Generators are categorized by their Wattage, which indicates the amount of power that they are capable of generating. So, when sizing a generator that can run your air conditioner or heat pump, the first thing to consider is the maximum amount of power (watts) that the heat pump may require.
In this article, I’ll delve into the power usage (wattage) of 5-ton AC units and heat pumps, guide you through the process of determining it, and explain how you can reduce it and why you should do so.
Once we’ve covered that, I will explain how to use the wattage of your unit, combined with the wattage of your other appliances, to accurately calculate the appropriate size of the generator you need.
Additionally, for those of you who are interested, you’ll also find some estimates of how much fuel these units might consume running on the generator.
Let’s dive right in.
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How many watts does a 5-ton AC/heat pump use?
Depending on the model of the unit and its power efficiency, a 5-ton (60,000 BTU) air conditioner or heat pump will typically use between 5000 and 7000 Watts of power. However, this range only represents the “Running” Wattage of these units, or in other words, the amount of power that they use during normal operation.
During the startup phase, a 5-ton air conditioner could demand as much as 35,000 Watts (35 kW).
Let me explain.
The core component of a heat pump is its compressor—a motor that rotates and compresses the refrigerant, facilitating heat exchange.
Like any motor, the compressor needs an initial boost, or push, to initiate its operation. This push comes in the form of an instantaneous “inrush,” “surge,” or peak in current consumption (measured in Amps).
The inrush current required by the compressor when starting is significantly higher than the current needed during normal operation. Since power (Watts) results from the multiplication of current (Amps) and voltage (Volts), this surge in current during startup also reflects on the heat pump’s power usage:
Starting Wattage (Watts) = Starting Current (Amps) x Voltage (Volts)
In order for the generator to be able to run the heat pump, it should also be able to start it. Consequently, the wattage of the generator should be greater than the starting wattage of the unit.
So, how to determine the starting wattage of your 5-ton unit?
Now, several factors might influence how high of a starting current your 5-ton heat pump requires to initiate its operation.
However, using the highest possible current the unit might draw—referred to as Locked Rotor Amps (LRA)—we can calculate the maximum starting wattage for the unit. Sizing the generator based on this value guarantees the generator’s ability to consistently kick-start the AC/heat pump.
Locked Rotor Amps (LRA) is one of the electrical specifications of your unit’s compressor. It signifies the peak current the compressor could need if, for any reason, it encounters difficulty during startup due to a “locked” rotor.
As previously mentioned, the LRA rating of the compressor helps determine the “potential” starting wattage of the air conditioner:
Potential Starting Wattage (Watts) = LRA (Amps) x Voltage (Volts)
For example, let’s take a look at the specifications on this nameplate of a 5-ton (60,000) heat pump:
On the compressor section of the nameplate, the manufacturer provides the following details:
- 208/230 Volts AC: This means the compressor is compatible with both 208 Volts and 230 Volts. Since most residential air conditioners and heat pumps run on a split-phase circuit, they usually run on 230 Volts.
- 144.2 LRA: As discussed earlier, this rating signifies that if the compressor’s rotor becomes “locked,” it could need up to 144.2 Amps to “unlock” and initiate the compressor.
With these 2 specifications, the potential starting wattage of the unit can be calculated as follows:
Potential Starting Wattage (Watts) = LRA (Amps) x Voltage (Volts)
Potential Starting Wattage (Watts) = 144.2 Amps x 230 Volts
Potential Starting Wattage (Watts) = 33166 Watts
Potential Starting Wattage (kiloWatts) = 33.16 kW
Now, it is important to emphasize that the starting wattage of these units should not normally reach this level, and should only be around 12 to 15 kW.
However, as indicated by the manufacturer, the starting watts of this unit could “potentially” reach 33 kiloWatts. Other 5-ton units have an even greater “potential” starting wattage. If the generator is not sized accordingly, it could fail to start the heat pump.
So, does that mean that you would need a 33kW+ generator to start and run the AC?
Well, opting for a generator of that size would not be optimal, because the heat pump would really only require around 5.5 kW of power to run. So, even with other appliances running simultaneously, the generator would still operate at less than 25%, except for those brief instances where it has to start up the heat pump.
This would lead to excessive fuel consumption and unnecessary expenses, considering you’d be paying for an extra 20 to 25 kiloWatts of generator capacity that you don’t really need.
So, what’s the alternative?
A solution to this conundrum would be to use a soft starter kit.
See, a device such as the EasyStart Micro-Air 368 could be installed on your 5-ton unit to limit the starting wattage of the compressor. In fact, this particular device is capable of reducing the potential starting wattage of the unit by up to 75%.
This means that the size of the generator that you need to run the 5-ton unit could also be reduced by up to 75%.
Using a 10 – 15 kW generator instead of a 30+ kW generator would not only reduce the cost of the generator, and make portability an option, but would also improve fuel efficiency, which would in turn reduce fuel costs in the long run.
