Calculator guide
Estimate generator capacity for a chosen group of electrical loads using continuous running wattage, the largest additional motor-starting surge, and an optional margin. Use actual appliance nameplates and an electrician-approved load plan rather than guessing what a whole home requires.
Formula or method
Estimated peak load (W) = total running watts + largest additional starting surge (W). Recommended capacity (kW) = estimated peak load × (1 + capacity margin/100) ÷ 1000. Enter the extra surge above running watts, not the motor's total starting watts, to avoid double-counting.
How to use this calculator
- List only appliances you plan to power at the same time and add their running watts.
- For motors or compressors, find the largest additional start-up surge above that appliance's running watts; enter it separately.
- Choose a capacity margin, then compare both the peak load and recommended kW with the generator's rated running and starting capacities.
Worked example
If planned appliances use 4,500 W continuously, the largest additional starting surge is 2,500 W, and the margin is 20%, the estimated peak is 7,000 W. The calculator recommends 7,000 × 1.20 = 8,400 W, or 8.4 kW. Check both the generator's continuous and surge ratings before purchase.
Understanding the result
The recommended figure is a planning estimate in kW, not a guarantee that every appliance can run or start. Staggering motor starts can change the actual simultaneous peak, while fuel type and altitude can reduce available output.
Limitations and assumptions
- The tool adds only the largest additional start-up surge; overlapping motor starts or other simultaneous transient loads may require more capacity.
- It does not check voltage, circuit distribution, power factor, transfer-switch sizing, installation codes, fuel supply, or manufacturer derating.
- Never connect a generator to household wiring without an approved transfer switch or interlock installed according to local rules.
Frequently asked questions
Is starting wattage the same as the surge input?
No. If the appliance's starting wattage includes its running wattage, subtract its running watts to find the additional surge used here.
Can I size a generator from the home's square footage?
No. List the actual loads you intend to power; square footage alone does not tell you their combined wattage or starting demand.
Does 8.4 kW mean any 8.4 kW generator will work?
Not necessarily. Compare manufacturer-rated continuous and peak output at your conditions and consult a licensed electrician about the transfer equipment and loads.
Sources
Planning estimate only. Verify equipment ratings, safe installation, and electrical code requirements with a licensed electrician.