Just tested this out. It's a bright clear day here. It took 2 or 3 tries to line everything up. But, after that it took about 10 seconds to create a burn mark on the end table. I tried to see if I could get a flame to ignite from a piece of paper, but it just smoked a lot and started to stink up the living room. So, I stopped. This could definitely start a house fire. Not sure if it would work the same on newer windows with any type of UV blocking or other filters. Our window is a large double pane picture window that is roughly 25 years old.