What Size Generator Do I Need for My House?

Square footage alone won’t tell you what size backup generator your home needs. The right size comes down to your home’s peak electrical demand, the startup surge required by large appliances, and how much of your home you actually want to keep running during an outage.

Typical Residential Generator Sizes

Home generators are rated in kilowatts (kW). As a rough starting point:

• 5–7.5 kW — refrigerator, lighting, internet equipment, and a handful of outlets
• 10–14 kW — a few chosen circuits, possibly a smaller AC unit
• 17–22 kW — central air conditioning plus kitchen appliances, lighting, and refrigeration for many homes
• 24–26 kW — larger homes running several major appliances alongside one central AC system
• 30 kW+ — homes with multiple AC units, electric heat, an electric water heater, a pool, a workshop, or full whole-home backup

These tiers are a starting point, not a final answer — actual sizing has to match your home’s real electrical load and the startup requirements of motor-driven equipment.

Running Watts vs. Starting Watts

Sizing starts with a list of what you want powered during an outage. “Running watts” is what a device draws once it’s already on; “starting watts” is the brief surge needed to kick on a motor or compressor. A sample household load might look like:

That adds up to roughly 7.1 kW of running load — but a 7.1 kW generator isn’t necessarily big enough. A refrigerator’s compressor, an AC unit, a sump pump, and other motor-driven equipment can briefly demand far more power at startup than they use while running. Size a generator only to the running total and the largest motor in the house may simply fail to start, overload the unit, or cause a voltage drop.

Houston homes need to pay particular attention to air conditioning load: a generator sized just for lights, a fridge, and a few outlets may not have the surge capacity to start a central AC system, though some newer variable-speed or soft-start units have lower startup demands (check the equipment’s specs to be sure).

Whole-Home Backup vs. Selected Circuits

A whole-home generator, wired through an automatic transfer switch, can power most or all of a home’s electrical system — useful for houses with central air, electric heat, medical equipment, pumps, an electric water heater, a home office, pool equipment, or other heavy loads.

Not every homeowner needs that much capacity. An electrician can instead wire a system that only powers a chosen set of circuits, which can lower both the generator’s cost and its fuel use — you just need to decide ahead of time what you want available in an outage.

Load-management equipment offers a middle path: it can let a smaller generator cover more of the home by briefly delaying a lower-priority load — an electric water heater or pool pump, for example — while the AC is running, rather than sizing the generator to run everything at once.

Generator sizing should account for the electrical panel, HVAC equipment, water heater, pumps, pool equipment, workshop loads, an EV charger, and any medical equipment — the actual electrical load, not just square footage or the home’s service rating. A 200-amp electrical service, for instance, doesn’t automatically mean the home needs a 48 kW generator: the service rating tells you how much power the home can draw from the utility, not how much generator capacity you need during an outage.

Let Electrical Works of Houston Help

Picking a generator is only the first step. Our electricians can review your home’s electrical system, identify the loads you want to keep running, calculate the generator capacity you actually need, and confirm your natural gas or propane supply can support it — then help you decide between selected-circuit backup and whole-home coverage, and install the transfer switch and related equipment to code.