Assuming you're installing a closed-loop, pressurized solar thermal system, a pressure gauge is absolutely essential. You can place it almost anywhere in the loop, but it's best to locate the gauge close to the charging ports.
Since a pressurized system has a solar loop which is completely filled with fluid (the industry standard being a mix of water and propylene glycol), it is very important to get all the air out of the system when you're setting up. This is doubly important because the solar fluid only needs to be checked every 10 or 15 years, so it's going to be in there a long time before you look at it again.
The single biggest use of solar thermal systems worldwide is to heat swimming pools. As a result, special technologies have appeared that facilitate this use, the best of which is a web-and-tube setup. The requirements for seasonal pool heating are much simpler than those for a whole-home, all-season solar thermal system: the water only needs to be heated to a little above ambient temperature, the collectors don't need to be climate-proof (as they're only laid out in hot months, when needed) and they don't need any glazing.
It doesn't matter whether you're installing a huge solar thermal system to heat the water for an entire community or a small residential setup to lower your power bills: you'll need a storage tank to keep hot water in. And since the sun has an annoying tendency to be weaker in winter and disappear at night, that tank needs to be insulated to keep your water hot as long as possible.
Solar thermal installations use liquid-to-liquid heat exchangers exclusively: on one side, there's the solar fluid that travels from the collector; on the other there's the water being heated.
The pumps used in solar thermal installations come in both AC and DC models. It is important to choose a model that not only fits your electrical supply but which is also compatible with the rest of the system, in terms of construction materials.
