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How Do I Calculate What Size Generator I Need? Generator Sizing Guide

Are you facing frequent blackouts? Is your area prone to extreme climatic conditions (storms, floods, blizzards, etc.)? Are you preparing for the next power outage? A Generator is now becoming an essential part of a household to provide power during emergencies. Another popular application of a generator is camping. If you continuously travel in your RV or go on frequent camping trips, then a portable generator can be a very good companion to power all your electrical and electronic devices. Construction and Worksites also use portable generators extensively. Regardless of the application, purchasing a generator is a huge deal and you have to get several things right. One common question many generator buyers ask is “How Do I Calculate What Size Generator I Need?”.

No matter the scale of your application, buying a generator is a major investment. You have to properly choose the right generator size. This is where the confusion begins. What is the meaning of Generator Size? How to calculate the size of the Generator? What factor should you consider while choosing a generator?

If you have these questions (or other similar questions regarding the generator size), then this guide is just for you. Here, we will discuss everything you need to know about the Generator Sizes.

A Brief Note on Generators

generator Even with developments in technology, we are still facing blackouts and power outages. If this happens for an hour or two, then the impact of the lack of electricity is not worrisome (rather it is an annoying situation).

But what if you have a storm and there is significant damage to the electricity infrastructure? In such emergencies, you have to face power outages for days together. The situation escalated from mere annoyance to catastrophe.

A blackout for days or weeks can cause severe damage (to both individuals and businesses) and could even be life-threatening. This is where a backup power system comes in handy. And the home generator is at the top of the list.

Portable and Home Generators are tiny electricity-producing devices for home usage (or small businesses). When we say “tiny”, we mean they are smaller than those big generators you see in commercial buildings or worksites.

Solar-powered or Wind-powered energy systems seem the main choice of alternative source of energy for those who are environmentally cautious. But the cost to generate a kilowatt of power is 50 to 60 times more than a generator.

We believe that solar or wind energy systems must work in tandem with existing infrastructure to lower the overall cost. But using them as a backup for days of blackout might not be possible (even if it is possible, the cost of such implementation will be astronomical).

What Do You Mean by Generator Size?

It is clear that a portable home generator is very useful for supplying electricity when there is an emergency or if you have any difficulty connecting to the grid. Now comes the important question. How to measure the size or capacity of a generator? How to determine the right size generator for me?

The first step in choosing the right size generator that fulfills your needs is to understand the basics of generator sizing. When we say the size of the generator, we are not referring to its physical measurements but rather the electrical power output of the generator.

Remembering high school physics, we know that electrical power is measured in Watts. As a generator produces electricity, we also measure its output in watts or W. If the number is larger than 1,000 watts, we can use the term kilowatts or kW (1 kilowatt = 1,000 watts).

If you look at online listings or catalogs, you will see generators with their power ratings such as 5,000 watts, 8,000 watts, 10,000 watts, etc.

This is the amount of power a generator can produce (or you can safely draw from it). Is there a thing such as a generator that is too large or too small? Yes. Without proper calculation, you might end up with a generator that is either small or large for your needs.

For example, if you bought a generator that has a significantly larger power output than your electrical needs then you spending a lot more as the cost of higher power generators is generally high.

Also, as you will be using only a small percentage of the power output of the “large” generator, there is a good chance that your generator will run with poor efficiency.

Having a generator that produces a smaller power than your total power requirements is also a problem. In this case, you might put the generator under severe stress which can lead to overheating or might even damage the generator.

There is also the case of a shortage of power to support all your devices and appliances. This can further cause an unstable or unreliable power output from the generator.

Things to Consider While Choosing a Generator

So, having the correct size generator will save you a ton of money and also help you run the electrical setup without any issues. But how do I calculate what size generator I need? If you are a beginner or a person with little exposure to generators, then this is the question you will be asking a lot.

Before answering this question, let us quickly take a look at some of the important things you need to consider while choosing a generator.

Where Will You Use the Generator?

The three common places where you will use a generator are your home, work site, and RV. A Home Generator can be of Standby type or Portable type. Regardless of the type, we use generators in our homes during power outages or blackouts. If there is a severe storm for days in your area, then the grid might not be able to provide you with electricity. In such situations, you need a generator to power all the essential electrical appliances and devices.

If you run a small business, then it will be difficult to work without power. Even in this case, you need a generator. Another popular and important application of generators is in construction or work sites.

