The control unit is mounted on our inner landing next to the airing cupboard. Very usefully, it comes with three temperature sensors, one monitoring the temperature of the panels, the other a pair that measure the temperature at the top and bottom of the hot water tank. Essentially, how it works is that if the temperature of the panels is greater than the bottom of the tank (and less than 60ºC), water is pumped round the panels, heating it up. [The astute will note that this uses electricity. The really astute will note that this uses electricity when the sun is out, and we (as mentioned) have solar PV for that!]
Since we've had it installed, we pretty much don't pay for hot water between mid-March and late October, as the solar heating is generally enough to maintain enough of the tank at 40ºC or better for baths, showers and washing up. Sadly, it's not possible to buy a hot-fill washing machine any more, though, so ours does heat water itself for laundry.
I'd go so far as to suggest that even if you don't get solar water heating installed, sticking a pair of contact temperature sensors on the top and bottom of your hot water tank that you can monitor is incredibly educational. Apart from anything else, it cures you completely of setting the central heating timer to heat your water twice a day for an hour, since:
- we certainly don't use that much hot water in a day
- the boiler will heat the top of the tank to 60ºC in about 20 minutes flat (and remember, the top of the tank is where you take water from!)
Sadly, our controller doesn't talk to a PC, so as of yet we don't graph the temperatures or track if the pump is running. As ever, however, watch this space, as I have plans.