They don't. You either have a solarthermal system to heat the water, or you use a tank to store hot water. I've previously installed an "iBoost" which diverted

They don't. You either have a solarthermal system to heat the water, or you use a tank to store hot water. I've previously installed an "iBoost" which diverted excess solar into an immersion heater. Heating slow(er) and storing is going to be easier than suddenly ramping up for a shower.