Combining solar and a heat pump

Homes with both solar PV and a heat pump have more variables than most systems. Mixergy is designed to work with both, but the way energy is prioritised depends on system configuration, availability of solar, and heat-pump operating limits. 

Understanding how these systems interact will help explain why heating sometimes waits, switches source, or progresses more slowly. 

Optimal configuration 

In a combined solar and heat pump setup, the system is usually designed so that: 

• Solar PV provides surplus electricity during the day 

• The heat pump heats hot water when required and within its efficiency limits 

• The immersion heater acts as a fallback when neither solar nor the heat pump can meet demand 

The exact configuration depends on how your installer has set up the system, including flow temperatures, priorities, and diverter behaviour. 

Heat source selection

Correct heat source selection in the app is essential so the tank knows what sources it is allowed to use.

We recommend always using 'heat pump' as your default heat source and ensuring that the 'Use spare PV to heat tank' option is turned on (see tank settings > PV diverter schedule). 

How Mixergy decides which energy source to use 

When solar and a heat pump are both present, Mixergy considers several factors together rather than following a single rule. The tank looks at: 

• Whether surplus solar energy is available 

• Whether the heat pump is able to deliver sufficient flow temperature 

• Your selected scheduling mode 

• Safety requirements such as cleansing and frost protection 

• Whether another heat source is needed to meet demand 

If surplus solar is available, the tank may delay heating to use it rather than drawing from the grid.  If solar is insufficient and the heat pump can provide heat efficiently, the heat pump is used. If neither can meet demand, the immersion heater may be used as a fallback. 

This balancing behaviour is intentional and designed to reduce energy use while maintaining hot water availability. 

Common pitfalls in combined systems 

Some issues are more likely when solar and heat pumps are used together. Common pitfalls include: 

• Heat source not set correctly in the app 

• Solar diverter not prioritising the tank 

• Heat pump flow temperature too low for cylinder heating 

• Space heating taking priority over hot water 

• Installer limits preventing full tank heating 

• Expecting rapid heating similar to a boiler system 

These issues are usually related to system design or configuration rather than faults with the tank itself.