PV solar systems convert daylight into D.C. current by means of solar panels. This D.C. current is then converted into everyday A.C. current via an inverter.
Wet solar systems (water heating), use a solar collector that has to face south and will only work with direct sunshine. Something that we are not overly blessed with in the UK. At least not anywhere near 12 months of the year!! They are not as efficient as PV solar systems and if the heated water is not used then the energy created is lost.