The attached diagram will give you some idea of how to calculate solar yield at different angles. The diagram was originally produced by NASA I think, and it relates to a site in N. America, and from the diagram, you can see that it relates to a lattitude of 28degrees (in the UK, I am 53degrees).
There are also some online PV efficiency calculators, that can help (ok, it is photovoltaic, but basically it is still solar energy) -
http://rredc.nrel.gov/solar/codes_algs/PVWATTS/version1/International/inputv1_intl.cgi(there are other simpler online PV energy calculators if you do a search for them)
Ivan