About this tool
Generator Sizing Calculator is a free, in-browser tool that sizes a portable or standby generator from the appliances you plan to run at once. Tick the loads and it adds their running watts and the single largest starting surge, then adds a 20% headroom margin to recommend a generator rating.
The required wattage is the sum of every selected appliance's running watts plus the largest starting (surge) draw among them — only one motor is assumed to start at a time, so just the biggest surge is added. The recommended size is required × 1.2, giving a safety buffer so the generator is not run continuously at its limit. Each appliance's typical running wattage is shown on its row.
Use it before buying a generator or an inverter for backup power, an RV or a job site: select the fridge, furnace fan, well pump, lights and tools you need simultaneously, and read off the minimum required and recommended watts. Everything is computed locally in your browser.
Frequently asked questions
Why add only the largest starting surge?
Motors draw a brief spike of extra current to start. In practice appliances rarely start at the exact same instant, so the standard method adds every device's running watts but only the single biggest surge — the worst realistic moment.
What is the difference between running and starting watts?
Running (rated) watts is the steady draw once a device is going. Starting (surge) watts is the short spike when a motor or compressor kicks on — often two to three times the running figure for fridges, pumps and air conditioners.
Why the 20% recommended margin?
Running a generator continuously at 100% of its rating shortens its life, leaves no room for an extra load, and struggles as altitude and temperature derate output. A 20% buffer keeps it working comfortably below its limit.
Are the appliance wattages exact for my devices?
They are typical values to get you in the right range. Your own appliances may differ — check their nameplate or manual for exact figures and adjust your expectations accordingly. All calculation happens in your browser.
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