Here’s a video that shows one of these soft starters being installed and their impact on the starting current of a 5 ton unit:
Having explored the power consumption of your 5-ton heat pump, understood how to calculate it, and learned how to lower it, let’s now delve into using this knowledge to accurately determine the appropriate generator size.
What size generator to run a 5-ton AC or heat pump?
Generators have 2 wattage ratings that represent their power capacity:
- Their Rated or Running Wattage Capacity: This represents the maximum amount of power that the generator can continuously and comfortably produce.
- Their Peak, Starting, or Max. Wattage Capacity: This represents the maximum amount of power that the generator can briefly produce if required.
Depending on the generator, its Peak Wattage capacity is usually between 110% and 130% of its Running Wattage capacity.
When sizing a generator for your heat pump, you’ll need to ensure that the Peak Wattage of the generator is greater than the starting wattage of the unit. Consequently, this would also ensure that the Running Wattage of the generator is also greater than the running wattage of the heat pump.
As demonstrated above, in case your 5-ton (60,000 BTU) AC/heat pump lacks a soft starter kit, the generator would need a Peak Wattage rating ranging from 30,000 to 35,000 Watts (30 – 35 kW) to guarantee its ability to initiate and operate the unit.
However, with a soft starter installed on the heat pump, the required Peak Wattage of the generator could be reduced to approximately 10,000 to 15,000 Watts (10 – 15 kW).
In any case, considering your other appliances and their power usage, the generator’s wattage in both scenarios will likely need to be higher.
The correct way of sizing a generator is to ensure its Peak Wattage rating is greater than the highest starting wattage that one of your appliances might require, combined with the running wattages of other appliances that will be in use simultaneously:
Generator’s Peak Wattage > Highest Starting Wattage + Running Wattage of Other Appliances
Regardless of whether a soft starter kit is in use or not, your 5-ton AC/heat pump’s compressor is likely to have the highest starting wattage. The formula becomes:
Generator’s Peak Wattage > Compressors’s Starting Wattage + Running Wattage of Other Appliances
Now, it is important to note that while the bulk of a unit’s power usage is attributed to the compressor, there are 2 more components that’ll also be drawing power during compressor startup:
- The condenser fan, often referred to as the outdoor fan.
- The blower (air handler) fan, often referred to as the indoor fan.
The power usage of these fans should also be taken into consideration when calculating the size of the generator. As a rule of thumb, these fans will pull up to 1500 Watts of power operating at full load (FLA).
To better understand this, and to emphasize the benefits of soft starters, let’s consider the following scenarios:
Scenario 1:
In this scenario, let’s say you have a 5-ton heat pump that has an LRA rating of 135 Amps. This means that the unit’s compressor could potentially require as much as 31 kW to start up:
Potential Starting Wattage (Watts) = LRA (Amps) x Voltage (Volts)
Potential Starting Wattage (Watts) = 135 Amps x 230 Volts
Potential Starting Wattage (Watts) = 31050 Watts
Now, let’s also assume that the following appliances might all be simultaneously running on the generator:
- A refrigerator that uses 250 Watts
- A freezer that uses 400 Watts
- A TV that uses 150 Watts
- A microwave that uses 1300 Watts
- A few lights totaling 250 Watts of power
- And of course, the condenser fan and the blower fan, which could demand up to 1500 Watts.
The Peak Wattage of the generator could be calculated as follows:
Generator’s Peak Wattage > Compressor’s Starting Wattage + Running Wattage of Other Appliances
Generator’s Peak Wattage > Compressor’s Starting Wattage + Refrigerator Watts + Freezer Watts + TV Watts + Microwave Watts + Lights’ Watts + Wattage of the Outdoor and Indoor fans
Generator’s Peak Wattage > 31,050 Watts + 250 Watts + 400 Watts + 150 Watts + 1300 Watts + 250 Watts + 1500 Watts
Generator’s Peak Wattage > 34,900 Watts
To ensure simultaneous operation of all these appliances and the capability to start the heat pump if its compressor’s starting current reaches LRA, we’d need a generator that has a Peak Wattage rating of about 35 kiloWatt.
Scenario 2:
For this scenario, let’s say that the heat pump is now equipped with an EasyStart Micro-Air 368, and let’s assume that the starting wattage of the compressor has now been limited to about 10,000 Watts (10 kW).
Let’s recalculate the required Peak Wattage of the generator:
Generator’s Peak Wattage > Compressor’s Starting Wattage + Refrigerator Watts + Freezer Watts + TV Watts + Microwave Watts + Lights’ Watts + Wattage of the Outdoor and Indoor fans
Generator’s Peak Wattage > 10000 Watts + 250 Watts + 400 Watts + 150 Watts + 1300 Watts + 250 Watts + 1500 Watts
Generator’s Peak Wattage > 13,850 Watts
With this upgrade, our generator would only have to have a Peak Wattage rating of 13,850 Watts or more.
