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The 2 Fool-Proof Ways of Calculating What Size Generator You Need

Powering your RV on camping trips, hosting tailgate parties, dealing with home emergencies, working at remote job sites - all are good reasons for owning a generator. And just as there are many reasons for owning a generator, there seem to be as many kinds of generators to choose from, all with different power outputs.

Let's say you want to power a TV and an electric cooler at a tailgate party. You'll probably want to go for a lightweight, low-watt generator with a built-in inverter. A generator with an inverter produces controlled AC electricity to safely power sensitive electronics, like TVs. Don't know what AC or DC are? Don't worry. We'll cover that in this article too.

If you need to power a miter saw, an air compressor, and a 1,000-watt standing work light at a powerless construction site, you don't need a generator that has an inverter, and you probably don't care about its noisiness, but you'll want a lot of power.

It can be confusing to find the right size and type of generator that's best for you, so by the end of this article, you'll be able to answer:

What size of portable generator do I need?
What kind of power do I need? 120V AC, 240V AC, 12V DC?
How do I compensate for a high power draw?
Do I need an inverter generator?

Pictured: A-iPower 5,000-Watt Portable Generator

What size of portable generator do I need?

Before jumping into how to calculate your power requirements, you'll need to understand a few terms.

The electrical energy produced by a generator is usually specified by 2 values - voltage and wattage.

Voltage

A generator's output voltage determines what kinds of devices the generator can power. Most generators produce 120-volt AC, which is the same voltage as your home's outlets that run common household appliances and power tools.

A bigger generator might be capable of producing 240-volt AC to run devices like household laundry dryers or electric welders.

Some generators have 5-volt DC USB ports and 12-volt DC outlets. 5V DC USB ports are used for charging your phone or tablet. 12V outlets are useful for charging a car battery or powering devices that can run off of your vehicle's 12V outlet.

Wattage

The wattage determines the maximum amount of power the generator can supply at one time.

A small generator of 1,000 watts can power devices that don't draw much electricity, like floor fans and lamps.

A much larger generator of 10,000 watts has enough power to run a 240V air conditioner or clothes dryer, or multiple 120V appliances.

To determine what size generator you need, you must consider both its starting and running watts. These 2 values are typically easy to find on the generator's packaging, product label, and online product description.

Starting Watts: Show how much power the generator can supply for a short burst. Manufacturers may refer to starting watts as maximum or surge watts. This is the power a device requires at startup.

Running Watts: Represent the power that the generator can produce continuously. Manufacturers sometimes refer to running watts as rated watts. This is the power needed to keep the device running after it is started.

Both starting and running watts are important to know because many electrical devices - especially those with electric motors or compressors - draw more power when they start than when they are running continuously.

A large appliance with an electric motor such as a furnace, refrigerator, air conditioner, or dryer may require 2 to 3 times more starting wattage than running wattage.

Pictured: Firman 10,000-Watt Portable Generator

Pictured: A-iPower generator connected to a camper

Calculate Power Requirements

To choose a generator that has enough power, you must match the size of the generator (that is, its starting wattage and running wattage) with the electrical demands of the equipment that you want to operate.

The output capacity of the generator must meet or exceed the demands of the devices it will power.

It is always best to start your generator with no load connected, then add each device one at a time.

Additionally, a good practice to consider is connecting your devices in order from highest starting watts to lowest starting watts. As you connect and start devices, your available wattage decreases.

This order of operation allows you to start devices with the highest power demands when there is more power available.

Running Watts and Starting Watts of Common Appliances

* The starting wattage on air conditioners can vary greatly depending on several factors, including the assistance of built-in capacitors and cold starts. Because of this, we can't list a standard starting wattage as every application can be different.

The two calculations below provide power estimations that will help point you to a generator that is right for your needs.

Calculation Option 1: Industry Standard Calculation

The Industry Standard Calculation is a quick and easy way to determine your required wattage. It may provide some padding in your wattage requirements, which could give you more power than you actually need, but you shouldn't have to worry about not having enough power.

The following steps will help you quickly calculate the correct size generator to meet your application.

As stated earlier, you should always start your generator without any load, and then connect each device one at a time. The following calculation requires this approach to work. However, using this calculation removes the worry of connecting devices in a specific order.

Step 1

Determine the starting watts and running watts for the devices that you want to plug into the generator.

a. Find the starting (surge) watts and running (rated) watts for each device that you want to plug into the generator. Look for a sticker or plate on each device or check your owner's manuals.

b. If your device or manual only shows amps and volts, watts can also be determined through a simple formula. (Amps x Volts = Watts)

c. List all of your starting watts and running watts as seen in Table 1 below.

Step 2

To find the total starting watts required, add the starting watts for the device with the highest value to the running watts of all the other devices from Table 1.

2,000 + 800 + 1,000 + 400 = 4,200 total starting watts

Step 3

To find the total running watts, add all of the running watts together from Table 1.

