Solar Projections

I made a speadsheet to try to project the total solar production.
Here is a chart from that spreadsheet:
red is actual, blue is projected

The weird hump in the data between june and july comes from the fact that the number of cloudy days in june spikes up and ruins an otherwise smooth curve of less cloud in the summer.
I pulled the formula(e) I used to calculate the average production each day out of the ether.
It factors in all of these:
1) length of the day – minus a constant factor because the sun is too low near sunrise and sunset
2) solar altitude – the closer to perpendicular to the panels the sun gets, the more electricity is produced
3) portion of the time shaded by clouds

The input data comes from a few sources:
This link provided length of the day and solar altitude for each day.
http://www.timeanddate.com/worldclock/astronomy.html?n=234&month=12&year=2011&obj=sun&afl=-11&day=1

This link provided days of cloud per month:
http://www.wrcc.dri.edu/htmlfiles/westcomp.ovc.html

Seattle’s data:

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Annual
23 19 19 16 14 15 9 11 13 18 21 23 201

Here is a link to the actual spreadsheet:
Spreadsheet file

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