Can a Heat Pump Water Heater Work with Solar PV?

Can a Heat Pump Water Heater Work with Solar PV? Yes, a heat pump water heater can work very well with solar PV. Solar photovoltaic panels generate electricity, and that electricity can be used to run the heat pump’s compressor, fan and controls while it heats domestic hot water. If the system is controlled correctly, the heat pump can be scheduled to operate when the solar panels are producing surplus electricity, allowing some of the property’s hot water to be heated using electricity generated on site.

How do solar PV and a heat pump water heater work together?

A heat pump water heater still needs electricity to operate, but unlike an immersion heater, it does not use electricity as its only source of heat.

Instead, the heat pump uses a refrigeration cycle to extract heat from the surrounding air and transfer it into the water stored in the cylinder.

This means the electrical input can produce considerably more useful heat than direct electric resistance heating.

For example, if a heat pump is operating at a Coefficient of Performance (COP) of 3, 1 kWh of electricity could provide approximately 3 kWh of useful heat.

If that 1 kWh of electricity comes from solar PV, the combination can be a very effective way of producing domestic hot water.

For compact domestic hot water installations, see our 100 litre wall mounted air source DHW heat pump.

For larger homes with higher daily hot water consumption, our range includes the 200 litre residential air source DHW heat pump and 300 litre residential air source DHW heat pump.

For larger properties or commercial use, see the 400 litre commercial air source DHW heat pump or 500 litre commercial air source DHW heat pump.

Can the heat pump run when there is surplus solar electricity?

Yes, and this is where the combination becomes particularly useful.

Domestic hot water demand is often highest in the morning and evening, whereas solar PV generation is normally strongest during daylight hours.

Rather than waiting until the evening to reheat the cylinder, the heat pump can be scheduled to heat the water during the day when solar electricity is more likely to be available.

Some heat pump water heaters and energy management systems can go further by responding to a signal when surplus PV generation is available.

Instead of exporting that electricity to the grid, the energy can be used to heat the hot water cylinder.

Can the hot water cylinder act like a battery?

In a sense, yes, although it stores energy as heat rather than electricity.

When surplus solar electricity is available, the heat pump can use it to heat the water in the cylinder. That hot water can then be used several hours later, after solar generation has fallen.

This is known as thermal energy storage.

A well-insulated cylinder with low standing heat loss can retain much of this heat until it is required.

It can therefore make sense to treat the hot water cylinder as part of the property’s overall energy strategy rather than simply heating it whenever the temperature drops.

Is it better to run a heat pump or an immersion heater from solar PV?

If the aim is to produce as much useful heat as possible from the available solar electricity, the heat pump will generally have the advantage.

An immersion heater supplied with 1 kWh of surplus solar electricity will provide approximately 1 kWh of heat at the point of use.

A heat pump operating at a COP of 3 could theoretically provide around 3 kWh of useful heat from the same electrical input because the remaining energy is taken from the surrounding air.

There can still be situations where a PV diverter sends surplus electricity directly to an immersion heater, particularly on an existing hot water system. However, where a heat pump water heater has suitable controls, running the heat pump from surplus PV can make better use of the available electricity.

Do you need a battery?

No. A battery is not required for a heat pump water heater to make use of solar PV.

If the heat pump operates while the panels are generating electricity, that electricity can be consumed directly within the property.

A home battery storage system can provide additional flexibility by storing surplus electricity for later use, but it adds another layer to the system and is not essential simply to run the heat pump from solar PV.

The best control strategy will depend on the size of the PV array, household electricity consumption, hot water demand, electricity tariff and whether battery storage is installed.

Is solar PV the same as solar thermal?

No.

Solar PV panels generate electricity. That electricity can power the heat pump water heater or other electrical appliances.

Solar thermal collectors use energy from the sun to heat water directly through a hydraulic circuit and usually require a cylinder with a dedicated solar heat exchanger coil.

Both technologies can contribute towards domestic hot water production, but they work in completely different ways.

Should the cylinder temperature be increased when solar power is available?

Some systems allow the hot water temperature to be temporarily increased when surplus solar electricity is available. This effectively stores additional energy in the cylinder and can increase the amount of usable hot water available later.

However, higher storage temperatures also increase standing heat loss and can reduce the heat pump’s COP because the compressor has to work harder to reach the higher temperature.

The controls should therefore balance solar self-consumption with heat pump efficiency, hot water demand and the required water hygiene strategy.

Is a heat pump water heater and solar PV a good combination?

Yes. Combining a heat pump water heater with solar PV can reduce the amount of electricity imported from the grid for domestic hot water production, particularly when the heat pump is scheduled around periods of solar generation.

The best results usually come from good controls rather than simply connecting the two technologies to the same electrical supply.

By combining solar PV generation, a high-COP heat pump, a well-insulated hot water cylinder and intelligent scheduling, the cylinder can store useful thermal energy during the day ready for showers, baths and other hot water demand later on.