Generator Size Calculator
Size a backup or standby generator from your total connected load in kW, including motor starting allowance.
Common conversions
| Input | Result |
|---|---|
| Small portable (camping, tailgating) | 2,000-3,500 W |
| Job site or power tools | 5,000-7,500 W |
| Home backup, essential circuits only | 5,000-10,000 kW |
| Whole-house standby, average home | 14-22 kW |
| Large home with AC and well pump | 22-26 kW |
| RV travel trailer, single AC | 3,000-3,600 W |
| RV Class A motorhome, dual AC | 5,500-7,500 W |
| Small commercial building | 30-60 kW |
| Mid-size construction site | 60-125 kW |
| Hospital or data center backup | 150 kW and up |
The math behind it
- kVA = (12 + 6) / 0.8
- kVA = 18 / 0.8
- kVA = 22.5
Everything you need to know
Every generator sizing job boils down to the same two questions: how much power do you need running at once, and how much extra can the largest motor demand for the split second it starts. Get those two numbers right and the rest is picking a standard model size. This page walks through the general method; if you already know which category you fall into, the whole-house, portable, and RV pages below go deeper on the numbers specific to that use.
The two-step sizing method
Start by adding up the running kW of everything you expect to use at the same time. That is your baseline load. Then find the single largest motor in the group, usually an air conditioner compressor, well pump, or air compressor, and add its starting surge on top. Motors typically draw 3 to 6 times their running current for a fraction of a second at startup, and a generator that cannot cover that spike will sag in voltage or trip offline even though it handles the steady-state load fine. Divide the combined total by the generator's power factor, commonly 0.8 for portable and standby units, to convert kW into the kVA rating printed on the spec sheet.
Match your generator type to your use case
A whole-house standby unit for an average US home typically lands between 14 and 22 kW and ties into an automatic transfer switch. A portable generator for storm backup or job-site tools usually runs 2,000 to 10,000 watts and is sized appliance by appliance. An RV generator has to match either a 30-amp (3.6 kW) or 50-amp (12 kW) shore power service and the starting surge of the coach's roof air conditioner. Each of those scenarios has its own worksheet with realistic numbers on the dedicated pages linked below, so use this page for the overall math and then confirm details on the page that matches your project.
Continuous vs. standby duty ratings
Nameplate kW is not one number. Gasoline and propane standby units publish a standby rating meant for outage duty only, not continuous full-time operation. Diesel gensets publish both a prime rating (unlimited hours, variable load) and a standby rating (limited hours, for emergency use, per ISO 8528). Always size to the rating that matches how you plan to run the unit, and check the manufacturer's derate tables for altitude and ambient temperature before finalizing a model.
Common applications
Add HVAC, water heater, range, and well pump kW, then add the largest motor's starting kVA spike, usually the AC compressor. Most homes land in the 14-22 kW range.
Sum welder, mixer, and tool kW at simultaneous use, then add 25% headroom for inrush from the largest motor on site.
Audio, lighting, and HVAC must be summed at peak draw. Tents and outdoor events also need fuel sized for the full run-time of the event, not just the load.
Combine well pump, refrigeration, and power tool loads, then size the generator (or generator plus battery bank) to the surge of whichever motor starts last in the sequence.
Common mistakes
Compressor and pump motors draw 3 to 7 times their full-load amps at start. Without a surge allowance, the generator's voltage sag will trip sensitive electronics or stall the engine.
Even a soft-start air conditioner adds 30 to 50% above running kW for a few seconds. Skipping that allowance causes nuisance shutdowns the first time the compressor kicks on.
A generator rated for sea level and 77°F can lose 10% or more of its output at elevation or in summer heat, turning a tight-fit size into an undersized one.
Two generators both labeled 20 can differ by 4 kW of usable power if one is rated in kVA at 0.8 PF and the other in kW. Always convert to the same unit before comparing.
Frequently asked questions
How do I calculate generator size in kW?+
Add up the running watts of every device you'll use at the same time, convert to kW, then add the starting surge of your single largest motor. That combined number, divided by the generator's power factor, gives you the kVA rating to shop for.
Should I size a generator by kW or kVA?+
Size by kVA when comparing generator nameplates, since that's how manufacturers rate output, then check that the kW rating at your load's power factor still covers your running watts.
Is panel amperage a reliable way to size a generator?+
No, panel amperage is only a wiring limit, not your actual load. Add up what you'll actually run at once, often 8 to 15 kW for a house, instead of sizing to the full panel rating, which would produce an oversized and overpriced unit.
What percentage headroom should I add for motor starting?+
Add 20 to 30% above your calculated running-plus-surge total as a safety margin. This covers nameplate tolerances, voltage sag under load, and future appliances you might add.
Is it safe to run a generator at 100% of its rated capacity continuously?+
No, standby and prime ratings assume headroom for surges and cooling. Keep continuous load at 50 to 80% of the rated capacity and reserve the top 20% for brief starting spikes only.
What's the difference between a generator's kW and kVA rating?+
kW is real power that does work, kVA is apparent power including the reactive component. They're equal only at power factor 1.0; at a typical 0.8 PF, a 20 kVA generator delivers 16 kW of usable real power.
How does altitude affect generator sizing?+
Naturally aspirated engines lose roughly 3% of output for every 1,000 feet above 500 feet elevation. A generator sized tight at sea level can fall short at 6,000 feet, so buy one size up if you're in the mountains.
Do I need a licensed electrician for a permanent generator installation?+
Yes, standby units need an automatic transfer switch, a dedicated fuel line, and code-compliant clearances from windows and doors. Most jurisdictions require a permit and inspection for anything beyond a plug-in portable.
What's the difference between a portable, standby, and RV generator?+
Portable units are gasoline or propane, moved by hand, and sized in watts. Standby units are permanently installed, run on natural gas, propane, or diesel, and sized in kW. RV generators are built into or towed behind the coach and matched to its shore power amperage.
How much bigger should my generator be than my calculated load?+
Round your calculated kVA up to the next standard model size, which usually adds 10 to 20% of headroom on its own. Avoid buying more than one size up, since diesel and gas engines run less efficiently and can wet-stack when consistently underloaded.