OneTaskBy Tim King · Updated
Generator Size Calculator
Running and Starting Watts for Your Home
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How the Generator Size Calculator works
This generator size calculator adds up the loads you want to keep running during a power cut and tells you the generator rating you need. Enter how many of each appliance you will run, plus any other loads, and it returns the total running watts, the peak starting watts and a recommended generator size with headroom.
Motors need a burst of power to start. A fridge that runs on 200 watts can pull 1,200 watts for a second or two as its compressor kicks in, and a well pump can pull two to three times its running load. A generator has to cover everything that is already running plus the biggest single start at the same moment, or it will stall or trip.
How the generator size calculator works
The formula is peak watts = total running watts + the largest extra starting surge of any one motor. For example, a fridge (200 W running, 1,200 W starting), a chest freezer (150 W) and a furnace fan (800 W, 2,350 W starting) run on 1,150 W, and the peak is 1,150 + (2,350 − 800) = 2,700 W. The recommended size adds about 20% headroom, so a 3,300–3,500 W generator. A second example: adding a 1 hp well pump raises the running load to 2,150 W and the peak to 3,650 W.
Only the largest start counts because motors rarely start at the same instant; if you are worried they will, stagger switching them on. Appliance labels show the real figures, and a plug-in power meter measures running watts exactly, so use those when you have them.
To see how many hours a generator will run, check the fuel use at 50% load in its manual; most portable units run 8–12 hours on a tank at half load. Inverter generators give cleaner power for electronics. Never run a generator indoors or in a garage.
Typical running and starting watts for household loads
| Load | Running watts | Extra starting watts |
|---|---|---|
| Fridge | 200 | 1,200 |
| Chest freezer | 150 | 900 |
| Sump pump (1/2 hp) | 1,050 | 2,150 |
| Well pump (1 hp) | 1,000 | 2,500 |
| Furnace fan (1/2 hp) | 800 | 2,350 |
| Window air-con (10,000 BTU) | 1,200 | 3,600 |
| Central air-con (3 ton) | 3,500 | 9,000 |
| Kettle | 2,000 | 0 |
| Microwave | 1,000 | 0 |
| Toaster | 1,000 | 0 |
| Coffee maker | 900 | 0 |
| Electric oven | 2,400 | 0 |
| Space heater | 1,500 | 0 |
| Washing machine | 500 | 1,500 |
| Tumble dryer (electric) | 3,000 | 3,800 |
| Dishwasher | 1,500 | 1,500 |
| LED lights (per 10) | 100 | 0 |
| TV and router | 200 | 0 |
| Laptop | 60 | 0 |
| Circular saw | 1,400 | 2,300 |
| Hair dryer | 1,500 | 0 |
| Electric vehicle charger (slow) | 2,300 | 0 |
FAQ
What size generator do I need to run a house?
A portable generator of 3,000–5,000 watts runs the essentials: fridge, freezer, furnace fan, lights, a sump pump and phone chargers. Running central air-conditioning, an electric water heater or an electric range needs a standby generator of 10–20 kW or more.
What is the difference between running watts and starting watts?
Running watts are the steady power an appliance draws once it is going. Starting watts, or surge watts, are the brief peak a motor needs to get moving, often two to three times the running figure. Generators list both, and you need to check both against your loads.
Can a generator run a fridge and a freezer at the same time?
Yes. Together they run on about 350 watts, and the biggest start is about 1,200 watts, so even a 2,000-watt inverter generator handles them with room left for lights and a router, as long as nothing large is started at the same moment.
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