I am glad that these questions, since they are the ones that many people wonder about.
The life expectancy of a solar water heater depends on the quality of its construction and design.
So some of them you will only have a duration of five years and others may at the ripe age of 25 years.
Please note that separate of a solar water heater components also have a different life expectancy. So the pump can only reach last four years and the fluid heater that has to be replaced every two years.
Best thing is to buy a system with a generous warranty because shows that the manufacturer relies on your system is going to last. As you might expect, the greater the guarantee more expensive the SWH it will probably be.
In terms of performance in terms of cloudiness, no doubt suffering because the water will reach higher temperatures than when the Sun is out.
So still works, but not so well. The reinforcement of backup will have to come into play to achieve the temperature of the water to where it must be, but even so it will be much hotter than the ambient temperature.
The only thing that can be done in this regard is to increase the size of your water storage tank to keep it warm for longer periods without additional heating. But 2 or 3 days of time cloudy in a row will mean the reduction of the temperature of the water no matter what you do.