Generator Size Calculator
Running and starting watts of your appliances and the generator size you need.
About this tool
Pick appliances from the list, which fills in typical running and starting watts, or choose Custom and type the figures from the label. Set quantities and add or remove rows. The calculator shows total running watts, the largest start-up surge, the peak you need and a recommended generator size rounded up with your chosen headroom.
How to size a generator
Generators are rated in running (continuous) watts and starting (surge) watts. To size one, first add up the running watts of everything you want to power at the same time. Then add the extra starting surge of the single largest motor-driven appliance: fridges, freezers, pumps, furnace fans, air conditioners and compressors draw two to three times their running power for a few seconds when the motor starts, but they rarely all start at the same moment. That gives the peak the generator must handle. For a typical home backup of a refrigerator, sump pump, furnace fan, LED lighting, TV and router, that's about 3,080 W running and 4,630 W peak; adding 20% headroom suggests a 6,000 W generator. Headroom keeps the generator out of overload, allows for estimates being low and lets you add a small appliance later. Resistive loads such as kettles, heaters and toasters have no surge but draw a lot of power continuously, so you may need to take turns using them. Honest limits: preset figures are typical and real appliances vary widely, so check labels or manuals. Never run a generator indoors, in a garage or near windows, because of carbon monoxide, and connect it to household wiring only through a transfer switch fitted by a qualified electrician. The electricity cost calculator, the battery life calculator and the BTU calculator are related.
Frequently asked questions
What size generator do I need to run a house?
Essentials such as a fridge, sump pump, furnace fan, lights and electronics typically need 5,000โ7,500 W; whole-house backup with air conditioning needs much more.
What is the difference between running and starting watts?
Running watts are needed continuously; starting watts are the brief extra surge when a motor starts.
Can I plug a generator into my house wiring?
Only through a properly installed transfer switch or interlock. Back-feeding is dangerous and often illegal.
Is my data saved?
No. The calculation runs in your browser and nothing is stored or sent.