Generators are categorized based on their wattage, which indicates the amount of power in Watts (or kiloWatts) that the generator can handle.
Therefore, a generator capable of running your 3-ton AC or heat pump should have a wattage rating higher than the maximum power consumption (measured in watts) of the heat pump, along with any other appliances that might be running concurrently.
In this article, I’ll begin by talking about how much power a 3-ton AC/heat pump uses, offering some estimates, and guiding you on how to figure this out accurately.
After we’ve covered that part, I’ll walk you through using the wattage of your air conditioner and other devices to figure out the right generator size for your requirements.
Let’s get started.
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How much power (watts) does a 3-ton AC (heat pump) use?
Depending on the model of the unit and its efficiency, a 3-ton (36000 BTU) AC/heat pump will use between 3000 and 4000 Watts of power. However, that is the “Running” Wattage of these air conditioners.
When starting up, a 3 ton unit might briefly require as much as 20,000 – 25,000 Watts (20 – 25 kW) of power.
Therefore, rather than focusing on how much power the AC or heat pump uses when running, the generator should be sized based on the potential starting power requirements of the unit. If the generator is capable of starting the unit, it’ll definitely be able to run it.
So, does this mean you’ll need a 20kW+ generator to run your 3 ton unit?
Well, not necessarily, and I will explain why in a moment.
But first, let me show you how you can determine the potential starting wattage of your heat pump.
How to determine the starting wattage of your 3 ton unit?
The main component in an AC or a heat pump, is its compressor. This component utilizes a motor to “compress” the refrigerant, enabling it to carry out heat exchange.
Now, during motor startup, a relatively higher amount of current (measured in amps) is typically needed to initiate motion. This initial current demand could be as much as 7 times the regular current that the motor draws when it’s already operational.
This initial current demand for the compressor is termed “Locked Rotor Amperage”, or LRA for short. When combined with the specified voltage, the LRA rating allows us to determine the “Potential” Starting Wattage of the air conditioner:
Potential Starting Wattage (in watts) = LRA (in amps) x Voltage (in volts)
For different AC units, the LRA rating of a 3-ton air conditioner usually falls within the range of 70 to 90 amps and is typically indicated on the outdoor unit’s nameplate.
For example, let’s take a look at this nameplate from a 3 Ton (36000 BTU) AC unit:
On the nameplate’s compressor section, the manufacturer provides these specific details:
- 208/230 VOLTS AC: This simply means the unit is capable of operating at either 208 or 230 Volts. If you have a split-phase system, as do most households in the U.S., your air conditioner will operate at 230 Volts.
- 14.4 RLA: RLA stands for Running (or Rated) Load Amps, and it indicates the maximum amp draw of the compressor when it’s already running.
- 77 LRA: As explained above, LRA refers to the “potential” starting amps of the compressor. In a worst-case scenario, this compressor could draw up to 77 Amps of current.
Using the RLA and Voltage ratings of the compressor, we can calculate the Potential Starting Wattage of this 3-ton unit as follows:
Potential Starting Wattage (Watts) = LRA (Amps) x Voltage (Volts)
Potential Starting Wattage (Watts) = 77 Amps x 230 Volts
Potential Starting Wattage (Watts) = 17710 Watts
It is worth noting that although this unit is capable of pulling 17710 Watts of power, the compressor will not necessarily require that much power whenever you turn the AC on.
Nevertheless, in a worst-case scenario, if the compressor does get “locked” for some reason, the unit’s power usage will spike to around 17 – 18 kW of power. If the generator can’t deliver this amount of power, the heat pump won’t initiate.
Now, the problem is that these 18 kiloWatts of power are only momentary; this particular air conditioner will actually only require about 3300 watts (equivalent to 3.3 kW) to operate (RLA multiplied by Voltage = 3312 watts).
If we do use an 18kW+ generator to run this 3-ton unit, we’ll end up paying for an extra 10 kiloWatts of power or more that we don’t really need. Plus, as I explain in this article, a generator that only runs at 20 to 25% of its capacity will be highly inefficient in terms of fuel consumption.
So, what is the solution?
Simple. We limit the air conditioner’s initial power needs.
How to reduce the starting wattage of your 3 ton unit?
There are two types of devices that can be added to your AC/heat pump to control the initial current when the unit starts:
- A hard start kit: these devices are generally cheaper and work by accumulating current and delivering it to the compressor when it requires that initial inrush during start-up.
- A soft start kit: these devices work by gradually starting the compressor and throttling the current until the compressor is fully started.
While soft starters are pricier, they generally offer better efficiency.
For instance, a soft start kit like the Micro-Air Easystart 364 can cut down the potential starting wattage of your air conditioner by 60% to 75%.
This means that the 18 – 25 kW potentially required by a 3-ton AC/heat pump during start-up, can be decreased to around 5 kW, allowing you to use a smaller, and portable generator.
