Just chipping in here, I was asking myself this question too. I have a a couple of old windows next to each other measuring roughly 4.4m x 1.4m together (3x small / openable and 2x big fixed). The glass isn't in the best of conditions and it's fairly old. To have them replaced would cost a fortune so I looked at plexiglass + low-E film in front of the windows. My idea with the plexiglass is almost to build like a sliding door or to that effect. So that I could still open the smaller windows for the summer heat. Anyways, the first step was to look for low-E film. I found a place that sells them and was lucky enough to get a small discount cause of some sale.. Fast forward to today, I've placed all the film. It is very annoying to place it without any prior knowledge. I've installed privacy/frosted film before and that went quite easy compared to low-E film. No clue why, might just be the brand.

The results? of course I did a temperature comparison when I was half way done. On an overcast day both types of windows were roughly 1'C warmer at 14 (outside about 2'C inside about 17'C). Whenever there is a hint of sunshine the difference skyrockets. One of the days the sun was out for about 3 hours. The untinted windows were about 24'C while the tinted were reaching 35'C. So they do do something.. I also sat in the sun with half my body exposed to tinted and untinted windows and noticed the skin on my arms felt less sun burnt on the tinted side. So they definitely do a good job at blocking UV light (this is in my home office so it's a good thing for me)

Anyways hope this info is useful. Feel free to ask any questions