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Heat pump and power consumption: influencing factors, calculation & tips

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The operation of a heat pump is based on a refrigeration cycle process in which the heat from the tapped environment is raised to the required temperature level. This requires use of scroll or piston compressors. These work efficiently and are barely audible, but usually consume electricity. The heating power consumption of a heat pump, the factors that it depends on and how you calculate the power consumption are explained in the following sections.  

Various factors influence the power consumption of a heat pump

Depending on the heat source, a heat pump needs about 20 to 25 percent electricity as drive energy to generate heat from air, the earth or water. This means that for the production of ten kilowatt hours of usable heat, about two kilowatt hours (kWh) of power are needed. The annual consumption of a heat pump depends on various influencing factors. Among the most important are:

  • Type of heat pump
  • Seasonal performance factor
  • Heat demand

The heat source that a heat pump heating system can draw on will affect the electricity costs that are incurred. As a general rule: An air source heat pump has a higher power consumption than a brine/water or water/water heat pump. This is because soil and water give off heat relatively evenly throughout the year. Ambient air temperatures fluctuate more. More energy is needed to meet the heat demand, especially in the winter months. On the other hand, air source heat pumps can be installed almost anywhere, do not require official approval and are cheaper to purchase and install. This is because neither drilling nor digging are required for heat recovery. 

The seasonal performance factor (SPF) is needed to roughly calculate the power consumption of a heat pump. This sets the heat energy generated in relation to the electrical energy used. For example, with a seasonal performance factor of four, the heat pump produces four kWh of thermal energy from one kWh of electricity. Consequently, the higher the SPF, the more efficient and power saving the heat pump. 

Please note: The calculated seasonal performance factor is a theoretical figure. The calculation is based on standard values such as room temperature, domestic hot water consumption, climate zones and ventilation habits. In practice, the SPF may deviate from the theoretically determined value.  

In addition to the seasonal performance factor, the coefficient of performance (COP) may also be specified for heat pumps. This also represents the ratio of supplied to dissipated energy. However, the COP is only valid for a certain point in time, for example for an air temperature of 15 degrees Celsius and a flow temperature of 35 degrees Celsius. This means that it is merely a snapshot.

 

In addition to the seasonal performance factor and the heat source, the individual heat demand is a decisive factor in the power consumption of a heat pump. The level of the heat demand depends on the individual user behaviour of the occupants and the energy condition of the building. Older buildings that have not undergone modernisation, for example, have a higher heat demand than well-insulated new buildings. Last but not least, it makes a difference whether the heat pump is only used to generate space heating or also to heat domestic hot water. It is not, therefore, possible to make a blanket statement.

Calculating the power consumption of a heat pump

Three variables are needed to roughly determine the power consumption of a heat pump: heating output, seasonal performance factor and operating or heating hours. The calculation is performed with the following formula:

Power consumption of the heat pump = heating output/SPF x hours run

Sample calculation: If a brine/water heat pump with 10 kilowatts and a SPF of 4.0 runs for 2000 hours a year, it requires 5000 kilowatt hours (10 / 4.0 x 2000 = 5000 kWh).

If system owners would like to calculate the annual electricity costs, they can multiply the total by the kilowatt price:

Electricity cost of the heat pump = power consumption x cost per kWh

Heat pump electricity can reduce costs

Although the special tariffs do not reduce the heating power consumption of a heat pump, it's still possible to save on costs. This is because with heat pump electricity, suppliers have the right to temporarily interrupt the electricity supply during periods of peak load. This enables better load management. In return, the heat pump tariffs are cheaper. However, to benefit from these tariffs, a separate electricity meter must be installed so that household and heat pump electricity can be billed separately. 

Heat pump electricity does not differ characteristically and qualitatively from traditional electricity. For system owners, only two factors are important – the cost and the origin. Many suppliers now offer heat pump electricity. Some of them also have green electricity tariffs in their range. It is always advisable to compare the tariffs before concluding a contract. Once system owners have found the best heat pump tariff for them, they can change tariffs as normal, taking into account the standard notice periods. The process can be handled in exactly the same way as with traditional electricity.

What do blocking times mean for heat pumps?

If power supply utilities (EVU) disconnect the system from the grid, this is referred to as a blocking time or power-OFF for the heat pump. An electrically operated heat pump heating system cannot operate during this time. However, buffer cylinders can ensure the supply of heating energy and DHW. System operators should take this into account in their planning. As the heat pump not only has to heat but also recharge the storage unit during this time, a higher output is required. This addition can be calculated using the blocking time factor.

Blocking time factor = 24 hours / (24 hours – sum of blocking times during the day)

Sample calculation: If the power supply utility interrupts the power supply for heat pump heating three times for two hours each time, the output must be one third higher 24 / (24 - 6) = 1,3.   

The number and duration of interruptions are regulated by law. A maximum of three blocking times of two hours per day are permissible.

There are ways to influence the energy consumption of a heat pump

A large part of the energy required in a household is used to heat rooms and domestic hot water. By replacing their outdated central heating boiler with a heat pump or hybrid system, homeowners can reduce their energy demand by up to 30 percent. If, in addition to modernising their heating system, they take further measures such as hydronic balancing or replacing the thermostats, they can achieve even greater success. To minimise energy consumption, however, the radiators should also be matched to the heating system. Efficient heat pump consumption is best achieved through combination with a panel heating system.  

Can I operate a heat pump only with underfloor heating?

An underfloor heating system is an area heating system that transfers heat to the room via radiant heat. In this way, the thermal energy is distributed evenly over large areas and only releases its effect when it hits solid bodies such as walls or people. Thanks to the large surface area, underfloor heating can cope with flow temperatures of around 35 degrees Celsius.

By way of comparison, a radiator requires temperatures of up to 70 degrees Celsius. Since the efficiency of a heat pump heating system increases further the smaller the difference between the heat source and the flow temperature of the heating system becomes, operation with underfloor heating is not only possible but also recommended. That's because this combination is where the heat pump achieves maximum possible efficiency. Heat pumps in dual mode operation or hybrid heat pumps are recommended for radiator systems with higher temperatures and for older buildings. You can find out more about this in the sections on heat pumps and heat pumps in new build and older buildings.

Covering the electricity needs of the heat pump yourself

Heat pump consumption can also be covered inexpensively with solar electricity from your own roof. Photovoltaic systems generate electricity from freely available solar energy. Storage units ensure that this energy is also available to you when the sun is not shining. The consumption of self-generated power thus offers more independence from power supply utilities. However, in order to take advantage of this, the correct design is important. For more information, see the section on Photovoltaics. An example of how well heat pump, photvoltaic and storage unit complement each other and increase the efficiency of the whole system is shown in the following video: 

 

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