Generator Sizing with an EasyStart Soft Starter | Calculation Examples

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Generator Sizing with an EasyStart Soft Starter - Calculation Examples

We often get questions about the size or performance of one generator model or another. Some people want to know about how small of a generator they can use. Others want to run two air conditioners at the same time or run an air conditioner with the 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 generators can exceed their peak ratings for the time it takes to start the motor. The real number to look at is a generators continuous capacity. This will tell you how much total power you have to actually run an air conditioner and possibly other devices at the same time. There are some caveats to this rating which we will talk about within this article.

The first thing to determine 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 printed on the plate. Add these values together to get the total current draw for your air conditioner. The two most common RV air conditioners are 13.5K BTU and 15K BTU. These units 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 shown on the manufacturers plates.

Now that we know how much we need, its time to convert that current to something we can use to compare to a generators continuous rating. Multiply the line voltage by the current requirement to translate amps to watts. Use 240 for a 220 to240 volt unit and use 120 for a 115 to120 volt unit. The RV examples above would then be 1800 and 1944 watts respectively which is 15 x 120 and 16.2 x 120.

There are some additional 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 as a result, generators then make less power. Many generators can put out their rated continuous output on a hot summer day at sea level but there are some low-quality generators out there that do not meet that specification. Reviews can help identify those that meet their ratings from those that fall short.

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 or “watts” per 1000 feet as a result. If we are using our RV air conditioner at 4000 feet then we lose 14% of our generators 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 generators continuous output rating to be sure our selection will work when we need it. The air conditioners dont actually draw more power when it is hot, it just makes the comparison a little easier to add it to the load when comparing multiple generators.

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 operating at high elevation. 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 generators rating when using natural gas, propane or other fuels since many times it will be lower than the gasoline ratings. Use these lowered ratings when comparing your power requirements.

Additional loads on the system will also add to your power 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.

Practical examples:
Home AC unit:
I want to size a generator for my home AC unit. What is the minimum size generator would I need for my compressor?

Compressor nameplate current: 20 amps RLA
Condensing fan nameplate current: 2 amps RLA
Circulation fan (on the air handler): 3 amps RLA
Total current = 20+2+3: 25 amps
Total power = 25(amps) x 240(volts) = 6000 watts
Loss operating at 2000 feet: 7% (3.5% x 2)
Required operating wattage: 6000(watts) x 1.07 = 6420 watts

A 7000-watt continuous rated generator would provide ample power to run the air conditioner. A 6500-watt generator could be used but it would be uncomfortably close on a hot day or at lower barometric pressures. Add more power if you are going to run additional loads. Typical wattages used by common appliances are often found in generator manuals and on the internet.

RV air conditioner:
I want to use a 2500-watt surge, 2200 watt rated generator with my RV air conditioner. I only camp at lower elevations below 4000 feet. Will it work?

Compressor nameplate current: RLA COOL: 12.4 amps
Fan nameplate amps: FAN RLA: 2.7 amps
Total current: 12.4+2.7 = 15.1 amps
Total power = 15.1 x 120 (volts) = 1812 watts.
Loss operating at 4000 feet: 4 x 3.5% = 14%
Required wattage: 1812 x 1.14 = 2066 watts
Generator continuous output: 2200 watts

Yes. At your full elevation, you will need to be sure other loads are off. If you operate near sea level, you will have some available power for other loads as well.

Selecting a generator when using EasyStart - Sizing Guides