Generator Size Calculator
Size a portable or standby generator in watts and kW from home size, HVAC, well pump, range, and dryer. Starting vs running watts.
How to use this generator size calculator
Motors need extra watts for a second when they start. A generator that covers running watts but not starting watts will stall when the AC or well kicks on. Whole-home standby units are sized in kW; portable units in watts (1 kW = 1,000 W).
Do not backfeed a panel with a suicide cord. Use a transfer switch or interlock. Sensitive electronics prefer inverter generators. This generator size calculator assumes you are not starting every motor at once except the worst single start on top of other running loads.
Formula
Worked example
Essentials in an 1,800 sq ft house with a 3-ton AC and a fridge often wants a 10–12 kW class machine; whole-home with range and dryer pushes 20 kW+.
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Frequently asked questions
What size generator for a house?
Essentials: often 5–12 kW. Whole home with central AC: often 16–24 kW. It depends on the AC tonnage.
Starting vs running watts?
Running is continuous. Starting (surge) can be 2–3× for a compressor or pump for a moment.
Can I run the AC?
Yes if the generator’s surge covers the compressor. Soft-start kits on the AC can shrink the generator you need.
Portable or standby?
Portable for outages you can babysit. Standby with auto-transfer if you travel or have medical devices.
Will 7,500 watts run a house?
It can run lights, fridge, and a window unit. It will not run a 4-ton central AC and a dryer together.
What size generator do I need for a house?
Essentials often 5–12 kW. Whole home with central AC is often 16–24 kW. Starting watts for the AC usually set the size.
Can a 7500-watt generator run a house?
It can cover lights, fridge, and some outlets. It will not start a large central AC plus a dryer at the same time.
Results are estimates for informational purposes and are not financial, legal, or professional advice. Last reviewed 2026-08-20.