Generators are rated in Watts (W), or kiloWatts (kW), indicating how much Electrical Power (Watts) they can handle.
When sizing a generator for your heat pump, your primary consideration should be the maximum power, or Watts, that the heat pump may at some point require. To ensure reliable operation of the heat pump, the generator’s Wattage rating should be greater than this maximum.
Of course, It’s also important to account for the power usage of other appliances that will simultaneously operate on the generator. This ensures that the generator can effectively handle the combined load.
In this article, I’ll first discuss the power usage (Watts) of heat pumps, offer some estimates, show you how to accurately determine it, and even how to reduce it, so you can use a smaller and portable generator.
Once we’ve covered that aspect, I will guide you on using the Wattage ratings of your heat pump and other appliances to calculate the appropriate generator size. And for those in a hurry, I’ve also included a generator size calculator below for quick reference.
Finally, we will discuss the fuel costs associated with running your heat pump on a generator.
Let’s get started.
I get commissions for purchases made through links in this post.
How many Watts does a heat pump use?
Heat pumps, like various other motor-driven appliances including refrigerators, pumps, washing machines, and fans, have two Wattage specifications to consider:
- Their Rated Running Wattage: This is the continuous power usage (Watts) of the heat pump, and it represents the maximum amount of power that the heat pump uses during normal operation.
- Their Rated Starting Wattage: This is the surge power usage of the heat pump, and it represents the maximum amount of power that the heat pump may require during its startup phase. The starting wattage of a heat pump is only instantaneous, however, it may be as high as 6 times its running wattage.
Both the Running and Starting wattage of your heat pump depend on its Heat Exchange Capacity, commonly expressed in tonnage or BTU rating. Essentially, the higher the heat pump’s capacity, the more Watts it will require to start and operate.
To provide you with an initial overview, the table below categorizes heat pumps based on their capacity (Tons/BTUs) and offers estimates of their potential running and starting wattage:
| Heat Pump’s Tonnage or BTU rating | Potential Running Watts | Potential Starting Watts |
| 1 Ton (12,000 BTUs) | 1500 – 1700 Watts | 6000 – 7000 Watts |
| 13,500 BTUs | 1600 – 1900 Watts | 6800 – 8000 Watts |
| 15,000 BTUs | 1700 – 2000 Watts | 7500 – 9000 Watts |
| 1.5 Tons (18,000 BTUs) | 2100 – 2500 Watts | 9000 – 11000 Watts |
| 2 Tons (24,000 BTUs) | 2800 – 3400 Watts | 12000 – 14000 Watts |
| 2.5 Tons (30,000 BTUs) | 3400 – 4000 Watts | 15000 – 18000 Watts |
| 3 Tons (36,000 BTUs) | 4000 – 4800 Watts | 18000 – 22000 Watts |
| 3.5 Tons (42,000 BTUs) | 4600 – 5400 Watts | 21000 – 26000 Watts |
| 4 Tons (48,000 BTUs) | 5500 – 6500 Watts | 24000 – 29000 Watts |
| 5 Tons (60,000 BTUs) | 6500 – 8000 Watts | 30000 – 36000 Watts |
Related: How much electricity does a heat pump use?
When sizing a generator that’ll run your heat pump, you should consider the “Potential” Starting Wattage of the unit. This figure represents the maximum amount of power the unit might demand during startup, and the generator should be appropriately sized to accommodate it.
This is particularly the case for non-inverter heat pumps.
For inverter heat pumps, also known as variable speed heat pumps, the starting wattage will not exceed the running wattage range.
This is because these types of heat pumps can regulate their motor speed, and instead of requiring that initial surge in power usage during startup, their power usage will slowly ramp up until it reaches the running wattage range.
But it’s important to clarify that the “Potential” Starting Wattage figures provided in the table represent maximums and worst-case scenarios. They may not accurately reflect the actual power requirement of these heat pumps during startup.
Let me elaborate on this.
Your heat pump has 3 main components that draw electricity:
1- The compressor
The compressor is situated in the condenser (outdoor) unit of your heat pump. It functions as a motor, pumping and compressing the refrigerant flowing through the coils to facilitate heat exchange.
Compressors have two crucial electrical specifications:
- RLA Rating (Rated Load Amps): This rating signifies the maximum amount of current (measured in Amps) that the compressor should draw when it’s running.
- LRA Rating (Locked Rotor Amps): The LRA rating represents the maximum Amps the compressor might require to overcome its initial “locked” state and gain momentum during startup.
Manufacturers typically specify both of these ratings on the nameplate of the condenser unit, except for inverter heat pumps, which have only an RLA rating.
