A generator has to carry the continuous load and survive the brief surge when a motor or compressor starts. This calculator adds the running wattages of the appliances you select, then adds the single largest extra starting load and your chosen planning margin.
Generator size calculator
Select the appliances that must run together. Starting wattages are planning estimates; use each appliance nameplate or manual before buying or connecting equipment.
Never run a portable generator indoors or connect it to household wiring without an approved transfer switch installed by a qualified electrician.
How to size a generator for appliances
Start with the wattage label or manual for every appliance that must run at the same time. Refrigerators, pumps, furnace blowers and some air conditioners can need substantially more power for a moment at startup. The preset values above are planning examples, not specifications for your exact equipment.
The method follows the principle in Generac’s generator wattage worksheet: add all running watts, then add the highest single starting-watt requirement. The calculator also shows a headroom-adjusted figure so the generator is not planned at its limit.
What the results mean
- Running load is the combined continuous demand.
- Peak starting load adds the largest single motor-starting difference.
- Suggested running rating and minimum surge rating include the selected planning margin.
Compare both figures with the generator’s continuous and surge ratings. A result is not approval for a particular installation, fuel system, transfer switch or extension cord.
Generator safety
Portable generators create lethal carbon monoxide and must never run inside a home, garage, shed or other enclosed or partly enclosed space. Do not backfeed a home through a receptacle. A qualified electrician should install an approved transfer switch for household circuits. See the U.S. Consumer Product Safety Commission’s portable-generator guidance.
Frequently asked questions
Do I add every appliance’s starting watts?
Normally, the planning calculation adds the largest single extra starting demand to the combined running load because motors are unlikely to start at exactly the same instant. A specialist should assess loads with unusual controls or simultaneous starting.
Where do I find appliance wattage?
Check the nameplate, EnergyGuide information and owner’s manual. If only volts and amps are shown, ask the manufacturer or a qualified electrician for the real running and starting demand rather than assuming a simple multiplication covers motor surge and power factor.