Best Generator Sizing Calculator

How Big of a Generator do I Need?

Before using our Ultimate Portable Generator Comparison Buying Guide, see how big of a generator is necessary.

How to Figure Surge Watts Into The Calculation

Now that you have a ballpark estimate of the running watts you will need, it’s time to factor in the surge watts that are required. Generally, appliances that contain large motors have a lot of inertia to overcome – e.g., a refrigerator compressor – and require an extra boost of electricity to turn them over. Once they are started, they settle back down to their lower running wattage.

In the calculator above, you can see that the **asterisked items are those that will have a higher surge wattage. To estimate how many surge watts you need, first check all the running watt boxes of those appliances that you plan on running simultaneously. Now look at the highest wattage item that has a surge watt requirement. Double this number, then un-check its box (so as not to double count it) and then add its surge wattage you just calculated to the remaining running sum at the bottom of the page.

For example, if you checked a Refrigerator/Freezer** (800w), Space Heater (1000w) and Sump Pump** (1500w), then your total running wattage would be 3,300 watts. To estimate surge wattage in this case, you would double the Sump Pump wattage (1,500 x 2 = 3,000 surge watts), un-check the Sump Pump field and add 3000 to back the remaining running wattage figure, which leaves you with a Surge Watt requirement of 1,800w + 3,000w = 4,800 watts. In other words, you would be searching for a generator that had at least 3,300 Running Watts and 4,800 Surge Watts.

Stagger Appliance Start-up For Additional Surge Watt Flexibility

Remember that Surge Wattage is only needed at start up, and is therefore within your control to some extent depending on how many things you are turning on at the same time. So, for example, in the scenario above, if you turned on the sump pump first, then you would need only about 3000 watts of surge power, because you could turn on the refrigerator later, after the pump settled back down to its 1,500 running wattage. And after the refrigerator settles down to its 800 running watts, then you can flip on the space heater, which has no surge watt requirement. Again, this is all possible if you don’t have to have everything turned on and running at exactly the same time.

In general, to make the most of your generator’s power, you want to start up your biggest surge watt appliances first, then go down the line with the second largest, third largest, and so forth, turning on appliances without surge watt needs last.

In addition, it’s also good to rotate appliances to conserve generator output. So rather than leave your fridge connected the whole time, you can let it run for several hours and then disconnect it, and the run the sump, AC unit or another high-draw appliance instead. By rationing power and rotating through your bigger appliances this way, you can often get away with a significantly smaller generator.

Err on The Side of More Power – Buy a Generator With a Bit More Wattage Than You Think You Need

Remember that all wattage “estimates” are just that. Unless you have the actual running and surge wattage of each and every appliance you want to power, you should account for the invariable under estimate here and there. You may also realize after you tallied up your running wattage that there was one more appliance you really needed, or that you are getting a newer model X that has significantly higher wattage requirements.

Simply put, like most things in life, its best in this case to err on the side of more, rather than less, generator capacity. Consequently, whatever running/surge wattage require you estimate, it’s not a bad idea to add an extra 20% on top of that for good measure. You may be very glad you did.