A: Solar photovoltaic panels are sometimes called solar P.V. panels to distinguish them from solar thermal panels, or solar hot-water panels, which use the heat from the sun to directly heat water. Solar thermal panels can be over 70% efficient, which sounds great compared to solar P.V. where the maximum commercially available efficiency is 22%. However, if you are using that solar electricity to power a heat-pump hot-water tank (please see page 66 in Chapter 2 of Zero Carbon Home), with its 400% efficiency, you get a total heating efficiency of 84% for the solar P.V. panel that is heating your hot water with a heat pump. This is better than the efficiency of a solar thermal panel.
Because of net-metering (please see page 82 of Zero Carbon Home), solar P.V. panels can generate the electricity in the summer, and you can use it in the winter. This is not possible with solar thermal panels, which don’t generate enough hot water in the winter and generate far too much in the summer. Also, the solar-P.V.-plus-heat-pump-hot-water-tank option has no pipes and hence cannot leak. Better overall efficiency, energy “storage” via net-metering, and no burst pipes make solar P.V., in my opinion, a far better solution than solar thermal panels.