The single most common mistake people make when buying a home generator is getting the size wrong. And it almost always goes in the same direction: too small. Someone buys a 7,500-watt portable unit thinking it’ll handle their house, fires it up when the power goes out, and then discovers that running one window AC unit, a refrigerator, and the lights pushes it right to the edge of its capacity. Add the microwave or a hair dryer and the breaker trips.
For homeowners in Houston, sizing a standby generator correctly isn’t just about comfort. It’s about having a system that can actually sustain your household through a multi-day outage in summer heat, when air conditioning isn’t optional, it’s a health necessity. I’ve done consultations on generator sizing for dozens of Houston homes, and the number varies a lot. Let me walk you through why, and how we figure out the right number for your specific house.
Start With Your Essential Loads
Sizing a home generator isn’t as simple as multiplying your square footage by a number. It depends entirely on what you’re trying to power.
The first step is identifying your essential loads. These are the appliances and systems that absolutely need to run during an outage. For most Houston households, that list looks something like this:
Central air conditioning is typically the biggest draw. A standard 3-ton central AC unit running on a single-phase residential system pulls around 3,500 to 5,000 watts when running, with a startup surge that can hit two to three times that figure for a fraction of a second. That surge matters because it’s what trips undersized generators.
Your refrigerator and freezer are essential. Combined, they usually pull 400 to 800 watts running.
Lighting throughout the house is surprisingly modest with LED fixtures, usually 300 to 600 watts total depending on how many zones you’re running.
Well pump, if you have one, adds another 750 to 1,500 watts.
Miscellaneous items like phone chargers, a TV, a few small appliances can add another 500 to 1,000 watts without much effort.
Add all of that up for a medium-sized Houston home with a single 3-ton AC system, and you’re looking at a continuous load somewhere in the 7,000 to 12,000 watt range. That’s the floor for a whole-home system, and it doesn’t account for electric water heaters, a second AC zone, or any other heavy appliances you might have.
This is where a lot of DIY sizing calculations fall apart.
Every motor-driven appliance, your AC compressor, your refrigerator, your well pump, has two power ratings: running watts and starting watts. The starting wattage, sometimes called the surge rating, can be two to three times higher than the running wattage. It only lasts a second or two, but the generator has to be able to supply it without dropping frequency or tripping a breaker.
For a 3-ton AC unit with a 5,000 watt running draw, the startup surge might hit 12,000 to 15,000 watts momentarily. If your generator’s surge rating can’t handle that, it will struggle every single time your AC cycles on.
This is one of the main reasons we don’t recommend sizing right at the edge of your calculated load. A generator running at or near its maximum rating all the time runs hotter, wears faster, and uses more fuel. Building in a 20 to 30 percent headroom buffer is standard practice for a reason.
The U.S. Department of Energy’s guide on backup power recommends calculating your total load and then choosing a generator with at least 20 percent capacity above that number to account for startup surges and future load additions.
Common Houston Home Scenarios and Typical Sizes
Let me give you some practical reference points based on the consultations we’ve done in Sugar Land, Cypress, League City, and the surrounding area.
Smaller home, single AC zone, gas appliances: A home under 1,800 square feet with one 2-ton to 3-ton AC system and mostly gas appliances (stove, dryer, water heater) can often be covered by a 14kW to 18kW air-cooled standby generator. This is one of the most popular sizes we install.
Mid-size home, two AC zones, mixed appliances: A 2,000 to 3,000 square foot home with two separate AC systems, an electric water heater, and standard kitchen appliances typically needs something in the 20kW to 26kW range. This is where we see the most variability based on actual appliance inventories.
Larger home or any home with all-electric appliances: Homes over 3,000 square feet, or any home that has an electric range, electric dryer, electric water heater, and a pool pump in addition to AC, can need 30kW or higher. These are typically liquid-cooled standby systems.
These are starting points, not final answers. The only way to get the right number for your specific home is to actually add up the loads, which is what we do during a consultation.
Natural Gas vs. Propane: Does Fuel Choice Affect Sizing
Fuel type doesn’t change your load calculation, but it does affect your options.
Natural gas is the dominant choice in Houston’s established suburban neighborhoods because the supply is continuous. You don’t have a tank to run dry. During an extended outage, you can run indefinitely as long as gas service isn’t disrupted. And in most Houston outage scenarios, the gas lines stay pressurized even when the power grid is down.
Propane is the right call when you’re in an area without access to a natural gas line. Tank sizing becomes important because you need enough propane to get through a multi-day outage. We factor that into recommendations when propane is the fuel source.
Generac and Kohler both make excellent standby generators in natural gas and propane configurations across the full range of residential sizes. Our generator installation page has more information on what’s available.
Why Getting This Wrong Is So Expensive
Undersizing a generator means running it at high utilization for extended periods, which accelerates wear and can cause premature failures. It also means the unit may not reliably handle startup surges, which leads to nuisance trips right when you need the power most.
Oversizing creates different problems. A generator running at very low utilization (under 30 percent load) can develop a condition called wet stacking in diesel units, where unburned fuel deposits build up in the exhaust system. Even in natural gas units, consistently light loads aren’t ideal for engine longevity.
According to Kohler Power Systems’ installation best practices , load bank testing and proper generator sizing are among the most important factors in system longevity. That’s not marketing. That’s engineering reality.
Getting the size right from the start means your generator runs in its optimal range, lasts longer, and is ready to perform when it counts.
Let Us Run the Numbers for You
Sizing a home generator in Houston doesn’t need to be a guessing game. We offer on-site consultations where we walk through your home, look at your electrical panel, and calculate your actual load based on your specific appliances and systems.
There’s no pressure, and we’ll give you a straight recommendation with our reasoning. If you want to power just the essentials, we’ll size for that. If you want whole-home coverage with room to grow, we’ll size for that instead.
Book a generator sizing consultation with Critical Power Solutions today. We serve Sugar Land, Richmond, Tomball, Spring, Cypress, Missouri City, Pasadena, League City, and the broader Houston area. Stop guessing and start planning.