Casting a little shade on a hot day

taken from http://en.wikipedia.org/wiki/File:Solar_altitude.svg

This formula will help to calculate the sun's angle in the sky. Simply add or subtract 23.5° to or from your position to achieve the maximum and minimum elevation taking into account seasonal change. In my case, I am at 46° North Lat. From this number, I would add 23.5° which gives me 69.5° of elevation. This is the angle the sun will be at 12:00 noon on June 21, or It is also the day in which the sun will be the highest in the sky. On December 21, the sun is the lowest in the sky, or the blue line. You dont want your overhang to obstruct the sunlight at this time of year since you will want to maximize solar heat gain. You can adjust the lenght of overhang to permit or obstruct more sun or to widen the time of shading. For example: if you calculate your overhang based on July 21st, this will also affect the shading for the same amount of time prior to June 21. In this case, you will get the same amount of shading May 21st as you will July 21st. This will also give a longer shade time on June 21st.

All these calculations are based on a south facing window. Adjustments can be made to the angle of the louvers equal to the bias from true south. For example: If you house is 45° towards the west or 225° SSW, you will need to angle the louvers or shade by 45° to cast a perpendicular shadow on the house. Otherwise, creating an overhang that is perpendicular to the house will cast a shadow that is at an angle equal to the bias from south. It is for this reason that I have angled the overhang on my house.

Too much math for you? You can alway use the trial and error method. Just hold up a board perpendicular to the house at noon in the summer. The closer to June 21st the better since you want to be accurate in your overhand and not have it come out too far so as to reduce solar heat gain in the winter. Play around with angles and lenghts for a while until you are comfortable with the result.

If you use an air conditioner, it will drastically reduce the heat load and potentially save you hundreds of dollars per season. It might even eliminate the need for one all together. ...an even bigger savings!