Updated: December 19, 2020

How Big of a Generator Do I Need?

by Charles Jasper

There are a few things in this world that you don’t want to be short on. Cash while you’re on vacation, gasoline while driving across the desert, and an under-powered generator during a power outage. If you’re faced with the choice between running your sump pumps or keeping your giant freezer going, what will you pick? You won’t need to make that tough choice if you can think about and determine the proper size of generator you need first.

When your power goes out, your generator can be your only line of defense between returning to normal quickly and ending up with spoiled food, a flooded basement, and an unhappy family. Sizing your generator starts by determining what exactly you need to run, when and how it starts, and what you can live without. Then, we’ll look closely at the difference between starting, or ‘surge,’ wattage and running wattage, and which can drastically alter the size of generator you need.

Next, we’ll look at where the lines blur a bit—are you better off getting something smaller and portable, or bigger and better equipped? Finally, we will take a look at some things to look out for as you shop and use your generator, and answer the question: how big of a generator do you actually need?

What Do You Want to Run?

How Big of a Generator Do I Need

The first step to sizing your generator appropriately and understanding how big of a generator you need is to take a careful look around your home and determine what exactly you’d like to run during a power outage. This differs greatly from person to person and depends a lot on where you live.

You’ll want to walk around your home with a pad and pen and write down things that are important for you to run while your power is out. These should be broken into three categories—these categories will vary a bit depending on where you live.It’s important to think critically here—what you write in each category can drastically change the generator you need to purchase.

Essential Systems

Sure, your television, living room lights, and Wi-Fi make your house feel like home, but don’t forget, they don’t necessarily make your house run. Systems are the absolute life essentials that keep your household going. If your power is out and you’ve got a generator, systems are things that you absolutely want to run, no matter what. There are a few things that we absolutely must run.

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• Sump Pumps and Well Pumps

If you’re in a flood-prone area or simply live downhill from some other homes, you’ll definitely want to keep the water out of your home. Additionally, if you are not near city water, you’ll be pumping water from a well—you need water, straight out. Running a sump pump or well pump on a generator can take a surprisingly large amount of power. You may not think this based upon how long a pump runs, but consider the force with which it has to push the water, a heavy substance, out through pipes.

• Medical Devices

If you require medical devices for your continued health and well-being, you need to account for the power that these require and then some to be on the safe side. Oxygen tanks, monitors, and other devices don’t tend to pull a lot of power, but they often do require a constant connection to function properly.

• Hot Water Heater, AC, and Heat

If you live in a temperate climate, you can likely survive just fine without these items. However, if you live in either a very hot or very cold area, or it’s a very hot or cold season, these things can mean the difference between making it through a power outage and not.

• Refrigerator and Freezer

Refrigerators, if kept closed and full, will keep food edible and cool for only four hours. Freezers, if kept full and closed, will keep food frozen safely for 48 hours. You can extend this time a bit by placing frozen water into these appliances. If, however, you require refrigeration for medical needs or have a large amount of meat or other items that you don’t want to spoil, you may want to consider having your fridge and/or freezer on your generator.

Nice-to-Haves

In this category, we’re looking at things that would make your power outage experience a bit more enjoyable. This category may include things like your hot water heater, air conditioning, and heat if you live in a more temperate climate. It also may include your refrigerator or freezer.

• Television

If you’re in the middle of a power outage caused by a natural disaster, a television can be a great nice-to-have item to stay informed. Understand too that you cable service may be out as well—having a backup antenna to get local stations is always a good idea.

• Microwaves and Ovens

If you have an electric range, having it operational during a power outage can be exceptionally helpful. Microwaves also make food prep quite a bit easier, especially if you’re in the dark.

• Lights

We don’t realize how much we use lights until they’re gone. A few lights can be helpful when navigating your home during a power outage.

Extras

Here, we’re really talking about the things you don’t need during a power outage. Having these things running would basically return your experience to normal.

• Blow dryers and other personal grooming tools- Nobody's looking at your hair during a power outage,so you don’t need to worry about powering your tools to get it coiffed perfectly. These types of small appliances draw a lot of electricity.

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• Wi-Fi - Yes, it can be hard to say, but Wi-Fi is a luxury that you likely will not need during a power outage. Ensure that your mobile phone is charged and you’ll have all the connectivity you’ll likely need. Remember too that service may be interrupted from your service provider.

• Outlets - These can get very complicated because they are attached to your circuit breaker.If you want to power outlets within your home, you’re more than likely looking at a whole home standby system.

The above items are provided as an example. You may have different items within your home that fall into each of these categories, and your list may be entirely different. If, for example, ensuring your hair dryer is running during a power outage is especially important to you, then by all means ensure it’s covered. Remember, however, that you will likely need a larger generator to accommodate it, especially for items that plug into outlets.

So I know what I want to run . . . what do I do?

Add it up! Take a look at each appliance you want to run and see what the paperwork lists as the wattage. For example, your sump pump will likely have some information listed right on it that tells you its starting wattage is X and its running wattage is Y. It may not be clear from looking at all appliances what the wattage is, and some research on your end may be necessary. You will often be able to find wattage information for each appliance online. If you cannot and need to instead work with ranges, consider the top end of the range for each appliance. Make a separate note of both running and starting wattage. This will be important.

Understanding a bit about volts, amps and wattage is important, as well. An amp is a unit of measurement for electricity that shows us how quickly electric current flows through a wire. A volt is the force with which that current moves. A watt is the combination of these items, the speed and force, to arrive at the full strength of electricity. Think of a wire like a river—the rate at which electricity flows is measured as amps. The pressure of that water moving along the river is calculated in volts. The watts, then, are the whole force of the water.

