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Concentration of CO2 in the Atmosphere

Pairing a Solar System with Heat Pumps

A 6.3kW grid-tied solar electric system paired with two single zone Mitsubishi heat pumps in Bath, Maine. Image: Maine Solar Solutions.

George Harvey

Things are changing almost everywhere energy is used. Increasing numbers of people are switching to solar photovoltaics (PV) to power their homes. And increasing numbers are switching to heat pumps to keep their homes warm. It is natural to consider whether it is wise to power the heat pumps with PV systems.

There are a few locations where heat pumps will not work because of geology. Assuming a house is a candidate for heat pumps, however, there is no technical reason not to combine it with solar power, which might be generated at or away from the home site. There are, however, questions about whether it is practical to use heat pumps at homes that are off-grid.

We should bear in mind that a net-metered solar system does not have to provide all the power a home uses. The fact that a grid-tied house has a solar system will not argue against its having a heat pump. In fact, the solar system would not necessarily have to be changed at all, though it might be worth considering upgrading it for the additional electricity load. The simple installation of a heat pump at a home with net metering would increase the amount of purchased electricity and a decrease in costs of fuel for heating. Normally, the increase is considerably less than the decrease.

While that is fine in theory, it is always best to talk with someone with experience. We asked Sam Zuckerman, the owner of Main Solar Solutions, what experience he had in combining PVs and heat pumps. He told us the two are increasingly being used together.

Zuckerman said, “We size systems to cover 100% of electric usage. Right now, a good 50% already have heat pumps or get them installed with the solar. The benefit of installing a heat pump and offsetting the electricity with solar power is that you can cover the cost of heating your home.”

Most customers who get heat pumps keep their old oil- or gas-fueled systems in place. Once the heat pumps are installed, they rely on heat pumps most of the time to keep costs down. Many install separate smaller heat pumps for hot water as well. The solar power allows customers to leverage their energy bills. Having a heat pump means somewhat more electricity is used, so it makes sense to install a larger solar system, if possible. But when the combination of solar and heat pumps is compared with other options, the results are so clear that one observer called them a “killer combination.”

Zuckerman told us, “It seems to be very compelling for folks who are retired or looking to retire who want to have stable costs. In the past, there wasn’t any practical way to do this, for either electricity or heat, but now there is.” A lot of people are choosing to invest in solar and heat pumps because when they are used together, they allow the homeowner to control costs in a way that is impossible for people depending on fossil fuels or grid electricity.

We asked whether heat pumps make sense for a home with an off-grid system. Zuckerman told us that he had some experience there also, but it is harder to justify because heat pumps draw a lot of electricity. Not only does the system need a large solar PV capacity, it also needs a lot of battery capacity. One system he installed was for three homes on an island, two of which were summer residences. The large solar and battery system required for the three homes in the summer was enough to supply a heat pump in the winter for the home that was occupied through the year.

Zuckerman would remind anyone considering heat pumps about the importance of insulating and sealing a house. He also said he believes we are moving into an era when houses using net-zero energy are becoming the most cost-effective option.

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