What Size Generator Do You Need? (4 Steps)

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To figure out what size generator you need in your situation you need to list the devices you want to run, figure out their power needs, add a margin of safety, and get a quality generator that produces the amounts listed.

For smaller houses with a smaller AC system, something like a 5000 watt generator could be enough.

A camping trip or DIY session can likely be successful with a smaller size than that.

On the flip side, bigger houses with a 4 ton AC system and other devices could need a 12000 watt generator or an even bigger size to successfully get through a power outage.

How to calculate what size generator you need?

To calculate what size generator you need you take the following steps:

  1. Figure out what devices you want to run at the same time
  2. Find out the power needs of these devices
  3. Add a margin of safety
  4. Get the right generators

1. Figure out what devices you want to run at the same time

Before investing in a generator, you need to figure out what electrical devices you will run with it.

The first thing to note about this is that how often you deal with power outages and how long they last can influence how much backup power you need.

For example, if the power outage lasts less than a day, a small generator for a fridge and freezer could be enough.

On the other hand, if you want to live somewhat normally during long power outages, you may need a big generator that can power your AC, a sump pump, and more.

At the same time, you still want to keep in mind that you likely don’t have to run all your devices together.

By turning these on and off in the right sequences, you will need to invest less in a generator.

At least if you are willing to do this extra effort.

2. Find out the power needs of these devices

Once you have your list, you can measure your power needs (or if necessary use the rough estimations of a generator size chart or something similar).

The first thing to note about this is that there are starting and running watts.

When it comes to running watts, you simply add up the requirements of the devices you intend to power.

On the flip side, some devices like an AC require an extra burst of power to get started.

Generators also come with the ability to produce more power for a short amount of time but there are still size limits.

Next, while the wattage needs of appliances tend to be the limiting factor, there are also devices like well pumps that need 240 volts to run.

Similarly, you want to look at an amp chart and the amp capacity of the generator outlets to see if they can handle enough for your devices.

Lastly, when it comes to calculating what size generator you need in terms of peak wattage, there are still two methods you can use.

Method A

If you don’t mind a bit of extra calculating in advance, you would “imagine” or note down starting up the devices with the highest peak watts first.

After each step, you consider the continuous watts of the devices already running and how much your generator has to produce to deal with the next starting watts.

Once you are at the end of your list, you should have a number of peak watts that is necessary for the appliances and tools you want to run.

Method B

Method B is somewhat simpler but also makes it so you likely get a bigger generator than necessary.

At the same time, this could be the right choice if you don’t want to deal with carefully starting up your devices in a certain optimal sequence.

This Method B simply involves adding up all the running watts of the devices you want to power at the same time and adding the highest additional starting watts.

3. Add a margin of safety

Before going ahead and getting a generator size that is theoretically just able to deal with the numbers from above, you want to add a margin of safety.

How big this margin of safety should be depends on a few details.

For example, longer extension cords will generally involve slightly more power loss.

It is typically smart to add at least a 10% margin of safety in terms of watts.

To play it safer, you could lean more toward 20% and even more.

4. Get the right generators

Lastly, you want to make sure you choose some of the best generators available.

Lower-quality models do not always reach the wattages, amps, or volts mentioned on the box.

Additionally, you want to get the right types of generators for your devices and fuel options.

Keep in mind that certain options like dual fuel generators tend to produce fewer watts when running on propane vs gasoline.

What size generator will run a house?

30 amp and 50 amp generators will run a house more easily due to the more convenient and safer connection.

However, it should be clear that what specific size generator will run a house depends on the devices inside of it and the power loss involved in getting the power to the appliances.

It is fair to say that 30 and 50 amp generators tend to be easier to connect safely to houses.

Besides that, it again really depends on the devices inside your house.

To run a 3 ton central air alone, you typically need a generator with more than 7000 peak watts.

On the other side, a refrigerator and a freezer will typically not need more than 2400 peak watts and 1400 running watts.


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Mats is the founder and head editor of Generator Decision. With a combination of critical thinking, tireless research, and a healthy interest in electronics he helps people find the right generators and how to use these. At this point in the journey, Mats has done research on hundreds of portable generators.