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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?

If you need a generator for home, you need more wattage to run multiple home appliances. Large dual fuel 10000-watt units like Westinghouse WGen9500DF are the best choice for homeowners.
It that’s a generator for RV you’re looking for – Champion 3400 Dual Fuel Inverter is a great choice. This quiet 3000-watt inverter generator can run everything in your travel trailer, including an AC, fridge, TV, and lighting circuits.

Use our generator wattage calculator to get a rough estimate of how many watts you need.

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

The most critical figure is the generator’s power output.

We measure it in watts (W). 

Small camping generators provide anywhere between 1,000 to 3,000 Watts.

Heavy-duty generators for home and construction sites can deliver more than 10,000 watts.

That amount of power is usually enough to run the most critical household appliances as well as demanding electric tools.

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

If you need a generator for your home, list all critical appliances you have to run during an emergency power outage.

Refrigerator, freezer, AC, and lighting circuits are some of the items you should include on this list.

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 – this is the amount of power the appliance needs to start up.

Appliances with motor usually require more wattage to kick off. One prime example is a window air conditioner.

Running watts (or rated watts) – these mean how much energy the appliance needs to run after the initial startup.

To give you a better example – a typical 10,000 air conditioner consumes 2,200 starting watts. That’s how much it needs to start up for the first few seconds. After that, its energy demand goes down. The AC then needs a constant supply of 1,500 running wattage to run.

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

In the above case, an inverter generator with 2,200 starting watts and 1,800 running watts such as Honda EU2200i could run the 10,000 BTU AC.

However, you won’t be able to use a generator with 2,000 starting watts and 1,600 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 slightly bigger than your needs

It is always a good idea to buy a generator offering some extra wattage. That 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 extra watts available than to end up with a generator that suddenly shuts off due to the overload.

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

Third – A generator running at a 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 that can provide at least 1,800 rated wattage. If the total running wattage of your selected appliances is 6800 and starting wattage is 8200 – get a generator with 7,500 rated and 9,500 starting watts.

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

What Size Generator Do I Need?

Just to be 100% on the safe side, you should manually check the exact wattage of your home appliances.

Those are usually listed on their labels and expressed either in watts or amps.

Continue reading to learn the difference and how to calculate these.

These will help you determine what size generator you need.

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 needs may vary.

There is one question you should 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 an excellent 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 heavy-duty portable generator to power these.

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

Watts vs. Amps vs. Volts – What is the Difference?

Amperes (Amps) (A) and watts (W) are merely 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, some electrical tools run at a higher voltage (most common being 220V). MIG or TIG welders are the most notable ones.

Each device has its energy consumption listed on its label. It is 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 a case, you would need a generator with at least 8,400 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 we would not recommend it.

That is because conventional heavy-duty generators usually suffer from higher 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, we can’t say the same thing about most 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, most inverter generators come with a parallel capability. That means you can use two inverter generators in parallel, doubling your total power output.

Eg. if you get two Hondas EU3000iS inverter generators and use them in parallel, you would end up with 5,600 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 not the best idea. That could not only damage your household appliances and the electric grid in the entire neighborhood but in the worst cases can also result in life-threatening electric shocks.