--- title: "What size generator does a house need? | Root House Mechanicals" url: https://roothousemechanicals.com/resources/generator-size updated: 2026-10-05 business: "Root House Mechanicals" phone: "(615) 426-9344" text: "(615) 426-9344" email: info@roothousemechanicals.com hours: "Monday to Friday, 8 a.m. to 5 p.m." --- # What size generator does a house need? > Size a generator to what has to run at the same time. Add up the running watts, then allow for the extra surge the biggest motor pulls to start. Covering the fridge, lights and a well or sump pump takes a much smaller generator than adding central air or electric heat. ## When to call a pro Call when you're choosing a standby generator or connecting any generator to the house wiring. Generators are where gas and electrical meet. We run the gas line. The wiring is electrical work, done by licensed and insured techs, and you have one point of contact for both. Call or text (615) 426-9344, Monday to Friday, 8 a.m. to 5 p.m. - Call (615) 426-9344 (Monday to Friday, 8 a.m. to 5 p.m.) - Text (615) 426-9344 - Email: info@roothousemechanicals.com ## How do I figure out what size generator I need? List what you want running during an outage, then add up the watts. Every appliance has running watts, what it draws once it's going. Anything with a motor, like a fridge, pump or air conditioner, pulls a short surge to start. Spec sheets call that starting or surge watts. The label lists the watts or amps it's rated for. For amps, multiply by the volts for a rough wattage. Add the running watts of everything that could run at once, plus the biggest motor's surge. That's a rough minimum. A portable's running watts should cover your running total, and its starting watts should handle the surge on top. Leave room above that, since motors can start together when the power comes back. For a standby unit, a licensed electrician's load calculation replaces the guesswork. ## What draws the most power in a house? Usually electric heat and central air. Heat strips or an electric furnace can be the biggest single load. A central air conditioner or heat pump draws a lot too, with a big surge each time its compressor starts. An electric water heater, range or dryer can draw as much as central air. A whole-house electric tankless water heater can outdraw most central air units. A fridge, lights and a gas furnace's blower take much less. Most well pumps pull a strong surge each time they kick on. A variable-speed pump or air conditioner, or a soft-start kit, shrinks that surge, and sometimes the generator. A heat pump with electric backup needs a far bigger generator in a January outage than a gas furnace's blower does. So settle heat and air first. That decision usually moves the size the most. ## Should I get a portable or a standby generator? If outages where you live are rare and short, a portable usually covers the essentials. With a well, a sump pump, medical equipment or someone who can't go without heat or air, lean toward standby. It costs more, but it starts itself and can run most or all of the house. You roll a portable out, well away from the house, and start it. Then you run heavy-duty outdoor cords to a few appliances, each rated for its load and free of cuts. Or you plug into a weatherproof socket on the outside wall (a power inlet) wired to your panel. In a long outage, you're refueling it in whatever weather knocked the power out. ## Can a generator run my whole house? Yes, with a large enough standby generator, or a smaller one with load management. That makes big loads take turns, like holding off the water heater while the air conditioner runs. Covering just the essentials means a smaller generator and a simpler install. ## What is a transfer switch, and do I need one? A transfer switch moves your house between utility power and generator power. Any generator connected to your home's wiring needs one, or an interlock. Either keeps generator power off the utility lines. A standby unit's automatic switch senses the outage, starts the generator and moves the house over. A portable uses a manual transfer switch or an interlock. You plug into the inlet and pick which circuits to run. An interlock is usually a sliding metal plate on the panel cover. It won't let the main breaker and the generator breaker be on together. Either one means working inside the panel. That's a licensed electrician's job, usually with a permit, and an interlock has to be a kit listed for your panel. ## What does a standby generator installation involve? A spot outside, a pad, a transfer switch tied into your panel and a gas line sized to feed it. Add the permits and inspections, and load management if you use it. Clearances from windows, doors and vents come from the manufacturer and local code. It's not something to eyeball. The gas line is easy to overlook, and we run it. At full load, a standby generator can burn more gas than your furnace and water heater combined. The line has to be sized for that, and sometimes the gas meter or propane tank does too. We'll look at your gas service and where the unit can go. A licensed and insured tech checks the panel. Then you'll have a firm price before we start. ## What can I figure out on my own before I call? Most of what goes into the sizing, and none of it takes a tool: - Write down what has to stay on, and what would just be nice. - Note your heat (gas furnace, heat pump or electric) and any well or sump pump. - Note whether you have natural gas or propane. - Open the panel door and take a photo. The main breaker's amp rating is usually on its handle. Some homes have it outside by the meter. If you see scorch marks or water, or smell burning, close the door and call. Stop at the photo. The metal cover behind the panel door stays on, and the gas fittings stay untouched. Leave the wiring and the gas work to us. The wiring is done by licensed and insured techs. ## Safety first **Never run a generator inside the house, garage, shed, basement or crawl space, even with the doors open.** Not on a porch, in a carport or under an overhang, either. The exhaust carries carbon monoxide, which you can't see or smell, and it can kill. Set a portable at least 20 feet from the house, exhaust pointed away from doors, windows and vents. Keep carbon monoxide alarms with batteries or battery backup on every level and outside the bedrooms. A plug-in alarm with no battery goes dead in an outage. If a carbon monoxide alarm goes off, or anyone has a headache or feels dizzy, weak or confused, get everyone outside and call 911. Stay out until the fire department says it's safe. Never plug a generator into a wall or dryer outlet. That's backfeeding. It can kill a line worker who thinks the line is dead, or start a fire. Connect only through a transfer switch or interlock installed by a licensed electrician. Keep a portable dry and on dry ground. Run it in rain or snow only if the manual allows it, under an open, canopy-style cover. Touch it with dry hands. Shut it off and let it cool before you refuel. Gasoline on a hot engine can catch fire. Store fuel outside the living space, away from anything with a pilot light. If you're buying a portable, get one with a carbon monoxide shut-off. ## Common questions ### How long can a standby generator run? On natural gas, as long as the gas keeps flowing at full pressure, but it isn't set-and-forget. In a long outage, many air-cooled models need a short daily shutdown for an oil check, and oil changes on the manual's schedule. On propane, as long as the tank lasts, which depends on its size and the load. ## Related services - [Whole-Home Generators in Middle Tennessee](https://roothousemechanicals.com/electrical/generators) Written by Root House Mechanicals. Last updated 2026-10-05.