How to use the generator size (wattage) calculator
- List each item you want to power: its name, running watts, extra starting watts and how many you have. The rows are examples to replace with your own.
- Set a safety margin and the voltage used for the amps estimate.
- Read the total running watts, the peak when the biggest starting load kicks in, and both figures with the margin added.
- Use the table to double check each line, then copy or print the result.
Formula
Running watts = sum of (running watts each x quantity)
Peak watts = running watts + the largest single extra starting load
Size to look for = running watts and peak watts, each x (1 + margin %)
Amps = watts divided by volts. The calculator adds only the largest single starting load to the running total, because motors rarely all start at the same instant.
A short outage kit
Using the example rows: a refrigerator at 700 running watts with 1,400 extra to start, a furnace fan at 500 with 1,000 extra, four lamps at 60 each and a charger group at 40. The running total is 700 + 500 + 240 + 40 = 1,480 W. The largest starting load is the refrigerator at 1,400 W, so the peak is 2,880 W.
With a 20 percent margin, the running need is 1,776 W (1.78 kW) and the starting need is 3,456 W (3.46 kW). The running amps at 120 V are 1,776 / 120 = 14.8 A. These numbers are only as good as the watt figures you enter.
Running watts and starting watts
Appliances with motors or compressors, such as a refrigerator, a pump or an air conditioner, draw much more power for a second or two when they start than when they run. A generator has to supply both. The running rating of a generator is what it can deliver continuously, and the starting or surge rating is a higher number it can deliver briefly. Pick one whose running rating covers your running total and whose surge rating covers the peak.
Lights, chargers and heaters have little or no extra start load, so enter 0. Take the numbers from the label or manual of each appliance. If a label gives amps and volts, watts are about amps times volts. The example rows in the form are only examples and are not typical for your equipment.
The margin
A generator that runs at its limit all the time is noisy, wears out sooner and may shut down. A margin leaves room for appliances you forgot, for the true draw being higher than the label and for later additions. The 20 percent in the form is an example value. Use a larger margin if you are unsure of your numbers, and check the generator's own documentation, because altitude, heat and fuel type reduce what it can deliver.
To estimate what an hour of use costs, try the electricity cost calculator. To check a circuit, use Ohm's law, and to see how long a battery bank would last use the battery life calculator.
Safety
Never run a generator indoors or in a garage, and keep it well away from windows, because the exhaust contains carbon monoxide. Have a qualified electrician install any connection to a house panel, with a transfer switch that stops power from flowing back to the grid. This calculator gives a size estimate only. It does not replace advice from an electrician or the generator maker.
Assumptions and limits
- The watts you enter are used as given. The example rows are examples and not figures for your appliances.
- Only the largest single starting load is added to the running total, which assumes motors start one at a time.
- The margin of 20 percent is an example value you can change.
- Power factor, altitude, fuel type, long cords and the generator's own surge rating are not modeled.
Frequently asked questions
What size generator do I need for my house?
Add the running watts of everything you want on at once, add the largest starting load to find the peak, and then add a margin. Choose a generator whose running and starting ratings both cover those numbers.
What is the difference between running watts and starting watts?
Running watts are what an appliance uses continuously. Starting watts are the extra a motor or compressor needs for a moment as it starts. A generator has to supply both.
Why do you add only the largest starting load?
Motors rarely start at the same instant, so the generator only has to cover the running load plus one starting surge at a time. If you start several at once, add their surges too.
Can I run an air conditioner on a portable generator?
It depends on the unit. Air conditioners can have a large starting load, so enter the starting watts from its label and see whether the peak fits the generator.