In the orientation optimization tool, I have a shift towards West or East.

[André Mermoud]

In the optimization diagram obtained by "Tools" / "Transposition Factor", the optimum may not be at azimuth = 0°.

First please observe that this optimum is extremely flat: a little deviation is not significant concerning the actual value of the transposition Factor.

Now this may have several causes:

With Synthetic hourly data, the optimum shoud theoretically be centered. It is indeed often slightly shifted towards west, probably due to an insignificant anisotropy in the Collares-Pereira model ?

With measured values in hourly values:

- If ground measurements, there may be an horizon shading, either in the morning or in the evening,

- Weather asymmetries like morning smogs (West shift) or evening storms (East shifts),

- With your own measurements, there may be a time shift in your recorded data by respect to the PVsyst time,

- With original POA measurements there may be an inaccurcy in the solarimeter orientation definition.

NB: This tool deals with irradiance only. The temperature of the array - which is higher in the afternoon - is not taken into account in this optimization, and could deviate the optimum towards east.