I have 2 south facing windows on the front of my house that I use to heat my home in the winter. They are 4 x 4 feet and I have made 3 panels 2 x 4 foot out of 1 x 2's and aluminum printing plates. They are painted flat black and have a 1 inch air gap at the top and bottom. I also put screws in to keep them about an inch from the glass. On a sunny day it can get to 85 degrees in that small room. I have some batteries charged by my solar panels and connected to an inverter and use this setup to run a 20 inch box fan pointed at the black panels. This forces the heated air into my living room and my furnace does not run from about 10 in the morning til about 4 in the afternoon. These panels get hot enough to burn you. If you have ever gotten into a car with a black interior on a sunny day and burned your leg, you know what I mean. It's not unusual for my thermostat to read 76 degrees with this free heat in the winter. I have been using these for about 4 years now and think I save alot on my utilities. My utility bill is way less than my neighbors. The only reason I don't have 4 of these panels instead of 3 is so I have a window to look out.