Generator Sizing Calculator — Running & Starting Watts
Size a generator from your appliance nameplate watts. Compare steady load, controlled motor starts and overlapping starts against both generator ratings.
Will your generator carry the loads when a motor starts?
List the appliances you actually plan to run. Compare continuous watts with two start cases, then check both ratings printed on your generator.
The example generator has enough running watts and can cover the largest single start. Its 3,200 W peak rating is 20 W below the all-start stress case. Only rely on the controlled case when starts can truly be sequenced.
Portable generator? Run it outdoors at least 20 ft from the home, exhaust away. CPSC safety guidance
What runs during the outage?
Use watts from each appliance's label or manual. “Starting W” means its total watts during startup, not extra watts to add again.
These are labelled example watts, not appliance defaults. Replace them with the ratings for your equipment.
Compare an actual generator
Enter both ratings, or leave both empty to see the load targets alone. A reserve applies only to the continuous running target.
A running rating and a peak rating are separate limits. Surge duration, fuel, altitude and installation can change the usable output; use the manufacturer's instructions.
How the two start cases work
Steady: sum running watts × quantity. Controlled: add the largest single appliance's extra starting watts (starting minus running) to the steady total. This assumes you can prevent motor starts from overlapping. Concurrent stress: add every motor's extra starting watts, including quantity. Both start cases must still fit the generator's peak rating, and steady load plus your optional headroom must fit its running rating. The result is a planning comparison, not an installation approval.
Generac portable-generator manual describes largest-motor starting and says to use appliance data labels. CPSC guidance: run a portable generator outdoors at least 20 feet from the home with exhaust directed away. Ask a qualified electrician about connecting to building wiring.
