We rented a house for a couple of years while ours was being built. We only got hot water when the sun was shining on the panel. No hot water in the mornings and none at night. We had to use an immersion heater.
You always need to boost solar hot water systems with gas or electric heaters.
The value of the solar hot water system is not using the water directly from the solar collectors. The value is that the booster heater only has to heat the water once it is already 50C degrees (plus) not from 20C degrees.(how do you type the degrees sign)
It sounds like your solar collectors worked because you had hot water when the sun was out but your booster heater was a dud. Solar collectors may outlast several booster heaters.