Home Generator Size Calculator
Estimate separate continuous and starting/surge generator requirements after listing the loads that may run together and the largest additional motor-starting demand.
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View all saved project estimatesWhat does the generator size calculator calculate?
Add the running watts of loads that may operate together, enter only the largest additional starting demand, and choose a reserve. The calculator reports separate continuous and starting/surge wattage requirements. Confirm both against manufacturer ratings because it does not verify appliance behavior, fuel performance, transfer equipment, installation, or code compliance.
Continuous and starting/surge generator requirements
The continuous requirement applies the reserve to loads expected to operate together. The starting/surge requirement adds the largest single motor demand above its already-counted running watts, then applies the same planning margin. A candidate generator must satisfy both distinct ratings; these estimates do not establish compatibility, connection safety, fuel performance, or compliance.
How to measure for an accurate estimate
- Create an essential-load list and record operating watts from current labels, manuals, or measured data for each specific appliance.
- Decide which loads may run simultaneously, then add only those running watts instead of totaling every appliance in the building.
- Find the largest starting watts among motor loads and subtract that appliance’s running watts to obtain its additional starting demand.
- Calculate alternative load schedules when high-demand equipment can be operated separately during an outage.
Essential outage loads with one motor surge
A homeowner totals 3,500 running watts for loads expected to operate together. The largest motor needs 1,800 additional watts while starting, and a 20% planning reserve is selected.
The continuous requirement is 4,200 watts after reserve. Starting peak is 5,300 watts before reserve, and the required starting/surge rating is 6,360 watts, or 6.36 kW.Before you purchase material
- Compare continuous rated watts and surge capability under the intended fuel; do not rely only on a model name or maximum headline wattage.
- Review manufacturer limits for temperature, altitude, fuel consumption, runtime, duty cycle, noise, maintenance, and sensitive electronics.
- Use qualified professionals to evaluate listed equipment, approved transfer arrangements, placement, permits, and property-specific requirements.
- Follow CPSC carbon-monoxide guidance: generators must remain outdoors and far from doors, windows, vents, and other building openings.
Common questions
What is the difference between running and starting watts?
Running watts are the power equipment uses during normal operation. Motors and compressors may briefly need higher power when starting. Because an appliance’s running watts are already in the simultaneous total, enter only its extra starting demand, not its full starting rating again.
Should I add the starting watts of every appliance?
Usually the planning shortcut uses the largest single additional starting demand because multiple motors are not assumed to start at exactly the same moment. If controls can start several loads together, a qualified professional or manufacturer should evaluate that sequence rather than relying on this shortcut.
Why include a reserve in the generator estimate?
A reserve allows for uncertain labels, load changes, and small additions, but it is not a safety certification or substitute for product limits. Too little margin can cause overloads; excessive oversizing can increase cost, fuel use, and maintenance concerns depending on generator type.
Does this calculator tell me how to connect a generator?
No. It provides a preliminary wattage estimate only and intentionally gives no connection or wiring procedure. Incorrect generator use can cause electrocution, fire, utility backfeed, or carbon-monoxide poisoning. Follow CPSC and manufacturer instructions and use qualified professionals for property-specific selection and installation.
Sources and reference standards
- U.S. Consumer Product Safety Commission — Generators and Engine-Driven ToolsFederal safety guidance on carbon-monoxide, fire, shock, placement, fuel, and professional installation hazards.
- Ready.gov — Power OutagesFederal emergency-preparedness guidance for outage planning, generator safety, medical needs, food, communications, and carbon monoxide.
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