The What Size Generator Do I Need Question Answered


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generatorIn today’s article I’m going to be answering the age old question; what size generator do I need? Trust me when I say that there is more involved in answering this question than you might think. When we first moved off the grid, we made some costly mistakes because of our lack of experience. It’s my hope that some of you can learn from our mistakes and hopefully avoid making them yourselves. I’ve written about the mistakes we made here: How To Choose A Generator.

I won’t be talking about the type of generator you should buy in this article. I will, instead, share some tips about sizing a generator so that you will be able to power all the electronic devices you would like to run.

How To Size A Generator

To properly size a generator, you need to step back for a minute and take a close look at what your needs actually are. Some people’s instinct is to buy the biggest generator they can find with the logic that they will always have plenty of power for whatever they need to run. This strategy is flawed because a really big generator is going to use way to much fuel when you only need to power a small tool like an electric drill. Going this route can end up costing you much more in fuel costs than you could possibly believe.

Other people’s instincts are to take the minimalist approach and buy the smallest generator they can find with the intention of saving money on fuel. This method is also flawed because they’ll almost always end up running the generator at it’s maximum capacity. This can dramatically reduce the lifespan of the unit.

Rather that follow these examples, I would suggest that you spend a little time thinking things through before you plop your hard earned cash down on a generator that won’t work out for you.

The first thing I suggest you do is decide how you’ll be using the genset. Will you be using it as your primary source of electricity? Will you be using it occasionally to run power tools when you’re working at a remote job site? Will you be using it as a backup for your home in the event that your power goes out? Will you be using it to supplement your alternative energy charging system on cloudy days or on days when the wind isn’t blowing? All of these questions need to be answered because they play an important role in how you should size your generator.

When you have answered what the primary use of the generator will be, it’s time to do some detective work. Whatever you do, DON’T SKIP THIS STEP. This isn’t the time to be guessing! Your task for this stage of the decision making process is to make a list of every electronic device that you will be powering at the same time. To help you with this stage of the process, I’ve created a handy form that you can print out and use. I’ve also created a handy spreadsheet to help you calculate the watts needed for your devices if all you know is the amp requirements. You can download these files by clicking on either of the links below.

Free Printable Generator Sizing Form
Free Printable Generator Sizing Spreadsheet

Now, after you have listed all of the devices that you plan on running at the same time, you need to figure out how much electricity they consume. Most electronic devices will have a sticker or metal plate riveted to them that will tell you how much power the devices use. This will either be reported is watts or amps. If the device’s power requirements aren’t stated on stickers or plates, you can find this information in the owner’s manual that came with it. Record the energy requirements for each device on the form that I have provided for you in the “watts column”.

Please note that generator manufacturers most commonly rate their units with wattage ratings. For example, our Honda EU2000i has a maximum rating of 2000 watts. To simplify things, it would be a good idea to do a quick conversion for any of the tools or devices that you only have the amp rating for. The formula to convert amps into watts is Watts = Volts x Amps. So, if you have a power drill that needs 120 VAC (volts AC) power at 3 amps, it will need 360 watts to run (120 x 3 = 360).

When you are collecting this information, keep in mind that many appliances or tools that use electric motors (refrigerators, air conditioners, power tools, etc) need more energy to start than they need to continue running. This is known as the surge rating or start up rating. If you size your generator but you don’t account for the surges of power that some devices need to get started, you may find out that your generator is too small for your needs.

After you have completely filled out my generator sizing form, you’ll want to grab your calculator and add up all the the numbers that you have recorded in the watts column. Whatever this number is will be the minimum size of generator you will need. Keep in mind that a generator that is run at or near it’s maximum rated capacity all the time is going to be headed to an early grave. You should also know that generators are often rated with two rating numbers. The first number is the “continuous” rating and the second is the “surge” rating. For example, our little Honda is rated at 2000 watts (16.7A) surge but only 1600 watts (13.3A) continuous. Manufacturer’s like to list the maximum surge rating for marketing reasons but if you don’t understand this and you run your genny at the maximum surge level for more than a few minutes, you’ll probably end up damaging it.

Most generator engines run at a set RPM (revolutions per minute) which is 3600 RPM. They will run at this speed whether they are powering something as small as a light bulb or something as large as an electric welder. They have to maintain a constant speed to put out the correct voltage. If a generator is designed to output 120 VAC power and someone fiddles with the carburetor to decrease the RPM, the voltage will drop and your devices won’t run or they will be damaged.

Knowing this, you’re more prepared to pick the right size of generator. If you know that a generator is rated to continuously output 4500 watts and consume the same amount of fuel regardless of the load you put on it, buying a generator that is too large for your needs will just mean that when you are running it, you’ll be wasting fuel.

Our Solution To The Ideal Generator Sizing Dilemma

Most of you would agree that there is a tool for every job. Some tools, like adjustable wrenches, will work on different sizes of nuts and bolts but if the bolt is really stuck, an adjustable wrench will probably slip and round the corners of the bolt off. An adjustable wrench might fit on a particular bolt but a properly sized wrench really is a better tool for the job.

Our family has learned that this is also true with generators. Instead of buying a monster generator that can power everything from my husbands welder to my cell phone charger, we own several. We have the little Honda EU2000i that purrs along while just sipping gasoline, we have a cheap light-weight 1000 watt generator that is easy to carry to the far corners of our property when my husband needs to use a power tool, and we have a big 6000 watt generator that runs my husbands welder perfectly.

After living off the grid for 8 years, we’ve learned that this is what works best for our needs. Hopefully this article has helped you get a better idea of what will work best for your application.


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