This simple, free calculator helps you compare the electricity consumption of a heat pump versus a traditional immersion heater (including one powered by solar PV via a diverter). Just enter your cylinder volume, desired temperature rise, and the Coefficient of Performance (COP) of your heat pump, and the tool does the rest.

In households that utilise both solar PV systems and ASHPs, the necessity of adding a solar diverter becomes a point of debate. Could homeowners achieve greater efficiency and cost savings by simply relying on their ASHP for their hot water production?

This question becomes particularly pressing when existing solar diverters start to malfunction or fail. For instance, our Solar iBoost malfunctioned after a few years, and our Immersun device has ceased working twice in its first year. Given the substantial cost of solar diverters, it’s worth reevaluating their value in homes that use heat pumps.

The Components at Play

Solar PV System: Converts sunlight into electricity, providing a renewable source of energy to power home appliances, including heat pumps.

Solar Diverter (Immersun, Solar iBoost, eddi, etc.): Redirects surplus solar electricity not used by the household to an immersion heater in the hot water cylinder, thus utilising excess solar energy for water heating.

Air Source Heat Pump: Transfers heat from the air outside to heat water. Its efficiency is quantified by the Coefficient of Performance (COP), which in our case is 2.7. This means for every unit of electricity, our heat pump produces 2.7 units of heat.

The Efficiency Comparison

To understand the efficiency and potential savings, let’s perform a calculation comparing the electrical requirements for heating our 300-litre hot water cylinder to 45°C using both the heat pump (with its COP of 2.7) and a similar scenario using a solar diverter with an immersion heater.

Heating Water with a Heat Pump

The efficiency of a heat pump is gauged by its COP, which in our case is 2.7. Full disclosure, maths isn’t my strong point, but I think that this makes sense: to figure out the energy needed to heat water, we used the following equation:

Q=m x c x ΔT

• Q stands for the heat energy needed, measured in joules.
• m represents the mass of the water, in kilograms (for 300 litres of water, this is 300kg).
• c is the specific heat capacity of water, valued at 4.186 Joules per gram per degree Celsius.
• ΔT is the change in temperature, in degrees Celsius (in this case, the increase to 45°C).

From this, to calculate the electricity that the heat pump requires, taking its COP into account, we used:

This formula helps us determine the amount of electricity in kilowatt-hours (kWh) that the heat pump would use to achieve the desired water temperature, allowing for a straightforward comparison with the solar diverter scenario. This approach aims to provide a clear understanding of the energy requirements and efficiency of heating water using these methods, helping homeowners make informed decisions.

Heating Water with a Solar Diverter

In this scenario, the electricity generated by the solar PV system is diverted to an immersion heater inside the hot water cylinder. The energy efficiency directly relates to the amount of surplus solar electricity available and the immersion heater’s ability to convert this electricity into heat.

The Calculation

Let’s crunch the numbers for both scenarios to determine the most efficient method for heating the water to 45°C.

Through this comparative analysis, we aim to see which system — solar diverter vs. heat pump — is more electricity-efficient for heating a 300-litre hot water cylinder to 45°C. The conclusion will help determine whether replacing the solar diverter is worth the investment or if relying solely on the heat pump, powered by the solar PV system, is the more prudent choice.

Results

Upon conducting the calculations, the results are quite revealing. To heat a 300-litre hot water cylinder to 45°C:

• Using the Heat Pump: It requires approximately 5.81 kWh of electricity. This calculation considers our heat pump’s COP of 2.7, highlighting its ability to efficiently convert electrical energy into heat.
• Using the Solar Diverter: Assuming the immersion heater is nearly 100% efficient at converting electricity to heat, it requires about 15.70 kWh of electricity. This scenario does not benefit from the efficiency multiplier provided by the heat pump’s COP.

Weighing Efficiency Against Sustainability

The comparative analysis demonstrates a notable difference in electrical consumption between using a heat pump and a solar diverter. The heat pump stands out for its substantial energy efficiency, necessitating less than half the electricity to reach the same heating level as the solar diverter method. The higher the COP, the higher the difference.

It’s crucial, however, to contextualise this efficiency within the operational dynamics of both systems. Unlike heat pumps, which can heat water as needed, solar diverters operate exclusively with the availability of excess solar energy. This means that the diverter’s capacity to heat water is directly tied to solar production, limiting its heating capability to periods of sufficient sunlight. In opting for a solar diverter, homeowners essentially choose to sacrifice some level of efficiency to utilise what is effectively free electricity generated by their solar panels.

But, this raises an important question: Is this sustainable use of electricity? Leveraging free, renewable energy directly from solar panels represents a pinnacle of sustainable living by reducing reliance on grid electricity and maximising the personal use of generated solar power. However, it’s worth considering whether it might be more beneficial overall to use the heat pump for water heating and then export any surplus electricity to the grid, as it is a far more efficient use of energy. This way, the unused electricity can support the broader community, potentially offering a balanced approach to personal efficiency and collective energy sustainability.