2- The Condenser Fan
Condenser fans, also known as Outdoor fans, are located on the condenser unit of your heat pump to allow heat to be exchanged with the exterior.
The outdoor fan’s Full Load Amps (FLA) rating is specified on the condenser unit, representing the maximum current the fan should draw during operation.
3- The Air Handler Fan
Air handler fans, also referred to as blower fans or indoor fans, are situated in the indoor unit of your heat pump and are responsible for recirculating and conditioning the air inside your home.
The indoor fan will also have an FLA rating, typically specified on the air handler unit.
Now, when you activate your heat pump, the outdoor and indoor fans will typically start before the compressor does.
Since these fans will be already running when the compressor kicks in, you can calculate the maximum current (measured in Amps) your heat pump might require as follows:
Potential Starting Amps = Compressor LRA (Amps) + Outdoor Fan Motor FLA (Amps) + Indoor Fan Motor FLA (Amps)
Since Power (measured in Watts) is the result of Current (Amps) multiplied by Voltage (Volts) (Watts = Amps x Volts), you can calculate the “Potential” Starting Wattage of your heat pump using this formula:
Potential Starting Watts = Potential Starting Amps x Voltage (Volts)
Let’s clarify this with an example involving a 4-ton (48,000 BTU) heat pump.
The image below displays segments of the nameplates on the outdoor and indoor units of our 4-ton heat pump, emphasizing the important specifications.
Using these specifications, we can calculate the Potential Starting Amperage of the heat pump as follows:
Potential Starting Amps = Compressor LRA (Amps) + Outdoor Fan Motor FLA (Amps) + Indoor Fan Motor FLA (Amps)
Potential Starting Amps = 109 Amps + 1.2 Amps + 6 Amps
Potential Starting Amps = 116.2 Amps
The manufacturer specifies 208/230 Volts as the voltage for these components, indicating that the heat pump and its parts are designed to operate on either:
- A 208 Volt 3-phase circuit.
- A 230 Volt split-phase circuit.
In the U.S., residential heat pumps typically run on a dedicated 230 Volt circuit.
To determine the Potential Starting Wattage of the heat pump, you need to multiply its Potential Starting Amps by 230 Volts:
Potential Starting Watts = Potential Starting Amps x Voltage (Volts)
Potential Starting Watts = 116.2 Amps x 230 Volts
Potential Starting Watts = 26,726 Watts
It’s crucial to emphasize that this figure represents the “Potential” starting wattage, representing a worst-case scenario. In reality, this 4-ton unit should not, and will not necessarily require the full 26 – 27 kilowatts every time it starts up.
Typically, this unit will only need around 12 to 15 kilowatts (12,000 – 15,000 Watts) of power during startup and about 5 kilowatts (5,000 Watts) during regular operation.
Nevertheless, as indicated in its electrical specifications, the heat pump might require the amount of power we’ve calculated at some point. To ensure its smooth operation and avoid potential issues, the generator should be appropriately sized.
For heat pumps that are 2-ton (24,000 BTU) or smaller, dealing with startup power requirements might not be an issue. However, for 3-ton heat pumps or larger, you may face limitations in your choice of generators, likely necessitating a standby (whole-house) generator instead of a portable one.
Standby generators come with several advantages, but they are stationary, more costly, require installation fees, and although they offer more power range, the extra watts will only be useful in those instances when the heat pump’s compressor is starting up.
Plus, as I’ll later explain, having an oversized generator is inefficient in terms of fuel consumption.
Alternatively, you have the option to reduce the startup power demands of your heat pump using a device like a soft starter, which would in turn give you the option to use a smaller, and portable generator.
See, a device such as the MicroAir EasyStart can be installed on the condenser unit of your heat pump to limit the compressor’s initial inrush current (Amps) to 20 – 30% of the LRA value. This reduction can potentially decrease the heat pump’s starting wattage by 75%.
For example, take a look at the effect that a MicroAir EasyStart 368 (ASY-368-X72) had on the starting current of the compressor on a 5-ton central heat pump:
Nevertheless, before deciding on a soft starter, ensure it is compatible with your heat pump’s specifications, including Voltage and BTU ratings. It’s also recommended to have a professional handle the installation for you.
Here’s a video that demonstrates the installation of one of these soft starters on a 5-ton heat pump, along with before and after amperage tests:
Now, what we’ve been primarily discussing is the power usage of your heat pump. While it represents a significant portion of your electrical load, you should also consider the power consumption of your other appliances.
In the following section, I’ll explain how to use the wattage of your heat pump in conjunction with the wattage of your other appliances to calculate the appropriate generator size for your needs.
How to size a generator that’ll run your heat pump and other appliances?
