calcs.homes
calcs.homes
Enter the running watts you need to power at once and the starting surge of your largest motor, and this generator sizing calculator returns the continuous and peak watts and a recommended generator size.
Estimate only — motor surges, altitude, and fuel type all affect a generator's usable output.
Add the running (rated) watts of everything you want on at the same time, then add a safety margin so the generator is not pinned at 100%. On top of that, add the starting surge of the single largest motor — a well pump, air conditioner, or refrigerator compressor briefly draws two to three times its running watts as it spins up. The running total plus that one surge is the peak the generator must handle.
Only the biggest motor's surge is counted because they rarely start at the same instant. Portable generators list both a running and a peak (starting) rating — match the peak figure here to the generator's starting watts, and confirm its running rating covers your continuous load with margin. For a whole-house standby unit, a load calculation or a transfer switch with load management is the proper path.
Running watts is the steady draw of a device. Starting watts is the brief surge — often 2–3× higher — when a motor or compressor spins up. Generators are rated for both.
Essential circuits (fridge, furnace blower, well pump, some lights and outlets) usually fit a 5,000–7,500 watt portable. Running central AC or an electric range too pushes you toward 10,000+ watts or a standby unit.
Running a generator continuously near its maximum shortens its life, wastes fuel, and leaves no headroom for a motor kicking on. Sizing about 20% above your load keeps it in its efficient range.