Dear phren,
If you are creating your own TMY3 files, you should familiarize yourself with the TMY3 file format, which is described in the TMY3 User's Manual (
PDF
). Table 1-3 in the manual describes the Time field as follows: "Time of data record (local standard time)."
SAM does not use the time stamp data from the weather file. It assumes that the first row of each column of data contains a value for the hour ending at 1:00 am on January 1 (local time), regardless of the values in the Date and Time columns.
The reason your results are different using the wrong time stamp assumption is that SAM makes solar angle calculations based on the time of day, assuming the time convention I just described. If you assume that the first row of data is for the hour ending at 1 am in London (universal time) instead of local time, then the solar angles and solar resource data for a given hour will be mismatched, and the simulation results will be meaningless.
Best regards,
Paul.