Approximate number of Btu through 1 square foot of single-pane window on a clear day for windows facing different directions at three latitudes (January 21 and July 21 are representative of winter and summer).
| Direction window faces | 32° Latitude (e.g.,El Paxo, TX) | 40° Latitude (e.g.,Columbus, OH) | 48° Latitude (e.g.,Spokane, WA) | |||
|---|---|---|---|---|---|---|
| Jan. 21 | July 27 | Jan. 21 | July 27 | Jan. 21 | July 27 | |
| North | 160 | 460 | 120 | 450 | 90 | 450 |
| East/west | 650 | 1,150 | 510 | 1,190 | 360 | 1,230 |
| South | 1,710 | 500 | 1,630 | 700 | 1,400 | 950 |