Solar heat through windows

When designing how large an air conditioning system should be, one of the factors to consider is how much heat is coming from the sun shining through the window glass.

The simple number we would like is a number of btu's per hour per square foot of window. Then you multiply by the total square feet of windows and you know how many btu's per hour are coming in, and therefore how many btu's per hour your air conditioner must have to remove that heat. Of course your air conditioner must also remove heat that conducts through the walls, etc.

The theoretical model and calculation of this number is so complex that you might just want some rules of thumb. Here are six:
Use 60 btu/hr / square foot of window, and skip the rest.

Always sunny, curtains usually open, majority of windows get sun during hot parts of the day, use 120 btu/hr / square foot of window.

Always sunny, curtains usually shut, majority of windows don't get sun during hot parts of day, use 60 btu/hr / square foot of window.

Not always sunny, curtains usually open, majority of windows get sun during hot parts of day, use 60 btu/hr / square foot of window.

Not always sunny, curtains usually shut, majority of windows don't get sun during hot parts of day, use 40 btu/hr / square foot of window.

Clear skylight, use 150 btu/hr / square foot of skylight.

There's a relatively easy on-line calculator for this at www.susdesign.com. (The results, however, are per day or per month, so use judgement of how many hours to use when converting it back to "per hour."

Here are some more rules of thumb (from Manual J), but I strongly question them:
18 btu/hr / square foot for north facing windows
27 btu/hr / square foot for south facing windows
48 btu/hr / square foot for east or west facing windows

Now, here's a little bit of why it is so difficult to get the correct number.

Sunlight reaching the earth's surface transports about 450 btu/hr / square foot.
It's more in January, less in July.
It's more near the equator, it's less near the poles.
It's more when the sky is clear and clean, it's less when the sky is not clear and not clean.
It's more at mid-day, it's less near sunrise and sunset.
See what's going on here? You have to consider all of the above to adjust that 450 btu/hr value.

Here are some minor variations to consider:
Southern hemisphere summer:
468 btu/hr at 15 degrees south latitude in January.
455 btu/hr at 65 degrees south latitude in January.
Northern hemisphere summer:
442 btu/hr at 15 degrees north latitude in July.
430 btu/hr at 65 degrees north latitude in July.

How clear and clean is your local sky when you are at your peak need for air conditioning? Perhaps your local weather service can discuss how much solar heat is blocked by your local sky on hot days.

In many cases, you need the most air conditioning at 3 pm or later because that is when the outside environment is the hottest. But the solar radiation is less than it's peak at mid-day.

Which windows are really taking in sunlight during the hottest part of the day. Windows may face any direction, it may be surprising to calculate that (in the northern hemisphere) a north facing window gets quite a bit of direct sunlight in the summer late afternoon while a south facing window is getting none. Do you have the time and power to calculate the solar factor separately for windows facing each separate direction? If not, what average value should you use?

Remember that 450 btu/hr /square foot value for sunlight reaching the earth's surface. That is measured perpendicular to the sunlight. Your window is almost never perpendicular to the sunlight. The direction of the sun relative to a stationary window is constantly changing and repeats itself twice a year at most. Your square foot of window may receive most of a "square foot" of sunlight if it is facing the sun at sunrise or sunset, but during the mid-day it may only be receiving half or less of that "square foot" that is perpendicular to the sunlight. Thus there is a drastic reduction from a potential 450 to a more realistic 300 btu/hr / square foot of window during the hottest part of the day, for those windows that "face" the sun.

How about local shading. Trees? Outside overhangs or awnings? The curtains and blinds inside - does someone shut them to block the sun during the hottest part of the day? These factors all make a big difference, but are sometimes very hard to model.

Is it ok to average the heat rate over time? Often the tabulated values are an average over a time period that is not defined. Is it averaged over the entire time you need air conditioning in a day? How does someone else know how many hours you need air conditioning in a day?

There are very detailed computer programs to assist modeling the solar heat gain through windows, but if they are to be accurate at all in your case, you can see that you will have to provide a great amount of detail about your building's circumstances, and then use the computer results with caution. Is the answer really for your situation?