Quick answer
Choose essential-circuit or whole-home coverage, list the loads that may run together, total their running watts, and add the largest additional motor-starting requirement when only one motor starts at a time. Then verify voltage, phase, fuel derating, generator continuous and surge ratings, and the transfer-equipment load plan. Use actual equipment nameplates whenever possible.
Interactive sizing worksheet
Estimate your generator wattage
Select only the loads you expect to run at the same time. The example wattages are editable planning values; replace them with the nameplate or manufacturer values for your equipment.
Calculation: 275 W running + 1,000 W largest starting addition = 1,275 W estimated peak.
The planning target also checks your selected 20% reserve. It assumes only one motor starts at a time and does not verify voltage, outlet capacity, fuel derating, altitude, transfer equipment, or installation conditions.
Nameplate decoder
Convert volts and amps before adding a custom load
Use input-side voltage, phase, amperage, and a published power factor. Do not enter welding-output amps, breaker size, or a guessed motor multiplier.
This conversion does not provide motor starting kVA, acceptable voltage dip, duty cycle, harmonics, or a generator recommendation. Use manufacturer starting data and add the verified running watts to the load table.
Define what “run the house” means
Essential-circuit coverage may include refrigeration, selected lighting, communications, a furnace blower, sump or well pump, and necessary medical equipment. Comfort coverage may add more receptacles, cooking, laundry, or one air-conditioning system. Whole-home coverage attempts to support a much broader set of circuits but still requires load management in many installations.
Write the outage operating plan before selecting equipment. “Everything” is not a measurable load, and a service-panel rating is not the same as the demand that will occur at one time.
| Coverage level | Inventory approach | Design consequence |
|---|---|---|
| Essential circuits | List critical loads and startup sequence | Smaller target with deliberate load management |
| Essentials plus comfort | Add selected cooking, laundry, or cooling loads | Higher running and startup requirements |
| Whole home | Evaluate service loads and coincident demand | Professional load calculation and transfer strategy |
Calculate running and starting requirements separately
Running watts are the continuous demand of the loads operating together. Starting demand is the brief additional requirement of motors and compressors such as refrigerators, pumps, furnace blowers, and air conditioners. Manufacturer worksheets commonly add total running watts to the largest additional starting load when only one motor starts at a time.
That shortcut is not sufficient when several motors can start together, equipment has unusual controls, or a load must never drop. Use nameplate and manufacturer data, and document which loads are intentionally sequenced.
- List every load that may operate simultaneously
- Replace generic chart values with actual nameplate data
- Identify 240V equipment and motor-starting requirements
- Define which loads will be locked out or sequenced
Check the factors that a watt total misses
The selected generator must provide the voltage and phase required by the backed-up loads. Outlet-level ratings can limit what a portable generator supplies even when the engine’s advertised maximum wattage is higher. Fuel type, altitude, temperature, and maintenance condition can also change available output.
Sensitive electronics, variable-speed HVAC, well pumps, medical devices, and electric vehicle charging deserve manufacturer-specific review. A planning reserve should not be used to hide missing load data.
Coordinate generator size with the transfer and connection plan
Generator capacity does not select the cord, inlet, transfer switch, conductors, breakers, or grounding arrangement. The approved connection system must match the generator output and the circuits it will supply while preventing utility and generator sources from being connected incorrectly.
Portable-generator installations commonly use a compatible power inlet and manual transfer equipment. Permanently installed standby systems commonly coordinate an automatic transfer switch, controls, fuel, and load management. Qualified design and local inspection are required for building wiring.
Use the calculator as an auditable worksheet
CordFind’s generator size calculator keeps running watts and additional starting watts separate. Enter actual loads, change example values to nameplate values, and save the resulting inventory with the planned startup sequence.
Treat the output as a screening target to compare with manufacturer continuous and surge ratings. Whole-home standby selection still requires a site-specific load and installation review.
Frequently asked questions
Common questions
What size generator will run an entire house?
There is no reliable square-footage-only answer. Whole-home sizing depends on simultaneous loads, motor starts, 120/240V equipment, heating and cooling, pumps, fuel, and the transfer/load-management design.
How do I calculate the generator watts I need?
Total the running watts of loads that may operate together, then account for the largest additional starting requirement when only one motor starts at a time. Verify the method against actual equipment data.
Can a portable generator run central air conditioning?
Only when the generator, outlet, connection system, transfer equipment, HVAC starting requirements, and operating plan are all compatible. Central HVAC often requires a professional review.
Should generator size match the home’s main breaker?
Not automatically. The service rating is not the same as actual outage demand. The design depends on which circuits are backed up and how loads are managed.
Primary references
CordFind summarizes connector specifications and public safety guidance. Always follow the instructions supplied with your equipment.