What Size Generator Do I Need?

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What Size Generator Do I Need?

A generator is a versatile, reliable power source that gives you the flexibility to keep your tools and appliances running in just any environment. From a portable genset for your town picnic to a standby power backup for your home, you can find generators in several different wattage sizes or capacities.

That leads to a very important question: What size generator do I need? In this article,

we discuss how much power you need from your generator, and what size of generator you need to run all your appliances simultaneously.

When talking about generator size, we don’t mean its physical size – rather, the size refers to the maximum possible amount of power it can supply.

How is Power Output Determined for a Generator

In electrical science, everything is measured in watts (W) or Kilowatts (kW), both of which are a unit of electricity. Put simply, the wattage of larger appliances like refrigerators, microwave ovens, and washing machines is a lot more than small devices such as fans, light bulbs, or radios that take next to no wattage to run.

In most cases, you will require generators that have an electrical output in the range of about 3,000 and 10,000 watts. Ask yourself first: What do you need the generator for? Is it for your home, for your camping needs, or for your business?

Should you need one for camping, a small portable one will do. Small to midsize inverter generators, meanwhile, are perfect for your RV.

If you’re shopping for a generator for use in your home as backup power, you can consider a medium-sized generator. You may also want to consider a stationary whole-house standby generator.

However, there are more than a few factors to consider when choosing a generator.

  • Too Small, you’ll not have enough power supply to properly run your appliances. This may result in generator overload, which can damage both your generator and the tools and devices.
  • Too Large, you’ll have to uselessly overpay for both the unit as well as the operational cost.

Focus on the Frequency of Use

You might also want to factor in the frequency of which you’ll be using the generator. Doing so will let you select the right size you’re sure to really meet your needs whenever they arise.

If you want a generator as backup power for your home, consider how common power outages in your area are. When the lights in your neighborhood often go out, probably because of natural disasters, large standby generators can prove a great investment. Otherwise, go with small portable ones.

If you’re shopping for RV generators or to take for your camping activities, ask yourself how frequently you will be doing these activities. Small to medium generators can do the task for you.

That being said, the power your generator needs to produce will depend on the type and number of electrical appliances you want to run. But before you start calculating the overall wattage that you’re going to need from your generator, you need to understand some key terms.

Understanding Starting Wattage, Running Wattage, and Surge Wattage

Did you know that all generators, including home backup generators and portable generators on construction sites, come with two very important power ratings that a generator user must be aware of.

  • Running Watts: The running wattage of an electrical appliance is the amount of electricity it needs to draw continuously to keep running. The running watts can easily be calculated using the rated voltage and current, which you can find stamped on the appliance’s user manual.

Running Watts = Rated Voltage x Rated Current.

  • Starting Watts: The starting wattage of an electrical appliance is the amount of electricity it needs to start up. Larger appliances with electric motors, such as refrigerators, furnaces, air conditioners, require a brief boost of power to get the motor to start turning. Note that most appliances take more wattage (usually three times more) to start than they do to run. Once the motor has turned on, the amount of power required gradually goes down until it reaches the specified running wattage.

Starting Watts for Appliances with Electric Motors = 3 x Running watts

  • Surge wattage: The surge wattage is the maximum power that your generator must supply to start the motors of the appliances.

Now that you have a better idea about how wattage works, you are a step closer to figuring out the right size generator that will fit your power backup needs.

Factors to Consider Before Choosing an Ideal Generator

Step 1: Make a list of Devices you Want to Power

To calculate the right size generator for your power requirements, start by listing all the important tools, equipment, and appliances you will need. Make sure to be inclusive, but more importantly, to be realistic.

If you’re calculating for home power backup, it is sage to make refrigerators and air-conditioners the top-most priority, along with lighting circuits. No matter if you stick to the essentials or go full-throttle A-Z home coverage, you’ll need to be thorough with your list.

Note that the more the number of devices you plan on powering, the more is the capacity you need from your generator.

