Portable Generator Sizing: In order to know if the size of generator that is best for you, you need to know two pieces of information, how much power will you need to run at one time and what is the max output of the generator.
What do the numbers mean?
Generators are rated with two wattage ratings. One is for constant power and the other is for starting power. Appliances often require more power when starting than when running. Generators supply a boost of power, while that happens, that is above what they can sustain long term. Usually these wattage ratings are right on the box or title of the generator if purchasing online.
How much energy do my appliances use?
Every appliance uses a different amount of energy. There is a label on nearly everything that consumes electricity, telling you the maximum power consumption of the appliance. Sometimes you will get a range but more often, it is just the maximum that is given. The label can give you this information in one of 3 ways. It can tell you the watts, the amps, or the kilowatts (thousands of watts). If it tells watts, you are good to go. If it tells you the amps, multiply by 115 volts, to get watts. If the label tells you kilowatts, multiply by 1000, to get watts.
Find the watts for all of the appliances you plan to run at the same time and simply add them up. This will tell you the maximum power consumption of all of your appliances together. Now see how that number compares to the wattage ratings of the generators you are considering buying. You want to have some wiggle room for plugging in extra things but the more wiggle room you have, the more fuel you will waste with extra capacity and the more money you will spend on capacity you don’t need.
For example: I found the label for my refrigerator inside the appliance, on the side wall. It tells me that the maximum power consumption of the appliance is 7.2 amps. 7.2 Amps x 115v = 828 watts. My stand alone freezer has a label, in the same place that tells me it consumes a maximum of 5 amps. 5 amps x 115 volts = 575 watts. 828 watts + 575 watts = 1403 watts. Note: I’ve used 115 volts in this example because US home voltage is usually between 110 volts and 120 volts. Labels usually have a voltage, in that range, on them.
So I want to be prepared for a power outage. I know that if I purchase a Generac iQ, a Honda EU2000I, or a Yamaha EF2000iS, which all have a 1600/2000 watt rating, I can run both, my refrigerator and deep freezer at the same time. I know that I won’t lose my food and that I have some extra room to run other devices, such as a lamp, a laptop, and/or maybe a TV.
On the surface it appears like I only have 200 watt of power left over to run these devices. The reality is, I have a bit more. Remember that these labels usually tell you the maximum power consumption of the appliance. Most of the time, the refrigerator and freezer are going to be consuming less. If they both happen to be starting at the same time, they are going to be consuming 1403 watts. During that start up, I have an extra 400 watts of startup power to use. After they appliance start, they will be using less power.
Getting technical but it’s easy.
If you really want to know how much power the appliances are practically using, to get your generator sizing just right, you’d need to measure their consumption. The Generac iQ mentioned above, has a meter, built in to tell you how much power you are using. You’d need to get both appliances to start at the same time to see this maximum usage measure on the meter.
For a more accurate reading and for the other generators, you can purchase a clamp on meter that can measure current flow in amps. The meters are safe and easy to use and don’t require any contact with electricity. They simply clamp on your wire and tell you the amps flowing through it. Though you will have to remove the outer insulation from your extension cord, exposing the three insulated wires inside. So, you may want to have an extra cord you use, just for this, as this will make the cord less durable.
You can then calculate watts as explained above. This will give you one or two readings for all of your appliances at once. (If you are using both outlets on the generator, you’d need two readings, one for each outlet). Make sure you measure while appliances are starting and while they are running, to get both the running and starting watts. Try playing a streaming video on your laptop, for maximum usage.
One final thought on generator sizing: inverters vs standard generators
Inverter generators tend to have variable speed, where conventional generators need to maintain a constant RPM to keep the voltage and hertz regulated. This means that if you run an inverter generator on a light load, it can slow down to save fuel. If you put a small load on a standard generator, it has to run at the same speed as if you ran it at a full load, consuming more fuel. This means getting the right size is probably a bit more important for a standard generator than an inverter.
With both types of generators, having a bit of extra capacity could improve the longevity of the machine. Having a bit of extra room means you are putting less stress on it. If you drive your car at 130 mph, every time you get in it, it probably won’t last as long as if you drive it 65 mph, every time. Usually, this won’t be a problem because your load will vary and most of the time, you won’t be running everything at once that you want to be able to run at once.