When sizing a generator that’ll run your air conditioner or heat pump, you’ll need to ensure that the generator can handle the maximum power (Watts) your unit may require, while also having enough extra capacity for your other appliances.
In this article, I’ll first discuss the power usage of 4-ton air conditioners and heat pumps, show you how to accurately determine it, and even how to reduce it so you can use a smaller and portable generator.
Once we have covered these aspects, I’ll guide you through the proper method of calculating the generator size you need based on the wattage requirements of your unit and other appliances.
P.S. If you’re short on time, you can scroll to the end of this article, where you’ll find a handy calculator for quick estimates.
Let’s get started.
I get commissions for purchases made through links in this post.
How many watts will a 4-ton ac or heat pump use?
Typically, a 4-ton AC/heat pump requires between 4500 and 6000 Watts of power while it’s operating, equivalent to 4.5 to 6 kiloWatts (kW). However, this range represents the “running” wattage of 4-ton units and does not account for the maximum power these units may require.
The highest power demand from a central air conditioner or heat pump occurs during startup.
In fact, during the startup phase, a 4-ton AC/heat pump might briefly require up to 30 kiloWatts (30,000 Watts) of power.
So, the key consideration isn’t just whether a generator can run your 4-ton unit but whether it can provide the capacity needed to initiate the unit’s startup. Therefore, the generator you choose should have a wattage capacity greater than the starting wattage of the unit.
So, to select the appropriate generator, the first step is to determine the starting wattage of your 4-ton unit.
How to determine the starting wattage of your 4-ton AC/heat pump?
Your 4-ton unit consists of 3 main components that draw power:
- A compressor.
- An outdoor fan motor.
- And an indoor fan motor.
The highest power usage to expect from your AC or heat pump is when both the outdoor and indoor fans are running while the compressor is starting up.
Therefore, the maximum, or “potential” starting wattage of the unit can be calculated as follows:
Potential Starting Wattage (Watts) = Compressor’s Potential Starting Wattage + Outdoor Fan’s Potential Running Wattage + Indoor Fan’s Potential Running Wattage
The compressor and condenser fan are located in the outdoor condenser unit of your system. To determine their power usage, you can refer to the electrical specifications provided on the condenser, specifically looking for Amperage and Voltage ratings.
The blower fan, on the other hand, is situated in the indoor air handler unit. To find out its power usage, you can refer to the electrical specifications on the air handler, which will provide the Amperage and Voltage ratings needed to calculate its Wattage.
Once you have the Amperage and Voltage ratings for each component, you can calculate their power usage (wattage) using the formula:
Wattage (Watts) = Amperage (Amps) x Voltage (Volts)
As an example, let’s take a look at this nameplate from a condenser unit of a 4-ton (48,000 BTU) heat pump:
On the condenser’s nameplate, look for the following specifications:
- Voltage: The voltage rating on these units is typically labeled as “208/230 Volts”. This indicates the unit’s capability to operate on either a 208 Volts 3-phase circuit or a 230 Volt split-phase circuit. Residential ACs and heat pumps usually run on a dedicated 230 Volt split-phase circuit.
- Compressor LRA (Locked Rotor Amps): This rating represents the maximum power the compressor’s motor might require during startup. The LRA rating can vary but typically falls between 100 to 140 Amps. In your case, the LRA is 117 Amps.
- Fan Motor FLA (Full Load Amps): This rating represents the maximum power the fan motor may require during operation. Indoor fan motors for units of this size typically have an FLA rating between 1 and 2 Amps. In this case, the FLA is 1.7 Amps.
