Whole-House Generator Size Calculator
This generator size calculator estimates demand from simultaneous running loads and one additional motor-start event. Replace the example values with your equipment data, adjust the planning margin, and download a worksheet for your installer.
1. Add the loads you want backed up
Use nameplate or measured wattage where possible. For motor loads, enter the extra starting watts above normal running watts.
2. Add sizing headroom
TRANSPARENT BY DESIGN
The formula behind your estimate
Divide watts by 1,000 to get kilowatts (kW). We show the continuous running estimate separately from the estimate that includes a starting event. The combined figure is a planning allowance—not a verified generator model recommendation.
Worked example
For 6,400 W running, a 3,000 W additional start event and 15% headroom: (6,400 + 3,000) × 1.15 = 10,810 W, or 10.81 kW. Running load with the same allowance is 7.36 kW. Displayed values are rounded to one decimal; the worksheet retains watts.
Use the result to compare specifications
Check continuous output, motor-start capability and fuel-specific ratings separately. Also verify voltage, phase, circuit or outlet limits, load management and installation requirements. A headline wattage alone cannot establish system suitability.
How to use the generator size calculator correctly
Start with the loads you actually want available during an outage. If you plan to use load management, do not automatically add every large appliance in the house as though it will operate simultaneously.
Running watts
Running watts are the power an appliance or system uses once operating. Use manufacturer nameplate data, equipment documentation, or measured demand where possible.
Starting watts
Motors can require substantially more power for a short period when starting. Air conditioners, well pumps, sump pumps, and compressors are common examples. Enter the additional starting watts above the device’s running demand.
Why we use the largest starting event
This calculator assumes only one motor starts at a time while the other listed loads are running. Simultaneous starts, compressor restart behavior and automatic restoration after an outage can require a different calculation. The tool does not model voltage dip, power factor or startup duration.
Headroom
Headroom provides room for uncertainty, smaller unlisted loads, and future changes. The default 15% is an adjustable editorial assumption, not a code requirement or manufacturer-prescribed margin. More headroom is not automatically better: unnecessary oversizing can raise equipment cost, fuel demand, and installation requirements.
Natural gas vs propane can change the answer
Many residential standby generators produce different power on natural gas and propane. A unit sold under a 26 kW headline may produce less than 26 kW on natural gas; other models retain equal output on both fuels. Compare the required kW figure with the exact fuel-specific output in the manufacturer’s current specification sheet.
Load management and soft starts
Load management can prevent selected high-demand circuits from operating simultaneously when generator capacity is limited. A correctly selected soft-start device can also reduce the starting demand of some air-conditioning compressors. Use verified post-installation data rather than guessing at a lower surge value.
Generator sizing questions
Are starting watts and extra starting watts the same?
No. If a device uses 700 W while running and 1,900 W total at startup, enter 700 running watts and 1,200 extra starting watts. If the source already says “additional starting watts,” use that figure directly.
What size generator do I need for a 2,000-square-foot house?
Floor area alone does not give a reliable answer. Enter the actual loads you want backed up, including heating, cooling and pumps. Homes of the same size can have very different electrical needs.
Will this tell me which generator to buy?
It gives you a load worksheet to compare against equipment specifications. It does not verify a particular generator’s transient performance, fuel supply, transfer equipment or installation. Have a qualified professional confirm the system.
Can I use this to size a home battery?
This calculator estimates power in kW. Battery runtime also needs stored energy in kWh, conversion losses and a charging plan. It does not calculate battery capacity or outage runtime.
Sources and methodology
Manufacturer resources support the distinction between continuous power and starting demand. The example loads and headroom are illustrative inputs, not universal appliance ratings. The formula, assumptions and limitations above describe this tool’s implementation.
- Honda: determining generator power needs and managing startup loads
- Generac: professional standby sizing and motor-start inputs
- Generac: rated power and individual outlet limits
Methodology version 1.1 · Reviewed by Backup Generator Guide. Report a calculation issue or suggest a correction.
Link to this calculator
Writing a homeowner guide or preparing an installation resource? Link readers to this free calculator so they can build their own load worksheet. Use the permanent address: backupgeneratorguide.com/tools/generator-size-calculator/.
Next steps
Once you have an estimated required capacity, compare the exact natural-gas or propane output, automatic transfer-switch requirements, load-management strategy, fuel infrastructure, and complete installed cost. Then find a qualified installer to confirm the design.