Generator size depends on load, not square footage
The right generator size is set by load: what has to run at the same time during an outage, plus the extra surge motors pull when they start. A well pump, central air or electric heat pushes the size up fast. A gas-heated house that only needs essentials can get by with a much smaller unit.
Home standby generators are rated in kilowatts (kW), the power they can supply at once. Your house's demand at any moment is the sum of everything that's on. Sizing means making sure the first number comfortably covers the second, including the moment a motor kicks on.
Essentials or whole house: the first sizing decision
Before any math, decide what you want running when the grid drops. In a Capital District winter, heat comes first: a gas or oil furnace is useless without power for its blower and controls, and a frozen house is the expensive outcome. In summer, the sump pump and the fridge move to the top of the list.
Essentials coverage keeps the house safe and livable: furnace blower, fridge and freezer, sump pump, well pump, some lights and outlets, the internet router. The transfer switch feeds a chosen set of circuits, and the generator only has to carry those.
Whole house coverage runs everything, central air, electric range and dryer included. That needs a service-rated transfer switch and a larger unit.
Load management is the middle path. You get a whole house transfer switch, but modules turn off big loads, like the air conditioner or an electric water heater, when the generator nears its limit, then let them back on when there's room. A smaller generator can then cover the whole house, with the trade-off that the AC may wait its turn. Our transfer switch installation page explains the switch types.
The appliances that drive standby generator size
A few loads decide most of the answer:
- Central air or a heat pump. The biggest load in many houses, with a hard start-up surge from the compressor.
- A well pump. No power means no water. The pump motor surges every time it cycles on.
- A sump pump. Small, but non-negotiable in a basement home, since the storms that take out power are often the ones that bring water.
- Electric heat, an electric water heater, an electric range or dryer. Steady, heavy loads that add up fast.
- The furnace blower and boiler circulators. Light loads, but a gas or oil furnace still needs power to run.
If anyone in the house depends on powered medical equipment, such as an oxygen concentrator, it goes at the top of the essentials list. Tell the installer, so that circuit is covered no matter which transfer switch you choose. The same goes for a home office you can't afford to lose for days.
If you have a well, the pump decides your generator size more than your square footage does. That's true across rural parts of the Capital District, from the hills of Brunswick to wooded lots in Wilton.
Motor start-up surge and generator sizing
Motors draw several times their running power for a moment when they start. A generator has to cover that spike or its voltage sags: lights dim, the motor stalls, and the unit may shut down to protect itself.
One motor starting is usually fine. Two big motors starting together, say the well pump and the air conditioner both calling the moment the transfer switch puts the house on generator power, is the classic undersizing problem. Installers handle it three ways: a larger generator, load management that staggers big loads, or a soft-start kit on the air conditioner to tame its surge. Ask which one fits your house.
Why square footage alone misleads on whole house generator size
Online charts that match house size to generator size are a rough guess at best. Picture two houses of the same size. One, in Delmar, has gas heat, a gas water heater, a gas range, public water and window AC units. The other, out in the country, has a well, an electric water heater, central air and an electric range. The second needs far more generator, or a much smarter switch.
Plan for what's coming, too. If you expect to add a heat pump, an EV charger or a finished basement, tell the installer now. Swapping a generator later costs far more than sizing it right the first time.
How an installer does the generator load calculation
A proper load calculation starts at your electric panel. The installer reads the breakers and the nameplates on major equipment, like the air conditioner condenser and well pump, and asks what you need running. Some also look at your utility usage or measure actual loads. From there they add up the running load, account for the largest motor start-ups, leave headroom, and choose a unit size, a transfer switch type and any load management.
Gas meter and propane tank sizing
The fuel supply has to keep up with the generator. A larger unit burns more fuel at full load, so the gas line, and sometimes the gas meter, has to be sized for it alongside your furnace, water heater and range. If the meter can't carry the added load, the installer coordinates an upgrade with the utility. On propane, a bigger generator needs a bigger tank to get through a long outage between deliveries. That's one more reason not to oversize.
Ask for the result in writing, so you can compare quotes on the same basis. Our standby generator installation page covers what happens after sizing, and the Generac generator cost guide explains how size feeds the price. When you request a free quote, list what must run during an outage and we'll match you with a local installer who can size it properly.