Generators have two wattage ratings that represent their power capabilities:
- A Rated, or Running Wattage rating: This rating indicates the maximum amount of power that the generator can continuously and comfortably supply.
- A Peak, or Surge Wattage rating: This rating indicates the maximum amount of power that the generator can briefly supply if necessary. The Peak Wattage of a generator is typically 110% to 130% of its Rated Wattage.
When sizing a generator that can run your heat pump, you’ll need to ensure that its Rated Wattage is greater than the maximum Running Wattage of the heat pump. But more importantly, you’ll need to ensure that the generator’s Peak Wattage is greater than the Starting Wattage of the heat pump.
For instance, consider a 3-ton heat pump that may require up to 4500 Watts of power in normal operation, and up to 20,000 Watts during startup.
To guarantee the operation of the heat pump, we’d need a generator that has a Peak Wattage rating of 20,000 Watts (20kW) or more, which would typically mean a Running Wattage of 15,000 – 18,000 Watts.
The problem with this is that the 10,000+ extra Watts that the generator offers would only be useful when starting the heat pump, and for the other 99% of the time, we’d have a generator that is 4 times oversized.
While using a generator significantly oversized isn’t unheard of, it’s still suboptimal and inefficient.
See, when a generator operates at just 25% of its capacity, it consumes twice as much fuel per unit of energy produced compared to when it operates at 100% capacity. To avoid this inefficiency, using a soft starter kit can be a cost-effective solution. As it can potentially reduce the required generator size by up to 75%.
To demonstrate this, here’s a table that estimates the size of the generator (Peak Wattage) that you would need to run a heat pump based on its capacity (Tons/BTUs), and whether or not it is fitted with a soft starter:
| Heat Pump’s Tonnage or BTU rating | Required Generator Peak Wattage without a Soft Starter | Required Generator Peak Wattage with a Soft Starter installed |
| 1 Ton (12,000 BTUs) | 7000 Watts+ | 2500 Watts+ |
| 13,500 BTUs | 8000 Watts+ | 2800 Watts+ |
| 15,000 BTUs | 9000 Watts+ | 3200 Watts+ |
| 1.5 Tons (18,000 BTUs) | 11,000 Watts+ | 3800 Watts+ |
| 2 Tons (24,000 BTUs) | 14,000 Watts+ | 5000 Watts+ |
| 2.5 Tons (30,000 BTUs) | 18,000 Watts+ | 6500 Watts+ |
| 3 Tons (36,000 BTUs) | 22,000 Watts+ | 7500 Watts+ |
| 3.5 Tons (42,000 BTUs) | 26,000 Watts+ | 9000 Watts+ |
| 4 Tons (48,000 BTUs) | 29,000 Watts+ | 10000 Watts+ |
| 5 Tons (60,000 BTUs) | 36,000 Watts+ | 12500 Watts+ |
Related: What size generator to run an air conditioner?
It’s important to clarify that the generator sizes presented in the table are based on the assumption that no other appliances will be running simultaneously when the heat pump starts up.
The correct way to size a generator is to add the highest starting watts that one of your appliances may require to the running watts of the rest of the appliances that’ll be simultaneously running on the generator.
The Generator’s Peak Wattage > Highest Starting Watts + Running Watts of the other appliances
Since your heat pump is likely to require the highest starting watts, this rule can be expressed as follows:
The Generator’s Peak Wattage > Heat Pump’s Starting Watts + Running Watts of the other appliances
For example, let’s say we’re trying to size a generator that should be able to simultaneously run the following appliances:
- A 3-ton (36,000 BTU) heat pump that, based on its “Compressor LRA” rating, could potentially demand up to 21 kW (21,000 Watts) to start up.
- A medium-sized electric water heater that uses about 2500 Watts.
- A large refrigerator that uses about 300 Watts.
- A large freezer that uses about 400 Watts.
- A TV that uses about 150 Watts.
- A few LED lights that use about 200 Watts in total.
Now, let’s consider two different scenarios.
Scenario 1:
In this scenario, our heat pump is left as it is and is not equipped with a soft starter.
The Peak Wattage of the generator is calculated as follows:
The Generator’s Peak Wattage > Heat Pump’s Starting Watts + Running Watts of the other appliances
The Generator’s Peak Wattage > Heat Pump’s Starting Watts + Water Heater’s Watts + Refrigerator’s Watts + Freezer’s Watts + TV’s Watts + Lights’ Watts
The Generator’s Peak Wattage > 21,000 Watts + 2500 Watts + 300 Watts + 400 Watts + 150 Watts + 200 Watts
The Generator’s Peak Wattage > 24,550 Watts
Now, this figure is calculated based on the “potential” starting watts of the heat pump.
