Generator Size Calculator
Create a preliminary portable-generator size estimate from continuous load, largest motor starting demand, reserve capacity and operating hours.
Default project preview
10,000 watts
Peak load before rounding
9,600 W
Apparent power
12.5 kVA
Output current
41.7 A
Motor-start extra
3,000 W
Calculator
Build the material order
Your order will appear here
Enter the project details and calculate a rounded material estimate.
Input and output guide
What each value means
The labels and units below are part of this calculator’s visible methodology. Replace every default with measurements or product data from the actual project.
Total continuous running load
Enter this project assumption in W. The current imperial default is 5000 W.
Largest motor running watts
Enter this project assumption in W. The current imperial default is 1500 W. Minimum: 0.
Motor starting multiplier
Enter this project assumption in ×. The current imperial default is 3 ×. Minimum: 1.
Reserve capacity
Enter this project assumption in %. The current imperial default is 20 %. Minimum: 0. Maximum: 50.
Output voltage
Enter this project assumption in V. The current imperial default is 240 V.
Planning power factor
Enter this project assumption in a unitless value. The current imperial default is 0.8. Minimum: 0.1. Maximum: 1.
Fuel use at rated output
Enter this project assumption in gal/kWh. The current imperial default is 0.12 gal/kWh. Minimum: 0.
Operating hours per day
Enter this project assumption in hours. The current imperial default is 8 hours. Minimum: 0. Maximum: 24.
Fuel price
Enter this project assumption in per gal. The current imperial default is 3.5 per gal. Minimum: 0.
How to read the outputs
Peak load before rounding
running load plus motor start. Default preview: 9,600 W.
Apparent power
power factor 0.8. Default preview: 12.5 kVA.
Output current
at 240 V. Default preview: 41.7 A.
Motor-start extra
3× multiplier. Default preview: 3,000 W.
Daily fuel guide
8 operating hours. Default preview: 9.6 gal.
Daily fuel cost
illustrative rated-output use. Default preview: $33.60.
Methodology
Transparent calculation
BuildMeter converts every input to a consistent internal unit, applies the selected allowance, and rounds products up to whole purchasable units.
Peak watts = continuous load + motor running watts × (starting multiplier − 1). Reserve is applied, then capacity is rounded up to the next 500 watts. kVA = kW ÷ power factor.
Reviewed project guidance
How to use this estimate in a real project
Last reviewed: July 29, 2026
Use this calculator to estimate generator running and starting capacity from selected loads. It helps compare scenarios but does not design transfer equipment, grounding, fuel supply or safe installation.
Calculation steps
- 1List only loads intended to operate during backup.
- 2Separate running watts from the largest likely starting surge.
- 3Consider load sequencing rather than summing every surge simultaneously.
- 4Add a modest margin and compare manufacturer ratings at site conditions.
Worked numerical example
Critical loads totaling 5.5 kW running with a 3 kW motor-starting increment create an 8.5 kW peak scenario. Adding 15% operating margin suggests comparing generators above about 9.8 kW.
Purchasing and rounding
Generators are sold by rated running and surge output, often with reduced performance on some fuels or at altitude. Transfer equipment and inlet ratings must match the selected system.
Interpret the result
A generator can have enough average power yet still fail to start a motor. Conversely, summing every nameplate surge can oversize the unit unnecessarily.
Page-specific assumptions
- Load list is complete.
- Starting data reflect actual equipment.
- Fuel and altitude derating are checked separately.
- Only intended simultaneous loads are included.
Common mistakes
- Using surge rating as continuous output.
- Ignoring well-pump or compressor starting loads.
- Backfeeding without transfer equipment.
- Operating combustion equipment in unsafe locations.
Primary and technical references
BuildMeter provides planning estimates, not engineering, architectural, electrical, mechanical, plumbing, permit or code approval. Review the methodology and verify final decisions with product documentation and qualified professionals.
BuildMeter Guides
Understand the measurements and assumptions
Use these visible reference guides before changing project inputs or planning allowances.
FAQ
Generator Size Calculator questions
Why are starting watts higher than running watts?+
Motors and compressors can draw several times their running power during startup.
Can a generator connect directly to house wiring?+
Only through approved transfer equipment installed according to electrical code. Improper backfeeding can kill utility workers and occupants.
Does this replace an electrician load calculation?+
No. It is a preliminary planning estimate and does not account for every load characteristic or installation requirement.