Might it be one of the many calculator links found in the thread linked below?

If so, I found it via a search on page one of this forum, and used the search box found just below the list of threads. I searched light calculator, and the 3 threads came up.

Sue

Here is a link that might be useful: Too many windows?

Don't know what thread that might be on, but here a couple sunlight calculators that I have used:

http://suncalc.net/
http://www.sunearthtools.com/dp/tools/pos_sun.php

Thanks!!!!

All you really need is a chart of sun angles, by season, for your site's latitude, plus knowledge of the window head and sill heights on the southern exposure. Since summer and winter are the two extreme seasons for high and low sun angles, you only have to use them to calculate the depth of sun for a given window.

Good luck on your project.