As explained above, a smaller generator may be able to start and run this heat pump and the rest of the appliances. However, to avoid any potential failures, the generator would have to have a Peak Wattage rating of 24,550 Watts or more.
Scenario 2:
For this second scenario, let’s say we’ve fitted the heat pump’s compressor with a MicroAir Easystart (ASY-364-X36), and let’s conservatively assume that the starting wattage of the unit is now limited to about 8500 Watts.
Let’s now recalculate the Peak Wattage of the generator that we’d need:
The Generator’s Peak Wattage > Heat Pump’s Starting Watts + Running Watts of the other appliances
The Generator’s Peak Wattage > Heat Pump’s Starting Watts + Water Heater’s Watts + Refrigerator’s Watts + Freezer’s Watts + TV’s Watts + Lights’ Watts
The Generator’s Peak Wattage > 8500 Watts + 2500 Watts + 300 Watts + 400 Watts + 150 Watts + 200 Watts
The Generator’s Peak Wattage > 12,050 Watts
By upgrading the heat pump, we were able to reduce the size of the generator that we needed by half. Instead of having to use a standby generator, we now have the option to use a smaller and portable generator.
A good example of this would be the DuroMax XP13000HX, which has a Peak Wattage of 13,000 Watts and a Running Wattage of 10,500 Watts when operating on gasoline, or a Peak Wattage of 12,350 Watts and a Running Wattage of 9975 Watts when operating on propane.
Now, to save you the hassle of having to determine the wattage of each of your appliances, I’ve put together a generator size calculator that lets you list your appliances, provides estimates of their wattages, and uses that to determine the size of the generator that you need:
How much fuel will a generator consume to run a heat pump?
The amount of fuel that a generator would consume running your heat pump will depend on the energy consumption of the heat pump, which itself depends on factors such as:
- The capacity of the unit (tonnage/BTUs).
- Whether the heat pump is cooling or heating.
- The usage duration.
Note: Power and energy represent different aspects of electricity and are not to be confused. Learn more here.
For example, a 3-ton (36,000 BTU) heat pump consumes around 2 kiloWatt-hours (kWh) of energy per hour of use in the cooling season and about 4 kWh per hour in the heating season.
In comparison, a 5-ton (60,000 BTU) heat pump will consume around 3.5 kWh per hour of use in the cooling season but may consume up to 7.5 kWh of energy per hour in the heating season.
Click here to learn more about the kWh usage of your heat pump.
However, other than the energy consumption of your heat pump, the amount of fuel that’ll be required to run it on a generator will also depend on how much of the generator’s capacity (wattage) you’re using.
The closer you are to maxing out the generator, the less fuel (gallons, liters, pounds, cubic feet, cubic meters) the generator consumes per unit of energy (kWh) that it produces. This applies to all types of fuels.
For instance, a 10kW (10,000 Watt) generator will consume the least amount of fuel per kiloWatt-hours (kWh) of energy that it produces when it’s generating 10,000 Watts of power.
Before I clarify this further, here’s a table that estimates the amount of fuel that generators consume to produce 1 kilowatt-hour (kWh) of energy, operating at different percentages of their capacity, for different types of fuel:
| 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 (0.16 – 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) |
Please note that these figures are rough estimates representing the amount of fuel consumed per unit of energy (kWh) produced, regardless of how big the generator is.
Bigger generators with greater capacity ratings will naturally consume more fuel producing more power, but the quantity of fuel consumed per unit of energy produced will be roughly the same.
For example, a 10,000-watt (10 kW) gas generator producing 10,000 Watts of power, will consume approximately 1.8 gallons of gas per hour and will produce 10 kWh of energy per hour.
A 5,000-watt (5 kW) gas generator producing 5,000 Watts of power, will consume approximately 0.9 gallons of gas per hour and will produce 5 kWh of energy per hour.
The 10 kW generator consumes twice the amount of fuel per hour compared to the 5 kW generator, but in turn, produces twice the amount of energy. Since both of these generators are operating at 100% capacity, they’ll consume roughly the same amount of fuel per kWh of energy that they produce.
1.8 Gal/hour ÷ 10 kWh/hour = 0.9 Gal/hour ÷ 5 kWh/hour = 0.18 Gal/kWh
However, if the 10 kW generator now only produces 5,000 Watts of power, which represents 50% of its capacity, it will use more like 1.2 gallons of gas per hour to produce 5 kWh of energy per hour. Operating at 50% capacity, the generator will now consume more fuel per kWh that it produces:
1.2 Gal/hour ÷ 5 kWh/hour = 0.24 Gal/kWh
This is another reason that you should consider installing a soft starter kit on your heat pump, as it would reduce the size of the generator that you need and make you more fuel-efficient.
