What size generator does my house need?

Generator sizing goes wrong in two directions. Too small, and the unit trips or stalls the moment the well pump and the air conditioner start together. Too big, and you've paid for capacity you never use. This guide explains how sizing actually works, why the transfer switch you choose changes the answer, and what to have ready before you get a quote. When you're ready for the install, our standby generator and transfer switch installation page covers how we do the work.

Start with the outage, not the house

Two houses with the same square footage can need very different generators. A house on public water with gas heat may only need to keep the furnace blower, the fridge, a sump pump and some lights going. A farmhouse in northern Harford County on a well, with an electric heat pump and a basement that floods, has a much longer must-run list. Start by deciding which of these you're after:

• Essential circuits. A chosen set of circuits: heat, well and sump pumps, refrigeration, some lights and outlets, anything medical. Life goes on, with some limits.
• Whole home. Everything in the house runs, central air and electric cooking included, as if the power never went out.

Running power and starting power

Every appliance has a running load, the power it draws once it's going. Anything with a motor or compressor, like a well pump, sump pump, air conditioner, heat pump or refrigerator, also draws a short surge when it starts, well above its running load. A generator has to carry the total running load and absorb the largest starting surge on top of it.

That's why sizing is done on paper first:

1. List every load you want on backup, and read its nameplate rating, or have an electrician measure it.
2. Add up the running loads of everything that could be on at the same time.
3. Add the starting surge of the biggest motor, because that's the moment the generator works hardest.
4. Leave headroom, so the generator isn't running flat out for days.

Online calculators and wattage charts are fine for a rough idea, but they use typical numbers. The nameplates on your own equipment, and the panel schedule, are what a real load calculation uses.

How the transfer switch changes the size

Every safe generator hookup keeps generator power and utility power apart, so the generator can't backfeed the utility lines. The device that does it also affects how big the generator needs to be.

That first row is a code rule, not just good practice. The National Electrical Code's article on optional standby systems requires an automatically transferred system to be able to supply the full load it transfers, unless a system automatically manages the connected load. With manual transfer, the user picks the loads, and the generator is sized to those.

The panel and service matter too

The generator connects through your panel, so the panel has to be in good shape and have room for the transfer equipment. A full or failing panel may need work first. If you're also planning an EV charger or a heat pump, think about the panel and the generator together. Our guide to upgrading from 100 to 200 amps explains how a load calculation decides service size.

Portable generators: smaller, with rules

A portable generator with an outdoor inlet and an interlock or manual transfer switch is often the lowest-cost way to keep essentials going. It's sized the same way, just to a shorter list. The U.S. Consumer Product Safety Commission is blunt about where it runs: outdoors only, at least 20 feet from homes and buildings, with the exhaust pointed away from doors, windows and vents, and never in a home, garage, basement, crawlspace or shed, even with the doors open. The CPSC also recommends a generator with a carbon monoxide shut-off and working CO alarms on every level of the house. Never plug a generator into a wall outlet.

Is backup power worth it here?

Outages in central and northern Maryland come from summer thunderstorms and winter storms alike. A single round of severe storms has knocked out power to more than 114,000 customers in central Maryland at once. If your house depends on a well pump or sump pump, or anyone relies on medical equipment, even a short outage matters. If you're still deciding between fuel and stored power, read generator vs. battery backup.

What to have ready for a sizing visit

• Your must-run list: well pump, sump pump, heat, fridge and freezer, medical equipment
• Whether you want essentials only or the whole home
• The fuel you have, or would add, at the house
• Where outside a unit could go, away from windows and doors
• Any plans for an EV charger, heat pump or addition

The connection is permitted electrical work in Harford County, Baltimore County and Baltimore City, and it's inspected before it's finished. Wyatt Electric has wired homes here since 1986. We'll walk through your loads, do the calculation and explain the options and the price before any work starts.