Can you use a solar water heater at night?

This is the most important thing for me. I want to get a solar heater. I only want to use it at night and in the morning. That is when the normal water is very cold. But I have read that solar water heaters actually heat the water then and there using sun-light.

water heater

Some houses in Australia do have Solar Power Perth, but the problem with this is that it cannot be depended upon. When the sun is shining it is fine, but even in Australia, it is not sunny all the time. When it is cloudy and raining the solar panels do not produce energy very well. Here in Perth, it has been raining most days for the last 10 weeks or so.