March 2012 Power Yield and How RETScreen is Doing

I've downloaded the data up to 13 April and I've put it in my spreadsheet to calculate the power yield for March 2012 (table below). It was marginally better than 2011. I am looking forward to the rest of April because so far the bright, cool days have been good energy producers. March 2012 was just 9 kWhr over last year. However, RETScreen calculations show that my system should have yielded 584 kW, so, two years in a row RETScreen is overestimating the system. Two years of data is not enough to say RETScreen is not accurate, but the overall trend with my system is underestimation. RETScreen calculated that my system should have produced 9128 kWhr to the end of March whereas my system did 8256 kWhr (10.6% under estimation). Most of the deviation came from the winter of 2010-2011 where snow substantially reduced the panel output. So, with this limited dataset, it points to people being conservative when estimating their payback expectations.

Sum - Daily kWhr	Years
Date	2010	2011	2012	Total Result
Jan		169.7	255.8	425.5
Feb		275.0	322.5	597.5
Mar		560.8	569.3	1130.1
Apr		492.9	313.5	806.4
May		534.3		534.3
Jun		707.2		707.2
Jul		769.0		769.0
Aug	164.0	658.5		822.5
Sep	438.0	528.0		966.0
Oct	450.0	391.1		841.1
Nov	355.4	316.3		671.7
Dec	78.2	219.8		298.1
Total Result	1485.62	5622.72	1461.16	8569.52