Generator Wattage Calculator
Add up running watts and starting watts for the appliances you want to back up and size a generator.
Last updated · View algorithm, constants and default inputs (JSON)
About this calculator
Add the appliances’ running load and the largest additional starting surge as a basic estimate; more than one motor may start at once, so check simultaneous loads and each appliance’s nameplate. This calculator is not an installation design. CPSC says portable generators must run outdoors at least 20 feet from the home, with exhaust directed away; never use one indoors or in a garage.
Worked example
calculated with the default values below
Total running watts needed 3,000 W · Running watts plus additional starting surge 2,200 W · Safety margin 20 % Recommended generator size 6.24 kW · Peak surge watts 5,200 W · Model starting-capacity estimate 6,240 W
General background references
Disclaimer
These calculators provide planning estimates only. Building codes, wiring regulations and safety requirements differ by country and change over time, and nothing here replaces a licensed professional or a permit. For structural, electrical or gas work, follow local code and have the installation inspected.
Implementation source
This record describes the implementation used by this page. It is not independent validation. External references, when available, support only the stated formula or definition; estimates and custom rules retain their own assumptions.
Formula and variables
W_surge = W_run + W_start; W_rec = W_surge x (1 + r/100); kW = W_rec/1000
- W_run
- total running watts
- W_start
- largest motor starting surge
- r
- reserve headroom %
- W_surge
- surge wattage
- W_rec
- recommended generator watts
- kW
- recommended kilowatts
Only the single largest motor-starting surge is added; r is extra headroom as a percent of surge watts to cover overload and future loads.
Frequently asked questions
Why add the largest motor surge instead of all of them?
The model adds the largest additional starting surge to running watts and assumes no other motor starts at the same moment. Check the starting watts of every load that may overlap and the generator manual. Correct wattage does not remove carbon-monoxide or electrical hazards; CPSC says portable generators must be used outdoors, at least 20 feet from the home, with exhaust directed away.