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Some labels will not have the direct wattage you need listed directly on them. For this, you’ll need to do a bit of math. Multiply the number of volts by the number of amps, and you’ll get the watts. A refrigerator for example, often runs on 115 volts at 1.1 amps., meaning that it has 126.5 running watts.

Note where you found these numbers and look to the right. You will find a second number marked with an A, noting amps. This is the starting or surge wattage. It will often list the type of motor the appliance is using to achieve that voltage—in a refrigerator, it’s often a locked rotor amp, so you’ll often find ‘LRA 5A’. This means you should multiple the volts by the amps again, giving you 575 watts, the surge wattage of the refrigerator.

Starting vs. Running Wattage

Next time you start your car, give a listen to your engine. As you turn the key, you’ll hear the starter—that’s the ch-ch-ch-ch-ch-ch-ch sound—and then hear the engine ignite. Notice that the engine revs quite high as it starts, then settles down into an idle, then revolves as it rests and when you’re not actively pushing the gas.

How Big of a Generator Do I Need

This serves as a very good analogy between starting and running wattage. Appliances consume more electricity starting than they do when they are already running. Some appliances’ surges when they start are more dramatic than those of others. For example, your refrigerator does not have a large difference between starting watts and running watts. A refrigerator may have a starting wattage of about 575 watts and then a running wattage of 126. This is fairly common and actually, quite a small difference. A window air conditioner, by contrast, uses 725 running watts but has a surge wattage of nearly 2,200 watts. That’s a huge difference and something you need to consider.

Make sure that you’re looking carefully at each of these items.

Example

Let’s take for an example, a small, two-bedroom home in Ohio, a fairly temperate location. The home, however, is at the bottom of a hill and has a basement, so sump pumps are installed. It is owned by a young couple with a three-month-old infant. During a power outage, they need comparatively little, but realize that what they do need requires quite a bit of power. The wife breastfeeds, for example, so a pump is necessary, as are the refrigerator and freezer for the safe storage of breast milk. The sump pumps need quite a bit of starting wattage when it runs to keep water out of the house.

We can see by our example that despite the relatively small number of electric appliances—refrigerator, freezer, sump pump, breast pump—the couple actually needs quite a large generator. In fact, in this case, a whole home application may be the best bet, considering that a number of items young parents need plug into outlets.

Types of Generators

How Big of a Generator Do I Need

Now that you have a better understanding of the wattage you need, it’s time to take a look at how big of a generator you actually need to purchase. There are four main categories.

• Recreational

These are the generators—often called inverters—you can take on a camping trip or tailgate. They have one or two outlets and usually run on gasoline. They are generally small and can be easily carried. This is perfect for someone who only needs to run a refrigerator and a light or two.

• Midsize

Still small and portable, and likely still running on gasoline only, a midsize generator or inverter often has the capacity of either a number of other small items, like the recreational, or the same number and a single large pull item, like a window air conditioner.

• Large

A large inverter is still portable but likely comes with wheels. You’ll see these on professional construction sites, as a number of workers use various power tools. Additionally, these generators may run on dual power. This means they will likely have a gas tank but also may run on propane, which is a great option. Here, you’ll be able to run a number of appliances, including your water heater and a sump pump. This generator can be attached to your circuit breaker, as well, though this should be done by a professional.

• Home Standby

The name of the game here is run everything. A home standby generator attaches to your circuit breaker and powers your entire home. It will kick on as soon as it senses power from the electrical lines has been interrupted. There is often a short 10 or 15-second delay as the generator gets going. These generators run on natural gas, and buying and installing a generator of this size requires professionals, unless you’re very handy. It also requires permits. The company you work with will help you size the generator to your home. You can choose to run your entire home or select items controlled through a separate circuit breaker, and, often nowadays, a mobile app.

What to Look Out For

There are a few things to look out for as you shop around for a generator and determine how big of a generator you should purchase. Keeping the below in mind can help you make the right purchase for you and your family.

• Fuel Source

How Big of a Generator Do I Need

Many larger generators run on both gasoline and natural gas. It can often be much easier to get and transport natural gas and it can last a bit longer, so this choice is up to you. Ensure, however, you have a plan for getting the fuel.

• Fuel Size

This can mean the difference between getting up every hour to refill the tank and not. Ensure you get a generator with enough fuel storage to give you peace of mind. Larger generators will allow you to connect a natural gas tank and fill a gas tank, alternating between the two sources as necessary.

• Carbon Monoxide & Safety

Ensure that you follow both the instructions that come along with your generator as well as any appropriate regulations. Keeping your generator away from open windows is critical. Never operate your generator inside. Carbon monoxide can kill quickly without warning.

• Wiring

You should not DIY attach your generator to your home’s electrical system unless you’re extremely competent. For one thing, this is often illegal in many jurisdictions. Additionally, the power you generate can back-feed to the lines that normally feed power to your home. This is incredibly dangerous to line workers who may be in the area attempting to restore power. If you’re running the generator separate from your circuit board, ensure that you take precautions regarding rain and water.

Conclusion

How big a generator you need is a function of exactly what you need to run. If you have just a few items that need power during an outage, and those items are relatively small draws, you can get away with a smaller generator. If, however, you are using a number of appliances, have appliances with high starting wattages, or simply want to power your whole house, you’ll need a larger generator. Doing your research before you head to the store or invite a contractor out for a quote will help ensure you get a generator that fits your needs.