousand watts on startup. When the pump and the AC fire in the same moment, that combined surge is what a too-small generator cannot handle.

A soft starter on the AC compressor limits inrush current and can cut the surge roughly in half. Micro-Air, a common brand, reports a 4-ton system that would otherwise need about a 17,000-watt generator can start on around 7,100 watts with a soft start installed. That one upgrade sometimes lets you drop a generator size and save more than the soft starter costs.

If your house is where the AC and pump math decides everything, that is a job for someone who can read your nameplates. Call (877) 473-1995 and we will connect you with a local electrician in San Angelo who can size a generator around your actual AC and pump loads.

Can Load Management Shrink the Generator You Need?

Yes. Load management, also called load shedding, lets a smaller generator back up a whole home by never letting all the big loads run at once. A Generac Smart Management Module (SMM) watches the generator’s output frequency and drops a lower-priority load, such as a second AC or the water heater, when it sags, then restores it once there is headroom. You can prioritize up to eight managed loads.

This is why a 22kW generator with load management can cover a house that would otherwise need 30 kW or more of raw capacity: you pay for the loads you use at once, not the worst case where everything starts together. An installer may quote a 22kW unit with a load module instead of a bigger, thirstier one. The transfer switch and modules are part of what a licensed electrician configures during professional Generac installation, and one of the pieces covered when you connect a generator to your house the safe way.

What Size Portable Generator Covers the Essentials?

A 3,500 to 5,000 watt portable covers essentials only, not a whole house. That range runs a refrigerator, a freezer, several lights, phone and internet gear, a sump or utility pump, and a small window AC, as long as you start them a few at a time.

What a portable cannot do is start a central air conditioner or power 240-volt appliances like an electric range, clothes dryer, or water heater. Those need far more surge and a 240-volt connection a portable is not built to feed safely into your panel.

A portable also means fuel runs, cords through a window, and keeping the unit at least 20 feet from the house because of carbon monoxide. Fine for a few hours, but not for keeping a West Texas home cool through a multi-day event like the February 2021 freeze. For that you want a permanently installed standby unit that starts automatically on natural gas or propane.

Why the Load Calculation Should Come From a Licensed Electrician

Sizing wrong is expensive both ways, which is why this belongs with a licensed electrician. Too small and the generator overloads, stalls, and leaves you dark during the exact heat wave or ice storm you bought it for. Too big and you overpaid by thousands, then run an oversized engine under light load that wastes fuel and shortens its life.

A licensed electrician walks your panel, reads the nameplate on your AC and well pump, factors your fuel type and any load management, and produces an NEC-compliant calculation that satisfies the City of San Angelo permit and inspection. A 22kW unit supplies roughly 86 amps on natural gas, enough to start and run a 3-ton to 4-ton AC while the fridge, lights, and household circuits stay on. If your home has 100-amp service or an older panel, the electrician will flag whether a panel upgrade comes first.

A standby install ties into both your electrical service and a fuel line, so it is regulated work, not a weekend project. In Texas, TDLR licenses the electricians who can pull the permit. The tradeoffs of doing it yourself are in whether you can install a Generac generator yourself, and the full installation timeline covers what to expect from deposit to startup.

Getting the Right Size for Your San Angelo Home

The right size is the one that matches a real load calculation of your home: your AC tonnage, your well pump if you have one, and how much you want running at once. Square footage gets you a ballpark. The surge math gets you the answer.

Call (877) 473-1995 and we will connect you with a local electrician in San Angelo who can run the load calculation, recommend a size, and give you an upfront price. You talk to a real local person, and there is no pressure and no obligation.