I tried out a 40w bulb and it got too hot. It was up to 170 within 2 minutes so I shut it off. Next I tried a 25w bulb. It heated the tin to 122 degrees and the temp stopped rising there. Consider the average human core body temp is 98.6, 122 is a tolerable temp. I was able to touch the tin without discomfort. I next put a glass on top with 16 oz of water in it at 65 degrees. Room temp was 72. I took about 30 min to bring the water to 75. This concludes the controlled environment testing. I'll see how it does whenever it gets below freezing.