How much generator do I need using EasyStart? ma-10094

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How much generator do I need using EasyStart?

We often get these questions about one generator or another. Some want to know about how small while other want to run two air conditioners or run a generator on propane. The answer gets complicated since there are quite a few things to consider. Here are some basic guidelines to help answer that question.

Generators are often rated by their peak output. This number is a relatively useless number since an air conditioner starts in a fraction of a second and many can exceed their ratings for that time. The real number to look at is a generator's continuous capacity. This will tell you how much total power you have to run an air conditioner. There are some caveats which we will talk about later.

The first thing to know is how much power does my air conditioner use? This information is printed on the name plate and found in the installation manuals. You will often find a compressor RLA (run load amps) and one or two fan RLA values. Add these values together and you get the total current draw for your air conditioner. For those of you with an RV, the two most common air conditioners are 13.5K BTU and 15K BTU. These draw 15 amps and 16.2 amps respectively on a hot summer day. There are a few that are a little more efficient but that is currently the most common current draws.

Now that we know how much we need, it’s time to convert that to something we can compare. Multiply the line voltage by the current requirement to translate amps to watts. 240 for a 220 to240 volt unit and 120 for a 115 to120 volt unit. The RV examples above would then be 1800 and 1944 watts respectively.

There are some considerations when using a generator. Generators ingest air from the atmosphere as part of their operation. Heat makes the air molecules separate so there is less oxygen and generators then make less power. Many generators can put out their rated continuous output on a hot summer day but there are some low-quality generators out there that do not meet that specification. Elevation over sea level is also a big factor. All generator engines lose 3.5% per 1000 feet of elevation. They will also put out 3.5% less power as a result. If we are using our RV air conditioner at 4000 feet then we lose 14% of our generator's power. To show this in watts, we can just multiply our air conditioner wattage by 1.14 so our 13.5K will need 2052 watts and our 15K will need 2216 watts. We can use that number to compare to our generator's continuous output rating to be sure our selection will work when we need it.

Generators are usually carbureted which is a device that meters the fuel and air into the motor. These devices are not compensated for higher elevations and can cause additional losses. If you are operating above 4000 feet, be sure to ask your generator manufacturer about that operation. Many offer high elevation kits that can regain some lost power and extend the life of your generator when operating in those environments.

Generators used with alternate fuels often have different ratings for the fuels used. Check with the manufacturer about the generator's rating when using natural gas, propane or other fuels since many times it will be lower thant the gasoline ratings.

Additional loads on the system will add to your requirements. Add the total power together to use two air conditioners on one generator. Be sure to identify other things you want to operate at the same time as the air conditioner when selecting your generator. This can include lights, a refrigerator, tv, or battery charger.

Additional example:
Home AC unit:

  • Compressor: 20 amps RLA
  • Condensing fan: 2 amps RLA
  • Circulation fan: 3 amps RLA
  • Total current: 25 amps, Total power = 25 x 240 = 6000 watts
  • Operating at 2000 feet: 6000 x 1.07* = 6420 watts

Rounding up, a 7000-watt continuous rated generator would provide ample power to run the home’s air conditioner in this example. A 6500-watt generator could be used, but it would be uncomfortably close on a hot day or at lower barometric pressures.
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*The factor of 1.07 reflects the power needed at sea level (1) and the additional power needed at a 2000 foot elevation (coming from the general principle of 3.5% lost engine power per 1000 feet of elevation). Therefore, at 2000 feet, 3.5% (lost power factor) x 2 (1000 foot increments) = 7% lost power and that translates to .07 in increased power needed by the generator to operate the compressor at this elevation.

1.07 x 6000 watts = 6420 watts total power needed from a generator to run the A/C unit, taking into account the RLA (combined compressor and fan) and the elevation.

 
 
 

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