Using the LRA and Voltage ratings, we can calculate the potential starting wattage of the compressor as follows:
Compressor’s Potential Starting Wattage (Watts) = Compressor LRA (Amps) x Voltage (Volts)
Compressor’s Potential Starting Wattage (Watts) = 117 Amps x 230 Volts
Compressor’s Potential Starting Wattage (Watts) = 26,910 Watts
Using the FLA and Voltage ratings, the potential running wattage of the condenser fan is calculated as follows:
Outdoor Fan’s Potential Running Wattage (Watts) = Fan Motor FLA (Amps) x Voltage (Volts)
Outdoor Fan’s Potential Running Wattage (Watts) = 1.7 Amps x 230 Volts
Outdoor Fan’s Potential Running Wattage (Watts) = 391 Watts
Now, all we need is the indoor fan’s running wattage. For that, we’ll have to take a look at the nameplate on the indoor unit:
Based on the nameplate specifications of the indoor unit, which include a Voltage of 230 Volts and a Fan Motor FLA of 6 Amps, we can calculate the potential running wattage of the blower fan as follows:
Indoor Fan’s Potential Running Wattage (Watts) = Fan Motor FLA (Amps) x Voltage (Volts)
Indoor Fan’s Potential Running Wattage (Watts) = 6 Amps x 230 Volts
Indoor Fan’s Potential Running Wattage (Watts) = 1380 Watts
Now, to determine the potential starting wattage of our 4-ton heat pump, we’ll add up the starting and running wattages of these individual components:
Potential Starting Wattage (Watts) = Compressor’s Potential Starting Wattage + Outdoor Fan’s Potential Running Wattage + Indoor Fan’s Potential Running Wattage
Potential Starting Wattage (Watts) = 26,910 Watts + 391 Watts + 1380 Watts
Potential Starting Wattage (Watts) = 28,681 Watts
In kiloWatts (kW), the potential starting wattage is:
Potential Starting Wattage (kiloWatts) = 28.68 kW
So, does this mean that you’ll need a 30 kW generator to operate your unit?
Well, not necessarily.
First of all, it is important to clarify that this potential starting power represents a worst-case scenario. In reality, your 4-ton unit should only use 10 to 12 kW of power when starting up.
However, if the outdoor and indoor fans are operating at maximum load, and if, for any reason, the starting current of the compressor does reach the LRA value, the starting wattage of your unit could potentially reach this theoretical maximum.
If this happens, and the generator is not sized accordingly, it won’t be able to start the AC/heat pump at all.
Using a 10,000 to 12,000 Watt portable generator to power your 4-ton unit is feasible, but it’s important to consider that the starting wattage of the unit should be limited to ensure reliable operation.
Limiting the starting wattage can prevent potential failures and guarantee that the generator has the capacity to start and run the heat pump.
How to reduce the starting wattage of 4-ton AC/heat pump?
One effective solution to reduce the starting wattage of your 4-ton AC/heat pump and make it compatible with a smaller generator is to install a soft starter kit on the compressor.
A soft starter device, like the MicroAir EasyStart 368 (ASY-368-X48), can significantly limit the current required by the compressor during startup to approximately 25% – 35% of the LRA value:
As a result, the potential 30 kilowatts of power required by your 4-ton unit during startup can be reduced to only 8 – 10 kilowatts. This would in turn allow you to use a much smaller and more portable generator.
Having the option to use a smaller generator not only lowers the initial cost of the generator but also makes your setup more fuel-efficient, saving you money in the long term.
By installing a soft starter kit, you can ensure the reliable operation of your 4-ton AC/heat pump while using a smaller and more cost-effective generator.
Here’s a video that shows the process of installing one of these EasyStart kits, and showcases the immediate effect these devices have:
Now that we have a grasp of your 4-ton unit’s power usage and how to reduce it, let’s explore how to calculate the generator size you would need based on the power requirements of your unit and other appliances.
What size generator to run a 4-ton AC or heat pump?
Generators have two wattage ratings that indicate the amount of power they’re designed to handle:
- Their Running (Rated) Wattage Capacity, indicating the maximum amount of power, or watts, that the generator can continuously and comfortably output.
- Their Peak (Surge) Wattage Capacity, indicating the maximum amount of power that the generator can briefly produce if necessary. The Peak Wattage capacity of a generator is typically 110% to 130% of its Running Wattage capacity.
