Are Solar Diverters Necessary for Homes with ASHPs and Solar PV?

Solar Diverter

As homeowners increasingly adopt solar photovoltaic (PV) systems and air source heat pumps (ASHPs) in their pursuit of greener, more cost-efficient energy solutions, the integration and optimisation of these technologies come under scrutiny. One component at the heart of this discussion is the solar diverter.

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:

heat pump calculation

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.


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.

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1122 kWhs
Reply to  Mars
1 month ago

@Mars Question. I have been wondering about something. Is the 2.7 cop an average seasonal cop or has anyone measured specifically what is the cop when you are heating only hor water with a machine that was cold and off completelyand only come on to heat up that 5 degree difference?  When u heat you hot water we all know that the ASHP reaches not to best cop. Unfortunately I can’t mesure my cop. Can anyone maybe run a wee test if they are able, to reset the cop to 0. Have the tank drop only 5 celsius let the heatpump heat it back up and measure only the cop for this one heating cycle. The reson I have been wondering becuse we also know that Heatpumps reach better cop if they are on for a long time. In this test make sure the heatpump is completely off not running the central heating as it would be “chating". I really want to know a true cop from a completely off ASHP only turning on to make that 5 c temperature difference in the DHW tank.
Does that make sense? 

1122 kWhs
Reply to  Mars
1 month ago

@Mars that’s is great. But could u also do a second test when it only re heat the 5c mayne later during the day when it dropps, as that would a very short cycle. Thanks so much! Or just use these two results together for a more realistic cop where there is a short cycle and a long together?

1122 kWhs
Reply to  Mars
1 month ago

@Mars Thanks for doing that! That is great result.

1122 kWhs
Reply to  Mars
1 month ago

@Mars You made me curious 🤔. 
I did two things to make mine a bit more efficient. One of them will sound a little crazy I know.
Here is the crazy one. I didn’t like how much the heat loss my tank. To boost it a little, I wrapped it around with some loft insulation and put a nice old heavy curtain as an outer layer. It cut my heatloss  to half.
The other one is that after my children have their showers, I don’t let the hetpump rehat the water. I set it to come on during cheaper rates and closer to the morning when I need it. The pump will run a lot longer more efficiently as it is reheating the tank from roughly 25c-44c.

1122 kWhs
Reply to  Mars
25 days ago

@Mars No, I made one 😀 
Loft insulation all around, then put an old curtain around it to seal the fibre glass. It is located under my stair case inside the house. Not the best as it is a horizontal tank. But I have definitely halfed the heat loss even though it is inside the house.

20240503 173636


1122 kWhs
1 month ago

Yeah, my diverter is off almost all the time since we have the cheap rates available. 
I only use my immersion  for legionel and  when price go negative. So that I can make money by using the less efficient method for hot water heating. 

994 kWhs
1 month ago

I do the same, 250L tank heated to 47C overnight, no reheat required to get us through a normal days usage.

514 kWhs
Reply to  Gary
1 month ago

I’ve had an iBoost for 6 years. In total it tells me I’ve ‘saved’ 2400 kWh to date. For the first 4 years it did a reasonable job of heating the water during the summer leaving me with minimal gas bills. Since buying an EV two years ago then installing a Powerwall last February the amount of savings have dropped (of course). The Powerwall is charged first and then the water. Having said that today, overcast but with PV generation, the iBoost is currently heating the water. The PW is now at 100% but there is not enough PV output to warrant plugging in the car. The iBoost has more than recouped its purchase cost if you assume an average electric tariff over that time. If you use an average gas tariff then it’s a moot point – however it has saved CO2 emissions. I’m on IOG so do not use the iBoost overnight. The saving is marginal. It would make no sense to have one with an ASHP – unfortunately Octopus have declined to fit one (apparently I need a 14 kWh HP and they only supply up to 11 kWh). If/when the iBoost fails I will not replace it. There would likely not be any payback during its life.

24 kWhs
1 month ago

The installer of my heat pump gave me that exact advice. Forget my Eddi and use the heat pump. My wife? “I don’t care. I like the hotter showers". Happy wife, happy life

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