1,000 + 800 + 1,000 + 400 = 3,200 total running watts

To run all of these devices simultaneously, you would require a generator with at least 4,200 starting watts and 3,200 running watts.

Calculation Option 2: Advanced Precise Calculation

While the above method will work, it isn't the most precise. If you aren't afraid of doing a little more math, you can use the Advanced Precise Calculation to get a more accurate idea of how much power you will need.

Because the calculation above pads in enough power to ensure you can start your devices in any order, it may point you to a bigger and more expensive generator than you need.

By following the more regimented, order-of-operations calculation below, you'll get a more precise idea of how much power you need, which could point you to a smaller and less expensive generator.

Again, you should always start your generator without any load, and then connect each device one at a time. The following calculation also requires that you start your devices in order from highest starting watts to lowest starting watts. When you connect devices in this order, you can start high demand devices when there is more power available.

Step 1

Determine the starting watts and running watts for each device that you want to plug into the generator.

a. Find the starting (surge) watts and running (rated) watts for each device that you want to plug into the generator. Look for a sticker or plate on each device or check your owner's manuals.

b. If your device or manual only shows amps and volts, watts can also be determined through a simple formula. (Amps x Volts = Watts)

c. List out these devices from highest starting watts to lowest starting watts as seen in Table 2 below.

Step 2

To find the total running watts, add all of the running watts together from Table 2.

1,000 + 800 + 1,000 + 400= 3,200 total running watts

Table 2

Example Devices

Starting Watts

Running Watts

Air Compressor

2,000

1,000

Refrigerator

1,600

800

Microwave

1,000

Blender

850

400

= Running Watts

Step 3

In Table 3, shown below, the starting watt values from Table 2 are ordered from the highest to the lowest.

To find the precise number of watts you need, add the starting watts of each new device you connect to the combined running watts of the already connected devices.

The highest of these numbers will be the total starting watts required.

Table 3

Device

Starting Watts

Running Watts of All Connected Devices

Total Starting Watts Required

1. Air Compressor

2,000 +

0 (nothing connected)

= 2,000

2. Refrigerator

1,600 +

1,000 (compressor)

= 2,600

3. Blender

850 +

1,800 (compressor + refrigerator)

= 2,650

4. Microwave

0 +

2,200 (compressor + refrigerator + blender)

= 2,200

Once you have the total starting watts required calculated, find the highest value. In our above scenario, our highest value is 2,650, meaning we need *2,650 starting watts from the generator we buy. Using the running watts value from Step 2, our generator should also have **3,200 running watts available.

*The starting watts are determined by the highest total starting watts required during the calculation in Table 3/Step 3.

**The running watts are determined by the total running watts from Step 2.

What Kind of Power Do I Need? 120V AC, 240V AC, 12V DC?

Now that you know how many starting and running watts you need from your generator, you should determine the electrical current output your devices require.

If you're going to connect your power tools directly to the generator, you need a generator with 120V AC outlets, the type of outlets you find in any home and business.

If you're going to be plugging in your RV's shore cord to your generator, you need a generator with either a 30-amp or 50-amp outlet. Some generators even supply both outlets. 30-amp outlets supply 120V AC, and 50-amp outlets supply 240V AC. Certain large appliances may require 240V power like some ovens, clothes dryers, and air conditioners.

Lastly, you may need a generator to plug in a few DC devices while camping like a 12V portable stove and a phone charger. In that case, you should look for a generator that has 12V outlets and a 5V USB port as well.

Check each appliance you plan on running off of generator power to determine which type of power, not just how much power, you require.

If you would like to understand 30-amp and 50-amp service better, click the button below to read our helpful article.

I'll say it again. Before you buy a generator, you need to have a good idea of the devices you'll be plugging into it.

I repeat this because some smaller generators (i.e. 3500 running watts) may not have outlets that deliver all available wattage to one 30-amp or 50-amp outlet. Some generators split the wattage between two 15-amp circuits.

If you tried to pull more wattage from a 15-amp circuit, you'll trip a breaker. Although, it's not going to damage your devices or generator, it's annoying and means you'll need a different generator.

Pictured: Table comparing 120V AC, 240V AC, and 12V DC power

How to Compensate for a High Power Draw?

You won't have an issue finding a portable generator for your specific power needs. For instance, if your setup requires a whole lot of wattage, some portable generators supply 10,000 or more watts of running power.

However, those portable generators are large and usually weigh over 200 lbs. Not mention, in general, the larger the generator, the noisier and more costly it is. You may have times where a small generator will work fine, like powering an electric grill, and other times where you need a bit more juice, like powering a camper.

Parallel kits give you the flexibility to use one generator or two generators, depending on your power needs at the time. Parallel kits work by running a cord from each generator to the parallel kit's box, and you plug in your devices to the parallel kit rather than the generators themselves.

Not every generator can be daisy-chained with a parallel kit though. Out-of-the-box parallel capability is a unique generator design. If you think a parallel kit is for you, look for generators that specifically state they can be run parallel - and they will always be an inverter generator. You can even purchase kits that include two generators and parallel kit.

