What Size Generator Do I Need? This Calculator Will Help You

Some of the most common questions people ask themselves before buying a generator are: how big of a generator do I need? What size generator is enough to run a refrigerator and freezer? Could I power an entire home with it? How many watts do I need to power my MIG welder/sump pump/air compressor?

In order to help you determine how much wattage should your generator have, we have prepared the generator wattage calculator. How to use it? First, choose where do you want to use a generator – home backup, camping or work? Then select all the appliances you want to run. The calculator will show you the total required starting and running wattage. Swipe down to see our recommended generators meeting your power demands. The list updates automatically based on your selections.

How to calculate what size of a generator is best for my needs?

First of all, generators are not measured based on the size of their engine but on their total power output. It is true that in most of the cases the bigger generator, the more power it can produce but this is not always the case.

The power output is measured in watts (W). Larger generators (above 10,000 watts) are often measured in kilowatts (kW, 1 kW = 1000W).

Now you need to estimate your energy needs and find out how much total wattage you need.

Step 1 – Make the list of devices you want to run

For homeowners: ask yourself first if you really need ALL of your appliances to run at once during an emergency power outage? Or perhaps only the most essential ones? Remember, the bigger generator you buy, the more you will have to spend on it. It’s often not worth to spend a few thousand bucks on a generator which you intend to run only as an emergency backup power source for a few days a year.

Step 2 – Use the calculator above to calculate the total power requirements for your selected appliances.

There are two figures you should pay attention to:

Starting watts (sometimes also referred to as surge watts) – these refer to the amount of power the appliance consumes the moment you turn it on.

Running watts  (or rated watts) – these mean how much energy the appliance needs to run continuously after you switch it on.

For example, a typical 10,000 air conditioner consumes 2200 starting watts (that’s how much it needs to switch it on). After that, it needs the constant supply of 1500 running wattage to run.

When choosing a generator you need to pay attention to BOTH numbers.

In the above case, an inverter generator with 2200 starting watts and 1800 running watts such as Honda EU2200i can run the 10,000 BTU AC.

However, you won’t be able to use a generator with 2000 starting watts and 1600 running watts to power it, even though it meets the running wattage requirements. The starting wattage provided by a generator is simply not enough in this case.

Step 3 – Choose a generator which is slightly bigger than your needs

It is always a good idea to buy a generator which offers some spare wattage. This is for a few reasons.

First – it’s always better to have a generator which is slightly too big than the one which is too small. It’s much better to have a few hundred spare watts available than to end up with a generator which suddenly shuts off due to the overload.

Second – Running a generator at full load can significantly shorten its lifespan.

Third – A generator running at 100% load produces much more noise. It can be a real nuisance, especially if you are planning to use it for camping or RV.

Therefore, if you calculate that your power requirements total 1600 running watts, get a generator which can provide at least 1800 rated wattage. If the total running wattage of your selected appliances is 6800 and starting wattage is 8200 – get a generator with 7500 rated and 9500 started watts etc.

How Big of a Generator Do I Need to Run the Whole House?

This is a very common question for which sadly there is no simple answer. It all depends on how big is your household and which appliances you want to run.

Check out the graphic below to get an idea of what would be your total wattage need.

What can different sizes of generators power

Image source: PopularMechanics.com

Use our calculator to get custom precise estimates and find out which generators are powerful enough.

Note: to be 100% on the safe side you should ideally manually check the exact power requirements of each of your appliances. Those are usually listed on their labels and expressed either in watts or amps. See below to learn the difference and how to calculate these.

The figures in the calculators are the average estimates and should be used to get a better idea about your power requirements. However since every house and the electric device is different, your individual requirements may vary.

There is one question you should absolutely ask yourself before buying a generator:

Do I need the one to power the entire house OR just the essential equipment? 

Do you live in a disaster-prone area where power outages are a common thing? If yes, then getting a larger generator even though it might be more expensive, would give you additional peace of mind and could prove to be a good long term investment.

On the other hand, if you only plan to run a generator a few times a year in case of sudden emergency blackout – you could save a lot of money by purchasing a smaller generator capable of running the essential equipment.

An air conditioner, a freezer, refrigerator, pressure pumps, lights, computers, and TVs are some of the appliances you might want to be able to run during a power outage. You don’t need an expensive standby whole house generator or a bulky ultra duty portable generator to power these.

A portable generator with around 7500 running watts of power would totally suffice.

Watts vs Amps vs Volts – what is the difference?

Amperes (amps) (A) and watts (W) are simply different units used to calculate the total power consumed or produced by electric equipment.

As for volts (V) – the vast majority of electric appliances sold in the US adhere to the same standard and are rated 120V. However, there are electric tools running at a higher voltage (most common being 220V) such as certain MIG or TIG welders.

The energy demands of each device are always listed on their labels and are expressed either in amps or watts.

If you know how many amps your tool or appliance requires to run, you can easily calculate its wattage requirement too. Simply use the following formula:

Wattage = Amps x 120

Example:

You want to power a 120V MIG welder with 70 Amp draw.

70A x 120V = 8400W

In such case you would need a generator with at least 8400 running watts to run your MIG welder.

A label found at a refrigerator. This one consumes 7.10 Amps, operating at 115 VAC (Volt AC power). Its total power consumption in watts would be 7.10A x 115V = 816.5 watts. Source: Kompulsa.com

Conversely, if you know the wattage and want to calculate the amperage of your portable appliances you can use this formula:

Amps = Wattage / Voltage.

Can I use a large portable generator to power sensitive electronics?

Technically yes but it is not recommended.

This is because conventional heavy duty generators usually produce more Total Harmonic Distortion (THD). The common consensus is that 3% THD is the maximum threshold for the power to be classified as “clean” and perfectly safe for modern electronics. State grid and standby generators operate well below this threshold. Unfortunately the same cannot be said about a lot of typical heavy duty portable generators.

Using them to run your laptops, flatscreen TVs or any other kinds of modern electronics for continuous long hours can potentially damage them.

There is good news – portable inverter generators do not suffer from the same problem and can be safely used to run sensitive electronics.

One problem with most of the inverters is that they are quite small. Don’t expect to run the whole house on a single inverter generator.

However, a lot of good inverter generators offer parallel capability. This basically means you can connect two inverter generators together and double the total amount of power you get.

Eg. if you get two Hondas EU3000iS inverter generators and use them in parallel, you would end up with 5600 running watts of power. This is already enough to power a few large essential appliances in case of an emergency. You could also safely run sensitive portable electronic equipment.

In any case, if you decide to use a portable generator, no matter if it is a conventional or inverter generator – DO NOT operate one without a TRANSFER SWITCH! 

Using a portable generator to power a house without a transfer switch is a really bad idea. It can not only damage your household and the electric grid in the neighborhood but in the worst cases can also result in life-threatening electric shocks.

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