You have to work in a place far from accessible electrical outlets. As using extension cords is not an option for running power tools, a portable generator is the only way to go.

Camping (either in cars, trucks or RVs) is also another popular application of tiny portable generators. If you have access to an electrical outlet for your RV, then no issues. But if you are a wilderness, a generator comes in handy.

Frequency of Power Outage

If you are thinking of buying a generator for your home or shop to provide power during power outages, then a couple of important questions you need to answer are how frequently do you face power outages and how long do those outages last?

Depending on the severity of the outage, you need to size the generator, its fuel, and other things.

Type of Devices or Appliances you intend to Power

Regardless of the application, the size of the generator will depend on the number of devices you want to run and also the type of device. When we say “type” of the device, we mean if the device, appliance, or tool has any motor in it or not.

While the “wattage” of the device is the essential factor in determining the total power requirement, there are two more important terms that will help us in accurately sizing the generators. They are Starting Watts and Running Watts.

We will see more about these terms in the next section where we will focus on the method to calculate the size of the generator by using these values.

How Do I Calculate What Size Generator I Need?

This is an important question. Let us quickly take a look at the meaning of Starting Watts and Running Watts of a device.

1. Keep an Eye on Starting Watts

Running Watts, also known as the Rated Power or Continuous Power, is the power rating of a device under stable operating conditions. In simple words, it is the power consumption of the device for normal operation.

Starting Watts, also known as Peak Power or Surge Power, is the power rating of the device while initialization or starting conditions. Generally, devices, appliances, and tools with electric motors have this surge power. It is a result of the high current that the motor draws while starting from a standstill (or off position).

This additional power is anywhere between two to three times that of the continuous power of the moto-based electrical device.

We can also define Starting Watts and Running Watts in terms of Generator as well. The Running Watts or Continuous Power (or Rated Power) of a Generator is the amount of power it can supply to its load (all the electrical devices, appliances, and tools) that are working normally.

If you connect a motor-based device to the generator, then the generator has to provide the necessary surge power that the motor needs. This rating of the generator is known as Starting Watts.

The surge power requirement of the device will stay only for a couple of seconds. As the motor picks-up speed and stabilizes its rotation, the surge power drops and the power rating falls to the continuous power of the device.

2. Generator Size Calculation

Coming back to the question “How do I calculate what size generator I need?”, both Starting Watts and Running Watts play an important role in this calculation.

The following table lists out some popular and commonly used devices, tools, and appliances along with their Running Watts and Starting Watts.

Remember that the Starting Watts that we mentioned in this table is the amount of additional surge power needed by the device for starting.

Device, Appliance, or Tool Running Watts (Rated Watts or Continuous Watts) Additional Starting Watts (Peak Watts or Surge Watts)
Large Halogen Lamp 300 Watts 0
55-inch OLED TV 100 Watts 0
Window AC (6,000) BTU 520 Watts 1,560 Watts
Microwave Oven (capacity of 1.1 cubic feet) 1,100 Watts  
1/2 HP Pump 1,000 Watts 2,000 Watts
Hair Dryer 1,000 Watts 0
Garage Door Opener (Motor Power is 1/2 HP) 900 Watts 2,400 Watts
Electric Drill with 1/2” Chuck and 6A Motor 660 Watts 1,320 Watts
Desktop Computer (with 24” Monitor) 800 Watts 0
Gaming Console 100 Watts 0

If you want to run all the 10 devices that we mentioned in the above table from a generator, then you have to first calculate the power consumption of all these devices.

Add all the running watts of all the appliances to the list. This gives the continuous power rating of all the devices. In the above example, this value turns out to be 6,480 Watts.

The next step is very important. Add the largest Surge Power value in the list to the total running power value we got in the previous step. So, 6,480 Watts + 2,400 Watts = 8,880 Watts. This is the Starting Watts rating of all the devices.

So, the generator you choose must be able to supply both these values i.e., 8,880 Watts of Surge Power and 6,480 Watts of Running Power.

An important question you might get is, why did we select only the device with the largest Starting Watts rating? It is very simple. we made this calculation based on the assumption that we will start only one motor-powered device at any time.

So, using only the device with the largest starting watts makes sense. Of course, this calculation might not work if you are certain that you will start two different motor-powered devices exactly at the same time.

In such a situation, you need to consider Starting Watts of both devices while calculating the power or size of the generator.

