What size generator do I need for my house?
It is the first real question every homeowner asks, and the honest answer is that whole house standby generator sizing comes down to one number: the kilowatts (kW) your home actually draws at the worst moment, plus a margin. In greater Houston that worst moment is almost always a 100-degree afternoon when the air conditioning is fighting to keep up and the grid has just gone dark. Size the unit for that and everything else takes care of itself.
There is no single “right” answer that fits every house, but there is a clear way to reach yours. Below is how to think about the number so you walk into a site visit already knowing roughly where you will land. The final figure always comes from a licensed installer running a real load calculation on your panel, and you can rough out a starting estimate with the generator sizing calculator first.
Sizing by home square footage (a rough starting point)
Square footage alone is a weak predictor because two homes of identical size can pull very different power depending on AC tonnage, electric versus gas appliances, and pool or well equipment. Still, it gives a starting frame:
- Under ~2,000 sq ft, one AC system: commonly lands in the 14 to 18 kW range.
- 2,000 to 3,500 sq ft, one or two AC systems: frequently 20 to 24 kW, often with load management.
- 3,500 sq ft and up, two or three AC systems: usually pushes the top of air-cooled or into liquid-cooled 26 kW and beyond.
Treat these as conversation starters, not a spec sheet. The surge from your largest AC compressor can move the number in either direction.
kW by appliance: building the number from the bottom up
A better method than square footage is adding up what you actually run. Rough running wattages for common loads:
- Central AC blower / furnace fan: ~800 W
- Refrigerator or freezer: ~700 W each
- Electric water heater: ~4,500 W
- Electric range or oven: ~3,000 W
- Electric dryer: ~5,400 W
- Well pump (1 HP): ~2,000 W running
- Pool pump: ~2,000 W
- Lights, outlets, Wi-Fi, TVs: ~2,000 to 2,500 W combined
Add your must-run loads, then add the single largest motor’s startup surge on top. That total, converted to kW, is your target before margin.
The AC compressor surge: the Houston factor
Here is the trap unique to hot, humid markets. An air-conditioning compressor does not just draw its running wattage; it pulls a brief startup surge, its locked-rotor amps, several times higher than steady draw the instant it kicks on. A 4-ton system that runs at a few thousand watts can spike far higher for a fraction of a second.
Stack two or three AC units in a larger Sugar Land or Cypress home, all cycling on a brutal afternoon, and a generator sized only for running loads gets blindsided. Correct sizing lands the largest compressor’s surge on top of everything already running. A soft-start device on the compressor can cut that surge sharply and sometimes lets you use a smaller unit, which is worth asking your installer about.
What size generator to run a whole house
To run a whole house, meaning every AC compressor, the electric range, the dryer, and everything else at once with margin to spare, most Houston homes need to be in true whole-home territory: often 22 to 26 kW air-cooled for a typical two-AC house, and 30 kW and up in liquid-cooled form for large homes with three or more AC systems. Whole-home coverage is the simplest experience, an outage becomes nearly invisible, but it is the most generator, the most fuel, and the most cost.
Managed loads vs whole-home: the smarter-money middle
Many homes never need the full whole-house unit. Pair a more modestly sized generator with a load-management (load-shed) system: smart switches in the panel prioritize what runs, so AC, kitchen, and key circuits stay powered while lower-priority loads are briefly dropped or rotated. The generator is never overwhelmed, and you get the comfort that matters for thousands less in equipment. A great many Houston installs land here, and for good reason.
Standby generator sizing: don’t forget fuel de-rating
Standby generator sizing is not finished at the appliance list. Nameplate ratings are typically given for natural gas, and delivered output depends on fuel and conditions. Natural gas usually yields slightly less than propane for the same engine, and gas-line pressure and pipe diameter matter; an undersized line can starve the unit under load. Heat and humidity, which Houston has in abundance, also shave output. A good installer factors this de-rating in rather than trusting the sticker. Fuel is worth its own read: see natural gas vs. propane.
Why a real load calculation beats every rule of thumb
Tidy formulas (“X square feet needs Y kW”) ignore AC tonnage, electric versus gas appliances, pool equipment, and EV charging. A real calculation walks circuit by circuit, inventories major loads, applies the largest motor’s surge, adds margin, and arrives at a defensible kW target. Rough it out with the sizing calculator, then compare brands in the best whole-house generator and Generac vs. Kohler vs. Cummins guides.
Where to go next
Once you have a target, work through fuel and installation: natural gas vs. propane, the install-day guide, and the permitting-by-county guide, since Harris, Fort Bend, and Montgomery counties handle approvals differently. When you want a real load calc, we will connect you with a vetted, licensed local installer. Start from the home page or your city page: Houston, Sugar Land, or Cypress.