What size generator do I need?
Add up what runs at the same time, add the start-up surge of the biggest motor, and leave a margin. Here is the quick way, with a calculator.
Updated 2026-10-16
The short answer
- A house during an outage: 20 kW covers the fridge, freezer, a well pump, lights, internet and a window AC. With central air, 36 kW.
- A small event or a job-site trailer: 20 to 36 kW.
- A film basecamp, a small store or office: 56 kW.
- A restaurant or grocery through a shutoff: 100 kW, sized from the peak demand on the utility bill.
The calculator
Tick what will run at the same time and set how many. It adds the running watts, adds the extra start-up power of the biggest motor, and puts a quarter on top.
Doing it by hand
- List what runs at the same time. Look at the plate on each appliance: it shows watts, or amps and volts (watts = amps × volts).
- Add up the running watts.
- Find the biggest motor — usually the AC or the well pump — and add the difference between its starting and running watts.
- Add 25% and convert to kilowatts (divide by 1,000). Pick the next size up.
For a business, skip the list: the utility bill shows the peak demand in kW for the month. Add 25% and that is the generator.
Single-phase or three-phase
Houses and most small shops are 120/240 V single-phase. Larger commercial buildings are 208 V or 480 V three-phase — the main panel's label says which. Rental units from 20 to 56 kW switch between the two; above that they are three-phase. Your electrician sets the voltage to match.
Questions
Is a bigger generator safer?
Not with diesel. A diesel engine run below about a third of its rating for days "wet-stacks": unburned fuel builds up in the exhaust, and the rental yard bills the clean-out. Aim for the load to sit at half to three-quarters of the rating.
What is the difference between kW and kVA?
kVA is what the generator can deliver; kW is the useful power. For rental diesel units, kW is about 80% of kVA, so a 25 kVA unit is a 20 kW unit. Size by kW.
Why does a motor need more power to start?
An AC compressor or a pump draws two to three times its running power for a second or two while it spins up. The generator has to cover that surge for the biggest motor that starts while everything else is running.
Want the size worked out for you?
Send the list of what has to run and the address. We text you back with the size, one all-in price and a delivery day.