A solar thermal system may seem to be the same as solar panels, but they are quite different. While solar panels produce electricity, solar thermals heat water to be used in your hot water heater. While solar thermals can be more efficient and are reliable even on cloudy days, PV systems are often a better option than thermals.
There are a few reasons for this. First, if you have a 2 story home, pumping water up to the 2nd story can be very energy intensive. Another reason is that since the cost of PV has dropped, thermals are no longer significantly cheaper than solar panels. Also, solar panels will last longer than solar thermals. Since there are moving parts with solar thermals, the systems are more likely to break and need regular maintenance.
Often times, if you’re set on having solar heat your water, it’s more cost effective and efficient to install a PV array and switch to an electric heater.