What is the rotation argument to bifacial.utils.vf_ground_sky_2d?

[erkkar]

What is the rotation argument supposed to mean in the function bifacial.utils.vf_ground_sky_2d? (https://github.com/pvlib/pvlib-python/blob/main/pvlib/bifacial/utils.py#L95) Is it the same as panel tilt? I’m having trouble understanding the docstring:

rotation : numeric
        Rotation angle of the row's right edge relative to row center.
        [degree]

What is the row’s right edge?

[cwhanse]

rotation is similar to surface_tilt; both describe the angle of the panel relative to horizontal. However, rotation is signed, where tilt is defined as positive in (0, 180). Here's an example where the two are different: rotation=-30, tilt=30.

[erkkar]

I see, thank you very much! Now I realised that for the argument x it says ”positive x is towards right” which in my case is northwards. The fixed panel rows are tilted 35° towards south, so in this case the rotation is also 35°, i.e. the right edges (north side) are higher than the left edeges (south side) of the panels. Right?

[cwhanse]

To confirm, the situation you describe is this? If the picture is correct, then yes rotation=+35 to emphasize the sign.

[erkkar]

Yes, thank you very much!