Home Generator Size Calculator

How to Size a Home Generator

Sizing comes down to a simple process — and this calculator does the math for you:

• List what must stay on. Start with essentials: refrigerator, freezer, some lights, router, furnace blower or sump pump. Then decide what comfort items (TV, window AC, microwave) matter to you.
• Find each appliance’s wattage. Check the rating plate or manual for running watts. Motor-driven appliances also list starting (surge) watts — or assume 2–3× running watts.
• Add running watts, then the biggest surge. Sum all running watts, then add only the single largest starting-watt difference, since usually only one big motor starts at a time.
• Add a safety buffer. 10–30% of headroom keeps the generator from running flat-out and leaves room for loads you forgot.
• Choose the generator type and connection method. Small portables run on extension cords; anything wired through your panel needs a transfer switch or interlock kit installed by an electrician.

Running Watts vs. Starting Watts: What’s the Difference?

Running watts are the steady power a device draws while operating — a refrigerator might run at about 700 watts. Starting watts are the brief surge needed when a motor or compressor kicks on — that same refrigerator may demand around 2,200 watts for a couple of seconds. Generators carry two ratings for exactly this reason: a continuous rating and a higher peak/surge rating. When sizing, your generator’s continuous rating must cover total running watts, and its surge rating must cover the biggest single motor start.

Portable Generators vs. Home Standby Generators

Portable generators are far cheaper and flexible: roll them out, start them, and run appliances with heavy-duty extension cords or through a transfer switch. Downsides: you must set them up in bad weather, refuel them by hand, and they typically can’t power a whole house. Home standby generators are permanently installed, start automatically within seconds of an outage, run on natural gas or propane, and can power an entire home — but they cost far more and require professional installation and permits. If your outages are short and rare, a portable often makes sense. If you lose power often, run medical equipment, or want full-home coverage, a standby unit is worth evaluating.

Why You Absolutely Need a Transfer Switch

A transfer switch (or an interlock kit on compatible panels) is the only safe way to feed your home’s wiring from a generator. It isolates your home from the utility grid so power can’t flow back onto the lines — backfeeding can electrocute utility workers repairing the outage and can burn down your house when the grid comes back. It also lets you power hardwired appliances like furnaces, well pumps, and central AC that have no plug. In most jurisdictions, a transfer switch or interlock is required by electrical code. This is not a DIY corner to cut — have a licensed electrician install it.

Generator Safety: The Dangers of Carbon Monoxide

Carbon monoxide (CO) is invisible, odorless, and lethal — generator exhaust can raise CO to deadly levels indoors within minutes. The rules are non-negotiable:

• Run portable generators outdoors only, at least 20 feet from the house, with exhaust pointed away from windows and doors.
• Never use one in a garage, basement, shed, or under a covered porch — even with the door open.
• Install battery-backed CO detectors near sleeping areas and test them regularly.
• Let the generator cool before refueling; gasoline spilled on hot parts can ignite.
• Keep fuel stored safely outdoors in approved containers, away from living areas and heat sources.