As explained above, for a generator to be able to operate your air conditioner or heat pump, it’ll need a Peak Wattage rating that is greater than the Starting Wattage of your unit.
Typically, a 4-ton AC 0r heat pump would require a generator that has a Peak Wattage rating of 25 to 30 kW (25,000 – 30,000 Watts) to guarantee the generator’s ability to start and run the unit.
But if the 4-ton unit is equipped with a soft starter kit, a generator with a Peak Wattage of 8 to 10 kW (8,000 to 10,000 Watts) should be enough.
However, the power requirements of other appliances that will run simultaneously on the generator should also be considered.
The correct way to size a generator is to add the highest starting wattage that one of your devices may require, to the running wattages of the remaining appliances that’ll be simultaneously running on the generator:
Generator’s Peak Wattage > Highest Starting Watts + Running Watts of other appliances
Since central air conditioners and heat pumps tend to have the highest starting power requirements, you can determine the required capacity of your generator as follows:
Generator’s Peak Wattage > AC/Heat Pump’s Starting Watts + Running Watts of other appliances
To visualize this, and to emphasize the benefits of installing a soft starter, let’s consider the following scenarios:
Scenario 1: Without Soft Starter
Let’s assume that you have a 4-ton heat pump, and using the method I explained above, you’ve determined that your unit could potentially require as much as 25 kW of power to start up.
Let’s also assume that the generator should also be able to simultaneously run the following appliances:
- A TV that uses 200 Watts of power.
- 2 Refrigerators that, combined, use about 400 Watts.
- A freezer that uses 250 Watts of power.
- A medium-sized water heater that uses about 3,000 Watts.
- Some electronics (laptop chargers, a Wi-Fi router, etc.) that use around 200 Watts.
- A few LED lights around your home that in total require 200 Watts.
Using the formula we discussed earlier, the required Peak Wattage of the generator is calculated as follows:
Generator’s Peak Wattage > AC/Heat Pump’s Starting Watts + Running Watts of other appliances
Generator’s Peak Wattage > AC/Heat Pump’s Starting Watts + TV Watts + Refrigerators’ Watts + Freezer Watts + Watter Heater Watts + Electronics’ Watts + Lights’ Watts
Generator’s Peak Wattage > 25,000 Watts + 200 Watts + 400 Watts + 250 Watts + 3,000 Watts + 200 Watts + 200 Watts
Generator’s Peak Wattage > 29,250 Watts
Generator’s Peak Wattage > 29.25 kW
In this scenario, you’d require a generator with a Peak Wattage rating of more than 29.25 kiloWatts, which would likely be a large, stationary generator.
Scenario 2: With Soft Starter
Now, let’s assume you’ve installed a MicroAir EasyStart 368 (ASY-368-X48) on your heat pump’s compressor, conservatively limiting its starting power to 9 kW (9,000 Watts).
Let’s now recalculate the size of the generator:
Generator’s Peak Wattage > AC/Heat Pump’s Starting Watts + Running Watts of other appliances
Generator’s Peak Wattage > AC/Heat Pump’s Starting Watts + TV Watts + Refrigerators’ Watts + Freezer Watts + Watter Heater Watts + Electronics’ Watts + Lights’ Watts
Generator’s Peak Wattage > 9,000 Watts + 200 Watts + 400 Watts + 250 Watts + 3000 Watts + 200 Watts + 200 Watts
Generator’s Peak Wattage > 13,250 Watts
Generator’s Peak Wattage > 13.25 kW
Instead of having to use a 30 kW generator, which would definitely have been a standby, immobile generator, we could now use a much smaller portable generator.
A good example of this would be the DuroMax XP15000EH, which has a Peak Wattage rating of 15,000 Watts (15 kW) and a Running Wattage rating of 12,500 Watts (12.5 kW).
Now, to save you the hassle of trying to determine the power usage of each of your appliances, I’ve put together a generator size calculator that lets you list your appliances, estimates their running and starting power usage, and uses that to determine the capacity of the generator that you need:
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