Some Generators With Starting Watts and Running Watts

In the previous section, we saw how to calculate the Size of the Generator using both Starting Watts and Running Watts of the devices or appliances. Note that while selecting the generator, make sure to leave some headroom for the generator.

As a general rule, we usually follow the 80% formula i.e., use only 80% of the power rating of the generator. So, in the previous example, we got the Starting Watts as 8,880 Watts and Running Watts as 6,480 Watts.

So, choose a generator with a running watts rating of 7,700 Watts and Starting Watts rating of 10,000 Watts to be on the safe side.

To help you out, we listed some of the best-selling portable generators for home usage with their ratings i.e., both Starting Watts and Running Watts.

Generator Starting Watts Running Watts
Generac 7676 GP8000E 10,000 Watts (Gasoline) 8,000 Watts (Gasoline)
Powermate P0081600 PM7500 7,500 Watts (Gasoline) 6,000 Watts (Gasoline)
Westinghouse 12500-Watt Dual Fuel Home Backup Portable Generator 12,500 Watts (Gasoline)

11,200 Watts (Propane)

9,500 Watts (Gasoline)

8,500 Watts (Propane)

Champion Power Equipment 100416 TRI Fuel Portable Generator 10,000 Watts (Gasoline)

9,000 Watts (Propane)

8,750 Watts (Natural Gas)

8,000 Watts (Gasoline)

7,200 Watts (Propane)

7,000 Watts (Natural Gas)

WEN DF1100T Dual Fuel Portable Generator 11,000 Watts (Gasoline)

9,500 Watts (Propane)

8,300 Watts (Gasoline)

7,500 Watts (Propane)

Pulsar G12KBN-SG Heavy Duty Portable Dual Fuel Generator 12,000 Watts (Gasoline)

10,800 Watts (Propane)

9,500 Watts (Gasoline)

8,550 Watts (Propane)

DuroMax XP12000HX Dual Fuel Portable Generator 12,000 Watts (Gasoline)

11,400 Watts (Propane)

9,500 Watts (Gasoline)

9,025 Watts (Propane)

Generator Sizing Guide – FAQ

1. How do I determine the size of the generator I need?

Ans: To determine the size of the generator, calculate your total power requirements by adding up the wattage of all essential appliances and devices you want to run during a power outage. Choose a generator with a capacity slightly higher than this total wattage.

2. How do I find the wattage of my appliances?

Ans: Refer to the product labels on your appliances or check their manuals for power specifications. Alternatively, you can use a wattage meter to measure the power consumption of each device.

3. What is starting (surge) wattage?

Ans: Starting wattage is the extra power a device needs to start up. Appliances like refrigerators and air conditioners may require a higher initial surge of power. Ensure your generator can handle the combined surge wattage of all appliances.

4. Can I run all my household appliances on a single generator?

Ans: It depends on the total wattage of your appliances. If the combined running wattage exceeds the generator’s capacity, you may need to prioritize or invest in a larger generator.

5. Are there standby generators specifically designed for homes?

Ans: Yes, there are standby generators designed for residential use. These generators are often connected to the home’s electrical system and automatically start during a power outage, providing seamless backup power.

6. Is it better to have a generator with more capacity than I currently need?

Ans: It’s generally a good idea to have a generator with some extra capacity to accommodate future needs or unexpected power surges. However, significantly oversized generators may operate less efficiently at lower loads.

7. Can I connect a portable generator directly to my home’s electrical system?

Ans: Connecting a portable generator directly to your home’s electrical system can be dangerous and is not recommended. It’s safer to use a transfer switch to isolate your home from the grid and prevent backfeeding.


Portable Home Generators or Standby Generators are very useful during power outages and blackouts. Apart from home and small business usage, we also use generators for camping (usually with RVs) and work sites (usually construction).

If you decided to invest in a generator as a power backup device, then the first question you get is “How Do I Calculate What Size Generator I Need?”. This is a very common question among first-time buyers.

In this guide, we saw the basics of generators and some important ratings such as Starting Watts, Running Watts, etc. In order to calculate the size of the generator, these two ratings of the devices, power tools, and other electrical appliances become crucial.

We saw how to find out the right generator size for your needs using the surge power and continuous power ratings. We hope that this guide could help you understand the basic calculations necessary to select the right generator size.

If you feel we missed something or want us to add anything, do let us know in the comments section. It will not only help us but also other readers.

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