What Size Generator Do I Need? (With Easy To Use Calculator)


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Last Updated: May 3, 2021

Power outages are one of the most common emergencies you’re likely to experience. You can sit out a short power cut, but a home generator is an essential addition to your prepping kit for prolonged power outages.

Go straight to the calculator

But what size generator do you need?

To figure this out, you need to add up the wattage of all the appliances you want it to power simultaneously, plus the highest-powered item you want to be able to use in addition to these.

For example, you may need to keep your refrigerator, freezer, well pump and lights running continuously and charge your laptop or use your microwave on top of this.

Now you could spend hours pulling out your microwave, craning your neck behind the refrigerator, and using a combination of mirrors to see behind the television to determine the exact wattage of each appliance.

Or you could use our quick wattage calculator to come up with a good estimate in 30 seconds.

Generator Wattage Calculator

Please note we have used estimated wattages. If you want an exact number, you will need to look at your own individual appliances.

This generator calculator is designed to give you a solid starting point to calculate your backup power requirements. It gives you both the running and starting watts you’ll require to run your appliances and electronics.

Our calculator assumes that you will start the appliance with the highest starting watts last rather than starting all appliances simultaneously. We discuss this assumption and other ways to make your power go further below.

Select Kitchen Appliances

  • Appliance Quantity Running Watts Starting Watts
  • Refrigerator 200 1200

  • Freezer 200 1200

  • Washing Machine 1150 2250

  • Clothes Dryer 5400 6750

  • Microwave (800w) 1300 1300

  • Dishwasher 250 600

  • Toaster 850 850

  • Electric Stove 2100 2100

  • Coffee Maker 800 800

  • Household Kettle 2200 2200

Select Heating | Cooling | Lighting

  • Appliance Quantity Running Watts Starting Watts
  • Standard Light Bulb 60 60

  • Sump Pump 800w 800 1300

  • Sump Pump 1000w 1000 2100

  • Electric Water Heater 4000 4000

  • Furnace Fan 1/2 HP 800 2350

  • Furnace Fan 1/3 HP 700 1400

  • Window AC (10,00 BTU) 1200 3600

  • Central AC (10,000BTU) 1500 4500

  • Heat Pump 4700 4500

Select Computers | Phones | TV | Radio

  • Appliance Quantity Running Watts Starting Watts
  • Radio 100 100

  • Phone\Tablet charger 12 12

  • Laptop Charger 90 90

  • Desktop Computer and Monitor 250 250

  • LED TV 120 120

What You Need to Know About Generator Sizing

Generator power output is measured in watts. However, there are two power output figures you need to be aware of when buying a generator:

Continuous power rating (running watts) – The amount of power the generator can consistently deliver.

Maximum or surge power (starting watts) – An additional boost the generator can deliver for a short time to assist with starting up appliances.

For example, a standard modern refrigerator requires 1200 watts of power to start up, but it only draws on 200 watts of power once it’s running.

You need to make sure the generator you buy is powerful enough to run your essential appliances AND be able to start them up. Fortunately, our calculator will calculate your requirements for both power outputs so you can make sure you buy a generator big enough for your needs.

Here are a few other things to bear in mind:

  • When browsing generators, the power rating given is typically the maximum power or starting watts. You may need to dig into the product description to find out the continuous power rating.
  • For dual-fuel generators, the generator will typically deliver less power (both running and starting watts) when run on propane than when fueled by gasoline.
  • Running a generator at full load can shorten its lifespan, so it’s worth overestimating your power requirements rather than planning to draw maximum power continuously.
  • When starting multiple appliances, the order in which you start them is important.

Making Your Power Go Further – Expert Tip

Smaller generators are cheaper, quieter, and more portable. When calculating the size of generator needed for your home, remember that you don’t need to run all your appliances and tools at once.

For example, you only need to turn the oven on when you’re cooking dinner, and you just need the washing machine on when you need to do laundry.

Even appliances that you think you may need to run continuously, such as refrigerators and freezers, can be turned off for a couple of hours without food spoiling. Find out how often you need to run your generator to power a refrigerator.

Thinking smart helps with the most power-hungry appliances.

An electric water heater uses around 4,000 watts of power. You may prefer to have your hot shower in the evening, but if you also need to have the cooker on for dinner and have the TV and lights running, your combined power demand will be very high.

Instead, shower at a different time of day when you can turn off all other appliances, so the water heater is the only load on the generator.

Bear this in mind when using the calculator, and you may find that you don’t need quite as large a generator as you think.

This efficiency will also make your fuel supplies last longer.

Watts vs. Amps

Watts (W) and Amperes (A) are different units used to calculate power. A device will always have its energy consumption listed on its label, but this could be in watts or amps.

If a device lists the power in amps, you can easily convert it to watts using the following equation:

Watts = Amps x Volts

In most cases, the voltage will be 120V (though some electric tools run at a higher voltage), so you need to multiply the amp rating by 120 to work out how many watts of power it requires.

Efficiency

You may wonder why your 800-watt microwave draws 1,300 watts of power from your generator. The answer is that devices and appliances are rarely a hundred percent efficient. Energy is lost to external heat and other factors, so you need additional power to account for this.