Running two generators connected to a parallel kit is a great way to have flexibility in your generator setup; however, it's not as simple as buying two generators and connecting some cables.

Like, I mentioned above, not every generator can be run parallel to another. When researching generators, check the manufacturer's information on a whether it can be run parallel. This is a special capability, so a manufacturer will make it obvious.

If you want to buy two generators and a parallel kit separately, we recommend purchasing the components from the same manufacturer. While you can parallel generators from different brands, there may be additional components or steps that complicate the process. It's easier to just go with the same brand.

Two generators with parallel kit product image

Pictured: Champion 5,000-Watt Inverter Generators w/ Parallel Kit

Pictured: Champion 9,000-Watt Dual Fuel Inverter Generators w/ Parallel Kit

Do I Need an Inverter Generator?

An inverter generator costs a bit more than a non-inverter version, but the higher price comes with some great perks.

Reasons why you may purchase an inverter generator include that it:

Protects sensitive electronics
- If you plan on powering sensitive electronics, like TVs, tablets, or computers, from generator power, you should use an inverter generator. An inverter will protect your electronics from power irregularities that damage the delicate electrical components.
Improves the generator's fuel-efficiency
- The inverter block controls the engine's RPMs depending on how much power is drawn from the generator at a given time. If you only have one, low-watt device plugged into the generator, your generator will conserve its fuel by running the engine at lower RPMs. A non-inverter generator does not downregulate the engine's RPMs depending on power draw, so it's always burning the same amount of fuel.
Makes less noise than non-inverter generators
- Try holding a conversation next to a standard generator, and tell me your voice wasn't sore afterwards because of how much you had to yell. Although an inverter generator still makes noise, the noise level is noticeably less than a non-inverter version. If you don't want to hear a massive engine chugging for hours on end, choosing an inverter generator is your best option.
Has the potential to be run parallel with another generator
- Calculating how much power you need from a generator means you'll want to think of every scenario you could use portable power. Sometimes you only need a few hundred watts, and other times you need a few thousand. Certain inverter generators can be paralleled with another, giving you the ability to use one generator at a tailgate and two generators when your home's power goes out.

Inverter generators with parallel kit outside

Pictured: Two A-iPower 3,800-Watt Portable Inverter Generators hooked up to an RV

Putting It All Together

After reading this article, you should have a better understanding of how much power and the type of power your devices require. You may even be toying with the idea of an inverter generator or a parallel kit.

Your generator setup is about what works best for you, not necessarily what is the most powerful, feature-rich generator you can afford.

Take the time to think about what your specific situation calls for. While you're at it, take a look at some of our portable generators to get a better sense of what other features you may want, like remote start or wheels.

More power to ya!

Still have questions? Did we forget an important-to-know detail about generators? Let us know in the comments!

I don't just put nice words on a page. Being a content writer at etrailer means I ask about a hundred questions every day to our experts, installers, technicians, and product managers. I ask the question before you know you have the question in the first place. It's my job to be always learning so I can serve you better through helpful, easy-to-read articles. Now and then, I also like to provide a bit of comedic relief around the office.

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Published On: 6/28/2021
Updated On: 10/24/2023

Tom N.

10/22/2023

A non-inverter generator does not downregulate the engine's RPMs depending on power draw, so it's always burning the same amount of fuel. This is not true the more power you use the more Fuel you will use. This is why they give run time on a tank of fuel at 1/2 load.

John S.

8/12/2022

my champion generator won't work with my on board charger to charge up my trolling motor batteries it makes it blink yellow and red on my charger diagnosis it is 36 volt and someone said I need a inverter or something else to make it put 120 volt to charge my batteries I use a 50 ft extension cord and it is 10 gauge. Please help me find a cure

David B.

8/17/2022

Has the generator worked in this way before or is this the first time you are trying to charge the batteries using a generator?

John S.

8/17/2022

@DavidB it is a new generator and been doing every since I bought it

David B.

8/17/2022

How many times does it flash red/yellow? What size is the generator? Does your owner's manual have a troubleshooting section in it?

See All (5) Replies to John S. ∨

Big 4.

8/12/2022

It is also a good idea to start the Generator with the main Breaker OFF, as some generators will produce a spike when started, and connected products could be damaged.

David B.

8/12/2022

That is a safe practice. I myself just don't plug anything in until the generator is up to speed and warmed up.

V. B.

8/12/2022

Why Amperage isn't shown anywhere, it's what campers work on mainly !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

David B.

8/12/2022

Amps are just the amount of electricity that can flow through a given conductor. It doesn't change from application to application. Volts are what push the electricity through the conductor. Things are listed in volts because that is something you can use to gauge the flow.

Dan B.

8/13/2021

Are they duel fuel generators 2000w to 3000 Watts How heavy are they

Victoria B.

8/18/2021

We do have one dual fuel generator in that wattage range, the Firman 3,300 Watt Portable RV Dual Fuel Inverter Generator # FIR54FR. This generator has a dry weight of 104lbs.

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