What Size Generator Do I Need Calculator
Most homes back up their essentials with a 7,500–12,500 watt generator; a true whole-house setup usually needs 20,000+ watts. A generator size calculator for house use works two ways — pick whole-house sizing by square footage, or add up your appliances' running watts plus the biggest starting surge.
Motor appliances (pumps, compressors, fans) surge 2–3× at startup — that surge, not the running watts, is what usually trips an undersized generator.
Estimates only. Fuel type, altitude, and temperature derate real output. Have an electrician confirm sizing and install a transfer switch.
How generator sizing works
Every generator decision comes down to two numbers. Running watts is the steady draw of everything switched on at once. Starting watts is the brief surge — two to three times running watts — when a motor appliance like a well pump, furnace fan, or AC compressor kicks in. The generator must cover running watts plus the single largest surge, with about 20% headroom on top.
The whole-house shortcut uses square footage because it correlates with how many circuits and big appliances a home has. It is a range, not a promise: two identical houses can differ by thousands of watts depending on electric vs gas heat, AC size, and well vs municipal water.
recommended W = (Σ running watts + largest starting surge) × 1.2 headroom
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What size generator do I need calculator FAQ
What size generator do I need for a house?
For essentials (fridge, freezer, furnace fan, lights, sump pump, internet), most homes land at 7,500–12,500 watts. A true whole-house setup with central AC and electric dryer usually needs 20,000+ watts. Use the whole-house mode above for a quick range, or the appliance mode for an exact number.
What is the difference between running watts and starting watts?
Running watts is what an appliance draws continuously; starting (surge) watts is the brief 2–3× spike when a motor starts. Size the generator for running watts plus the single largest starting surge — that is the number that trips an undersized unit.
What can a 7,500 watt generator run?
Comfortably: refrigerator, freezer, furnace fan, a dozen LED lights, TV, internet, and a sump pump — roughly 5,000–6,000 running watts with headroom. It will not run central AC and an electric dryer at the same time.
Portable vs standby generator — which should I choose?
Portable generators (3,500–12,500 W) cost far less and roll out when needed, but you refuel and connect them manually. Standby units (10,000–26,000 W) start automatically on natural gas or propane and can carry the whole house — at a much higher installed price.
How much fuel does a home generator use?
A 7,500 W gasoline portable burns roughly 0.75 gallons per hour at half load — about 12–14 hours on a typical tank. Standby units on natural gas use around 1.5–3 therms per hour depending on load.
Do I need a transfer switch for a generator?
Yes, for powering house circuits. A transfer switch (manual or automatic) isolates your home from the grid. Never backfeed through a dryer outlet — it endangers line workers and is illegal in most areas.
Can a portable generator run central air conditioning?
Usually not. A 3-ton central AC draws about 3,800 running watts but surges past 11,000 at startup, so it needs a 14,000+ watt generator. A window AC at 1,200 watts is the realistic portable-generator option.