Quick answer
Add the running watts for every load that may operate simultaneously, then add the largest extra starting requirement if only one motor starts at a time. Compare both the required running capacity and peak requirement with the generator’s published ratings. Use actual nameplate values 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.
Running watts and starting watts are different ratings
Running watts describe the continuous power a generator can supply. Starting watts describe a short-duration higher output used when some motors, pumps, compressors, and air conditioners start.
The calculator treats “extra starting watts” as the amount above the device’s normal running load. This makes the worksheet auditable: simultaneous running total plus the largest single starting addition, assuming only one motor starts at a time.
Replace example values with equipment data
Published appliance charts are estimates. Two refrigerators, pumps, or air conditioners with similar descriptions can have different electrical requirements. Read the equipment nameplate and manual for watts, volts, running amps, starting amps, or locked-rotor information.
When only volts and running amps are provided, volts multiplied by amps gives apparent wattage for a basic planning check. Motor starting behavior and power factor can require additional manufacturer information.
Interpret the planning target conservatively
The result is a screening value, not a promise that a particular generator will start every load. Compare the running total with the generator’s continuous rating and the estimated peak with its starting or surge rating.
Fuel choice, altitude, temperature, extension-cord voltage drop, generator outlet limits, load sequencing, and sensitive electronic equipment can change the practical result. Review the generator manual and obtain qualified help for building connections or large motor loads.
Generator size does not determine the house connection
Wattage alone cannot select a cord, inlet, transfer switch, or conductor. After estimating generator capacity, identify the actual generator receptacle and the approved connection equipment for the intended loads.
Never connect a generator to a household outlet. Building wiring requires approved transfer equipment and a suitable inlet or other manufacturer-specified connection installed for the electrical system.
Frequently asked questions
Common questions
How do I calculate what size generator I need?
Total the running watts of loads that may operate together, then account for the largest additional starting demand. Compare both figures with the generator’s continuous and starting ratings.
Should I add every appliance’s starting watts together?
Not when the operating plan ensures only one motor starts at a time. If multiple motors can start together, the simple largest-single-start assumption is not sufficient and the system needs a more conservative review.
Can this calculator size a whole-house standby generator?
It provides an initial load estimate. Whole-house generator and transfer-equipment selection also depends on service configuration, large motors, fuel, local requirements, and professional installation design.
Primary references
CordFind summarizes connector specifications and public safety guidance. Always follow the instructions supplied with your equipment.