The generator sizing calculator answers “how many kilowatts running, and how many watts of surge?” Fuel runtime is a different page. This guide walks the calculator fields: appliance list, custom loads, simultaneous starts, power factor, and why recommended kW is larger than running watts.
Running Watts vs Surge Watts
Every motor load has two numbers. Running watts is the continuous draw after the motor is up. Surge (starting) watts is the brief inrush — often 2–3× running on a well pump, compressor, or central AC. The calculator sums running watts across everything you selected, then takes the peak surge if several motors can start together.
Minimum size (kW) is running watts plus headroom, converted to kilowatts. Recommended size rounds up to the next common generator rating so you are not buying a 6.1 kW unicorn. Nameplate “peak” marketing watts are not continuous kW — use the running/surge split, not the carton.
Size capacity here. Then estimate tank hours with the generator fuel runtime guide and the fuel runtime calculator. Transfer equipment is separate — see the transfer switch guide.
How to Fill the Appliance List
Check the loads you will actually run on backup — not every receptacle in the house. Fridge, furnace blower or boiler circulator, well, sump, lights, networking, and maybe one window AC is a typical essential list. Adding the range, dryer, and two ACs at once is how a 7 kW portable becomes a 22 kW standby on paper.
- Use the catalog watts as a start — then override from nameplates when you have them.
- Custom loads — name, running W, surge W. A 240 V well that says 10 A running is about 2,400 W, not 1,200 W.
- Do not double-count — furnace already includes the blower; do not add a second “air handler” row unless it is a separate unit.
Fridge 200 W / 600 surge, furnace 800 / 1,600, well 1,000 / 3,000, lights 400, router 50. Running ≈ 2,450 W. Peak surge ≈ 3,000 W if the well is the largest start and others are already running. Recommended size steps up from ~3 kW plus headroom — usually a 4–5 kW class inverter or a 7 kW open frame, depending on what else you left off.
Simultaneous Motor Starts
The advanced toggle’s simultaneous starts control is the difference between “one pump at a time” and “well + AC compressor both kick when the thermostat and pressure switch line up.” If you leave it at one start, surge is the largest single motor. If you pick two or more, the calculator stacks those surges.
Interlock panels that dump the whole house onto a small generator make multi-start likely. A load-management or essential-only subpanel makes single-start realistic. Match the dropdown to how you will actually switch loads — not to a wish.
If the set stalls on motor start, that is inrush vs remaining headroom. Diagnose on generator sizing troubleshooting — do not only bump the recommended kW without checking simultaneous starts.
Power Factor and kVA
Fuel and engine size track kW (real power). Many standby nameplates also list kVA. The calculator’s PF field (default ~0.8) converts: kVA ≈ kW ÷ PF. A 10 kW / 12.5 kVA set at 0.8 PF is the same machine described two ways. If you only have kVA, convert before you compare to the recommended kW.
Use the kW / kVA / amps calculator when you are reading a panel in amps and need watts for the appliance list.
Fuel Tradeoff (Optional)
Tank size on this calculator is a side comparison — which fuel lasts longest at the load you just sized — not a substitute for the dedicated runtime tool. Gasoline, propane, diesel, and natural gas burn at different gal/kWh. For outage planning hours, switch to the fuel runtime calculator with a real load percent.
Size running and surge watts Add loads, set simultaneous starts, get recommended kW.
Frequently Asked Questions
Why is recommended kW bigger than running watts / 1000?
Headroom for surge, power factor, and the next standard frame size. A 4,800 W running total is not a 4.8 kW purchase — you still need start margin and a size you can actually buy.
Can I use peak carton watts as running?
No. Peak/surge on the box is the short inrush the set might survive, not continuous output. Enter running and surge separately.
Does this replace a load calculation for a standby ATS?
It is a load list for generator kW. Service/feeder demand is still NEC 220.82 or the feeder/service calculator depending on dwelling vs feeder.
Where do I go if the sized generator still sags?
Generator sizing troubleshooting — missed surge, stacked starts, starting vs running mix-up.
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