Geothermal Heat Pumps and Generator Sizing

Dan Frawley

Dan Frawley

Geothermal heat pumps require more power during start up than while running.  In order to choose a backup generator, you need to calculate the starting wattage of the heat pump plus the running wattage of all other electrical components.

Be Aware of Locked Rotor Amperage

Many customers with geothermal systems also need backup generators due to remote locations or frequent power failures.  Locked Rotor Amperage refers to the current required to start the heat pump from rest, and is much larger than the current required for operation.  Unfortunately, you will need to purchase a larger backup generator.
Calculate Load / Select Generator

The most important piece of information you need to know is how much electrical power you will need for your specific situation because nothing will be as disappointing as purchasing an underpowered generator. You need to determine all the electrical devices you plan on running with the generator.

Most small electrical appliances run on 120 volts but there are larger appliances, such as a geothermal heat pump, that run on 240 volts.  It is important to note that if you need to run any 240volt devices you need to get a generator that is capable of 240-volt output.

The next step is determining the power requirement of your appliances in watts.  This can be fairly easy as most devices are described by their wattage; in the case of light bulbs the wattage is printed on the bulb itself.  If the device doesn’t provide a wattage requirement, you can determine watts by multiplying volts times amps.

If you just can’t find the power requirement of a device, look at the table in the worksheet provided to determine the wattage of the desired appliance.  Keep in mind that the table included is just a guide, and wattage may vary based on manufacturer and model.

Example Calculation

It is important to be aware that some electrical devices have starting wattage requirements that are larger than the running watts. If so, use the starting wattage requirement for the largest of the devices you want to run and the running watts for all other devices.  In other words, the starting wattage would be the Locked Rotor Amperage (LRA) of the heat pump multiplied by the volts.  Let’s take, for example the Geomax Rainier QT072 Heat Pump:

LRA = 150 Amps

LRA X 240 Volts = 36,000 Watts of 36KW

In order to have a generator large enough to start and run this heat pump, you would need a generator capable of 36KW of output with 240-volt capabilities.  Keeping in mind that this generator would only be able to accommodate the heat pump, you will most likely need a 40-45KW generator, depending on the size of your home and the amount of electrical devices, to power your home and all the electrical components that you have.

It is important to note that the wattage needed is a general calculation, and that you should consult a licensed generator dealer to accurately size a generator for your needs.

Reduce Locked Rotor Amperage by 30%

Geomax offers a product that is capable of reducing the locked rotor amperage by 30%. This product is called the Geostart.  This system reduces the amount of amps needed to start the compressor thereby reducing the electrical load on the house.  Incorporating the Geostart system will not only reduce the electric consumption of the home, but also would reduce the size of the generator needed as an electric backup to about 15-20KW.

Questions, call Eagle Mountain at 1-800-572-7831

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