If you go for the more efficient type of panels (evacuated tube rather than flat panels)and they’re well placed, then 70% of your hot water should be a reasonable expectation. only on heavily cloudy days (& of course foggy ones)should it be inactive,although if you put enough panels on your roof even a day like those would do it,but then the problem is what to do with all that heat when the sun comes out (i’ve seen 167 degrees C on the panels in the summer