Suitable sensor for approximating sunlight on a roof?

I’ve got an outbuilding which I may make fully off-grid in the future, but it is a complex site on a sloping hillside with large trees that cause a lot of shading at different times of the year.

I was thinking of putting a small sensor (or even a tiny solar panel) on the roof and logging the data I get for the year. And I can combine that with the mains electricity use of the outbuilding to see how much battery I might need, area of solar panels, etc.

I don’t know if I should be using a simple LDR or a UV-sensor to best approximate useful sunlight levels, or something like the Sparkfun solar cell IC?

And presumably I’ll need some sort of waterproof case for it, so perhaps I’d be better off getting a cheap small solar panel that is already designed to live outside? Like

Many thanks