In my case I've got a contactor on the house power board that disconnects the hot water at night, so it's only heating off solar power. You can also get specialised solar diverters that do the same thing, but they cost literally ten times as much and only squeeze a tiny bit of extra efficiency out of the system.

If you do go down this path, make sure you insulate the crap out of the cylinder and surrounding water pipes, mine only heats once a day and that's once the sun's providing the power for it.