Find the generator wattage needed
Check running and starting watts, using one largest extra start demand when motors start one at a time.
Back to calculator ↑List simultaneous loads
Sum running watts for everything used together. Find the largest difference between one appliance’s starting and running watts.
List what will run in the same outage scenario, not every appliance in the house. If a label provides amps and voltage instead of watts, confirm how to interpret it for that device, especially motors and loads with power factor. Actual demand can differ from marketing or maximum figures.
Read both generator ratings
Generators have running and short-duration starting ratings. Compare each load with its matching rating. Do not sum every motor surge if they start sequentially.
Additional starting watts are the excess above one appliance’s running power. If it runs at 500 W and starts at 1,700 W, enter 1,200 W extra. This assumes other loads are already running and this appliance starts separately; adding the full 1,700 W again would double-count its 500 W running load.
Worked default example
Running load 1,800 W plus the largest extra start demand 1,200 W gives 3,000 W starting estimate. Sizing mode shows at least 1,800 W running and 3,000 W starting requirement. If you have a candidate generator, switch to comparison mode and enter both nameplate ratings.
Even when both running and starting totals fit, an individual outlet may have a lower limit than the whole generator. Check outlet ratings too.
Check limitations before use
This comparison does not check voltage, phase, surge duration or power quality. Run generators outdoors per the manual and use a qualified electrician for home connections.
If several motors start together or equipment is sensitive to power quality, have a professional assess the actual load, voltage, phase and frequency. Never feed generator power backward through home wiring. Operate the engine outdoors away from openings as its manual directs to prevent carbon monoxide exposure.
A negative running or starting headroom means the entered load exceeds that corresponding rating; a positive other headroom does not make the comparison pass. Remove simultaneous loads or consult the manufacturer about real capacity and startup sequence. This arithmetic does not derate for altitude, temperature or machine condition.
Common questions
Should I add every appliance’s starting watts?
This estimate uses one largest extra start demand for sequential starts; simultaneous starts need another assessment.
Does passing mean it is safe to connect?
No. Voltage, phase, protection and connection need checking.
Can I run it in a garage with the door open?
No. Operate outdoors following manufacturer warnings.
Sources and calculation guidance
Check these against your chosen product’s label and instructions.