Big difference between vertical blinds with BSDFs and static geometry

Hi Kim,

First of all, I would suggest before running an annual simulation try to understand the behavior of your BSDFs. It has 2 ways. One what @sarith did in here for you. The other is using BSDF viewer.
So, for your vertical slats 45° the BSDF makes sens to me but the way you apply it in your model might be influential as you got some comments in here.

Again I suggest you to run an point in time image based (rendering) by using the same model setup only for 2 hours (one winter day, morning time and one afternoon) and see the behavior of your shading.
you can use the HB+ example files.