For instance, we’d be able to use a generator such as the DuroMax XP15000EH, which has a Peak Wattage rating of 15,000 Watts on gas or 14,250 Watts on propane, and a Running Wattage rating of 12,000 Watts on gas or 11,400 Watts on propane.
To make things easier for you and avoid the task of determining the wattage of each of your other appliances, I’ve created a convenient generator size calculator.
This calculator allows you to list all your appliances, including the 5-ton unit, and provides estimates for both running and starting wattages of these appliances. The calculator will then automatically determine the size of the generator that you would need:
Now, let’s discuss fuel consumption and efficiency.
How much fuel to run a 5-ton AC/heat pump on a generator?
The amount of fuel that your generator would consume running a 5-ton air conditioner or heat pump will primarily depend on the energy consumption of the unit, which itself depends on factors such as efficiency, temperature, and the quality of insulation in your home.
However, as a rule of thumb, a 5-ton air conditioner or heat pump should consume between 3 and 4.5 kWh (kiloWatt-hours) of energy per hour of operation in the cooling season. In the heating season, a 5-ton heat pump will approximately consume 5 to 7.5 kWh of energy per hour of runtime.
You can estimate the hourly energy consumption of your unit using its energy efficiency ratings (SEER/HSPF). More on this topic here.
So, how much fuel will you need to produce that much energy?
The answer is that it depends on the percentage of Running Wattage capacity that your generator will be operating at.
Let me explain.
As hinted above, the amount of fuel that a generator consumes does not only depend on how much power it is producing but also on how much of its Running Wattage capacity that amount of power constitutes.
The closer a generator is to operating at full capacity, the more fuel-efficient it’ll be.
In other words, a generator will consume less fuel (gallons, liters, or pounds) per unit of energy (kWh) that it produces operating at 100% capacity than it would consume operating at, say, 50% or 25% of its Running Wattage capacity.
For example, let’s say we have 2 generators, a 20kW generator, and a 5kW generator, and let’s say both generators are producing 5 kW of power (5,000 Watts).
While 5 kiloWatts represents 100% of the 5kW generator’s capacity, for the 20kW generator, 5 kiloWatts only constitutes 25% of its capacity. As a result, the 5kW will consume less fuel to produce the same amount of power and energy than the 20kW generator would consume.
Here’s a table that estimates the amount of fuel that a generator consumes at different percentages of its capacity, and for different types of fuel, to generate 1 kWh (kiloWatt-hour) of energy:
| Fuel Type | Consumption at 25% Running Capacity | Consumption at 50% Running capacity | Consumption at 100% Running capacity |
| Gasoline | 0.28 – 0.32 US Gal/kWh (1 – 1.2 liters/kWh) | 0.22 – 0.25 US Gal/kWh(0.83 – 0.94 liters/kWh) | 0.16 – 0.19 US Gal/kWh(0.6 – 0.72 liters/kWh) |
| Propane | 1.3 – 1.6 lbs/kWh (0.3 – 0.38 US Gal/kWh) | 1 – 1.2 lbs/kWh (0.23 – 0.28 US Gal/kWh) | 0.7 – 0.9 lbs/kWh (16 – 0.21 US Gal/kWh) |
| Diesel | 0.11 – 0.13 US Gal/kWh (0.44 – 0.5 liters/kWh) | 0.08 – 0.1 US Gal/kWh (0.32 – 0.38 liters/kWh) | 0.06 – 0.07 US Gal/kWh (0.24 – 0.28 liters/kWh) |
| Natural Gas | 0.7 – 0.9 Cu. meter/kWh (25 – 32 Cu. ft./kWh) | 0.5 – 0.65 Cu. meter/kWh (18 – 23 Cu. ft./kWh) | 0.39 – 0.48 Cu. meter/kWh (14 – 17 Cu. ft./kWh) |
For instance, let’s say you’re using a 15kW generator, such as the DuroMax XP15000EH, to run your 5-ton heat pump and other appliances. Let’s now make the following assumptions:
1- When your 5-ton unit is operating, and your other appliances are running at the same time, your total power usage is around 9000 Watts (9 kW). Assuming you’re running the generator on gas, this amount of power constitutes around 75% of the generator’s capacity, since it has a Running Wattage of 12,000 Watts.
2- At this % of its Running Wattage capacity, the generator consumes 0.2 U.S. Gallons of gas per kWh of energy that it produces.
3- It’s the heating season, and your heat pump consumes around 6 kWh of energy per hour of operation.
This means that for every hour of operation, your heat pump alone will consume 1.2 Gallons worth of gas:
Gas Consumption Rate (Gallons/hour) = Energy Consumption Rate (kWh/hour) x Gas Consumed Per kWh (Gallons/kWh)
Gas Consumption Rate (Gallons/hour) = 6 kWh/hour x 0.2 Gallons/kWh
Gas Consumption Rate (Gallons/hour) = 1.2 Gallons/hour
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