Pro Tip: Go from room to room in your house and thoughtfully list all the important appliances. Don’t miss the trivial but important ones like a wireless router, blenders, coffee makers, etc.

Step 2: Note down Running and Starting Power of the respective items

Once you’ve identified the devices you want to run, you’ll need to make note of the wattage that each appliance requires to start and operate. Simply refer to the product’s user manual, which mentions this information right next to the serial number and service information.

To gain a deeper insight on where to find the default wattage details of any given appliance, you can check out this easy-to-use calculator on the US Department of Energy’s Energy Saver website. Please note that it only provides a rough estimate, and it is advised to refer to your specific appliance for an exact reading.

What’s more; You have to differentiate between the running wattage and surge wattage, because some appliances take more wattage (usually three times more) to start than they do to run, while others have a steady wattage from the start till the end.

As such, computing the total wattage for larger devices generally means multiplying the running wattage by 2 or 3 to get the starting wattage, and adding these numbers together.

For example, a typical coffee maker uses 500 watts to run. Its starting wattage, therefore, is 1500 (500 running wattage times 3). The total wattage is 1500 plus 500 = 2000 watts.

For easy reference, we have listed the most common household appliances and their wattage needs:

Appliance/Device Estimated Watts
Toaster 800-1500
Microwave 1000-2000
Hot Plate 750-1500
Electric Skillet 1000-1500
Toaster Oven 1200
Hair Dryer 1000-1875
Vacuum Cleaner 300-1500
Space Heater 750-1500
Refrigerator 600-800
Washing Machine 500-1000
Television 100-350
Clothes Iron 1000-1500
Household Fan 50-120
Furnace Fan 750-1200
Stereo 30-100
Cell Phone Charger 10
Blender 300-1000
Central AC 2000-4000
Water Heater 3000-4500
Water Pump 1000-2000

Step 3: Add up the total wattage

Kudos, you’re a step closer to finding the answer to “how big of a generator do I need?”. Now, simply select which appliances and systems you want to power at the same time.

Fill in the wattage numbers, including RUNNING watts and SURGE watts, in the designated worksheet columns below. Add up the running wattage of all your appliances together, and input the total in the space labeled as ‘A’. Likewise, select the individual appliance with the highest surge watts, and input the number in the space labeled as ‘B’.

Add the numbers in Spaces (A) & (B ), and you’re all set to determine the base level generator you need from that number.

Tools or Appliances Running Watts Surge Watts
3 Light Bulbs 210 0
Television 300 300
Mobile phone charger 25 0
Central Air Conditioner 2000 4000
Washing Machine 1500 2500
Radio 150 150


(A) Total Running Watts + (B) Highest Surge Watt Item


4185 + 4000 = 8185 Watts

Once you’ve identified the total wattage that you need your generator to supply, be sure to allocate a 10-20% margin so your generator can handle all your power needs without using more than 90% of its total capacity. It will also give you leeway in case you want to add more devices.

In this particular calculation, it will be safe to have a generator that produces 10kW of power to operate your devices simultaneously.

Note: The sample wattages provided in this table are simply a guide. Refer to the specific appliance for accurate wattages for your calculations.

The Verdict

Finding the best generator size for you becomes easy when you know what you need it for and when you need it. Choose the smallest generator size that can accommodate all the wattages you need, leaving enough margin just in case you need more.

For outdoor use, better go with portable ones so you won’t be stressed with transport and storage. But for home use, and in case you’re in a disaster-prone area, large generators can be your best investment.

Just remember some tips in installing your generator:

  1. Don’t use it inside your home or garage. Make sure there’s at least a 20-feet distance between your home and the equipment.
  2. You might want to consider splitting the power source between two or more smaller generators rather than using just a single one.
  3. Make sure you understand how to maintain your generator.
  4. Use a circuit breaker to avoid overheat, overload, and fires.

Now you’re all set. Tell us in the comments below what generator size you’ve opted for!