Having the option to use a smaller generator not only saves you money and expands your choices, but will also allow you to be more fuel efficient, therefore decreasing the cost of operating the generator.
Watch this video that illustrates how to install a soft starter on your unit and showcases the impact these devices can have:
Now that we understand how to calculate the highest wattage your 3-ton unit might need and how to possibly lower that requirement, let’s find out what generator size you’ll need to power your AC/heat pump along with other household appliances.
What size generator do you need to run 3-ton AC/heat pump?
Generators come with two wattage specifications that indicate the power they’re built to provide:
- Their Running (Rated) Wattage, which indicates the amount of power (Watts) that the generator can continuously output.
- Their Peak (Max.) Wattage, which indicates the amount of power that the generator can briefly output if required.
For a generator to handle your 3-ton air conditioner, it needs a Running Wattage rating that is greater than your AC unit’s Running Wattage. However, more importantly, the Peak or Max Wattage of the generator should be greater than the Starting Wattage of your unit.
In general, if your 3-ton unit is equipped with a soft starter kit, a generator with a Peak Wattage of around 5000 Watts (5kW) should be enough. Otherwise, without the soft starter, you’d need a generator with a rating of 20000 Watts (20 kW) or higher.
Keep in mind that other devices running alongside the heat pump also count.
The correct way to size a generator is to add the highest Starting Wattage that one of your appliances might have, which in this case is likely the Starting Wattage of the 3-ton unit, and add that to the Running Wattages of other simultaneously operating appliances.
Generator’s Peak Watts > Highest Starting Watts + Running Watts of other appliances
However, it’s worth noting that while the majority of a unit’s power consumption is associated with the compressor, there are two additional components that also contribute to the power usage of the unit:
- The condenser fan, commonly known as the outdoor fan.
- The blower (air handler) fan, also known as the indoor fan.
When calculating the appropriate generator size, it’s important to account for the power drawn by these fans. As a general guideline, these fans can collectively consume up to 1000 Watts of power when operating at their full load amperage (FLA).
To visualize this, and to demonstrate the impact of using a soft starter, let’s consider the following examples.
Example 1:
For this first example, let’s say you’re trying to figure out the size of the generator needed to run the following appliances:
- A 3-ton (36000 BTU) heat pump that has an LRA of 70 Amps, making its compressor’s potential starting wattage a little above 16000 Watts.
- A washing machine, that has a starting wattage of 3000 Watts, and a running wattage of 500 Watts.
- A refrigerator, that has a starting wattage of 1000 Watts and a running wattage of 150 Watts.
- A freezer, that has a starting wattage of 2500 Watts and a running wattage of 400 Watts.
- A TV that has a running wattage of 100 Watts.
- A few lights that together pull about 300 Watts.
- And of course, the condenser fan and the blower fan, which can use up to 1000 Watts of power.
Now, the highest Starting Wattage, in this case, is that of the heat pump’s compressor (16000 Watts). Let’s add that to the running wattages of the rest of the appliances to calculate the size of the generator that we’d need:
Generator’s Peak Watts > Highest Starting Watts + Running Watts of other appliances
Generator’s Peak Watts > Compressor’s Starting Watts + Running Watts of the washing machine + Running Watts of the refrigerator + Running Watts of the freezer + Running Watts of the TV + Running Watts of the lights + Wattage of the Outdoor and Indoor fans
Generator’s Peak Watts > 16000 Watts + 500 Watts + 150 Watts + 400 Watts + 100 Watts + 300 Watts + 1000 Watts
Generator’s Peak Watts > 18,450 Watts
With this much starting power, a single portable generator will probably not be an option, we’d need to install a standby generator such as the Generac 7043.
Example 2:
Let’s say the 3-ton unit is now equipped with a soft starter kit, and let’s conservatively assume that its compressor now only requires up to 5000 Watts to start up.
Let’s recalculate the required Peak Wattage rating of the generator:
Generator’s Peak Watts > Highest Starting Watts + Running Watts of other appliances
Generator’s Peak Watts > Compressor’s Starting Watts + Running Watts of the washing machine + Running Watts of the refrigerator + Running Watts of the freezer + Running Watts of the TV + Running Watts of the lights + Wattage of the Outdoor and Indoor fans
Generator’s Peak Watts > 5000 Watts + 500 Watts + 150 Watts + 400 Watts + 100 Watts + 300 Watts + 1000 Watts
Generator’s Peak Watts > 7450 Watts
With this upgrade, we can now opt for a smaller generator, such as the DuroMax XP8500EH, which has a Peak Wattage of 8500 Watts, and a Running Wattage of 7000 Watts.
If you’re uncertain about the wattage of your appliances, you can make use of our generator size calculator. This tool provides estimates for both the running and starting wattage of common household devices but also allows you to enter your own appliances and their power usage.
The calculator then uses these values to determine the suitable generator size:
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