[StavrosK]

Hmm, I see. I live in Greece, so I don't really have an intuition for how much sun one needs (we get eight months of sun or so). What you say makes sense, thank you.

[ovao]

The Norwegian Institute for Air Research has a fairly simple online calculator: https://fastrt.nilu.no/VitD-ez_quartMED.html

I can't personally vouch for its accuracy though.