If that’s the case, how do you know what figure to use in your calculation?

For some appliances, you can look at the data tag and calculate the running watts required to power the device. To calculate the operating wattage, multiply the amperage by the voltage. For a microwave that requires 13.4A and 120V, the operating wattage would be 1,608 watts.

Resistive vs. Reactive Loads

The appliances and devices you want to run off your generator will fall into one of two categories: resistive or reactive.

  • Resistive loads: Appliances that draw a consistent amount of power from the generator. For example, light bulbs, a water heater, coffee machine
  • Reactive loads: Appliances that draw a variable load from the generator. For example fridge, freezer, water pump

Appliances such as refrigerators, air conditioning units, and water pumps contain an electric motor that requires an additional boost of power to start up. Refrigerators also have an internal fan that comes on intermittently to keep the internal temperature steady.

In our generator size calculator, reactive loads have an additional wattage in the “starting watts” column to account for this variation, reactive loads have a higher wattage in the “starting watts” column to account for this additional power draw when starting up.

Be Smart When Starting Appliances – Expert Tip

When you are running reactive loads, the order in which you start appliances is important. In our generator calculator, we assume that you will start the appliance with the highest starting watts last. This gives you a lower maximum draw on the generator than starting them together.

Here’s an example to illustrate why this is the case.

Let’s assume you want to run a refrigerator, freezer and washing machine.

 Running wattsStarting watts
Refrigerator2001200
Freezer2001200
Washing machine11502250
TOTAL15504650

If you just add the starting watts together, you may think you need a generator with a maximum rating of over 4,650 watts.

That’s only the case if you’re starting all three appliances simultaneously. If you start them individually, then you only need to run one starting load at a time.

Once you have the refrigerator running, then you can start up the freezer. The maximum draw on the generator at this point will be 1,400 watts (200 running watts for the refrigerator plus 1,200 starting watts for the freezer).

Once the freezer has dropped to its running watts, you can add the washing machine. As you’ll only need to add 400 watts (the combined running watts for the refrigerator and freezer) to the starting watts for the washing machine, this gives a maximum demand of 2,650 watts.
You could reduce power demand even further by:

  1. Starting the appliance with the highest starting watts first. E.g. if you started the appliances above in the reverse order, you would have a maximum draw of 2,550 watts on the generator (1,350 running watts plus 1,200 starting watts for the refrigerator). In practice, appliances with higher starting watts are likely to be used more intermittently, which is why we set up the calculator to start them last.
  2. Turning off other appliances and electronics when running appliances with high starting watts. You can play with the calculator above to see what difference this makes.

Can a Generator Power a Whole House?

This depends on the size of your house!

Most households will struggle to find a portable generator large enough to power every appliance and electronic device. There’s also a balance between power and cost – high-powered generators are significantly more expensive.

At some point, you’re going to need to compromise.

If you need to power a refrigerator, microwave, sump pump, and lights, then a 5,000-7,000 watt generator should be sufficient. If you want to run more power-hungry items such as a water heater or air conditioning unit, you will need to look at a 10,000-watt model.

In general, if you want to power a whole house, you will need a diesel, gasoline, or dual fuel generator; we have reviewed the best options in each category; click the links below to read more.

You may also want a smaller, more portable generator to use for camping or to power an RV if this is part of your disaster response plan.

Connecting a Generator to Your House

If you’re using a portable generator to power some or all of your house, you’ll need to install a transfer switch. This allows you to switch the power source from mains power to your generator in an emergency.

Wiring a transfer switch involves working with your home’s main electricity supply, so in most cases, you’ll need to get an electrician in to do the work. You need to get this in place before disaster strikes!

You can find more information on installing a transfer switch here.

The Best Generator for Sensitive Electronics

Conventional generators produce so-called “dirty power.” The voltage and frequency produced by the generator fluctuate, creating harmonic distortion. This isn’t usually an issue for large appliances such as refrigerators or well pumps, but it can damage sensitive electronics such as laptops and mobile phones.

And the last thing you want in a disaster is to fry your mobile…

Inverter generators also run on gasoline or propane, but their mechanics are different. The high-frequency AC produced by the engine is converted into DC using an alternator. This is then converted back to AC by the inverter. As a result, the generator’s current is much more stable and “clean,” making it a better option for powering sensitive electronics.

You may decide to invest in two generators for your home: a high-powered conventional generator for powering large appliances and a smaller inverter generator for electronics. If that’s the case, you can use the calculator twice to work out what size generator you need for each purpose.

Can I Use a Generator in an Apartment?

Conventional generators need to be situated outdoors so you can’t use them in an apartment.

If you live in a ground-floor apartment with a garden, you may be able to use a generator to power your apartment (though your neighbors may not be too happy about you doing so), but this isn’t a realistic option for most apartment dwellers.

However, there is an alternative solution. Portable indoor generators can be used inside. Instead of using fossil fuels to power them, they can be charged from mains power or a solar panel.

Indoor generators deliver much less level power compared to conventional generators. Even high spec models only generate around 2,000 watts of continuous power, so you’ll have to prioritize which appliances and electronics you want to keep running.

Use our generator wattage calculator to work out your total essential wattage and find the best indoor generator for your needs.

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