I exchanged the size XL vest for a Large, as it was a bit too roomy over a base layer (6', 210lb). I used the vest yesterday evening. over a base layer and under a wind breaker. As the temps dipped into the low 40s, I turned on the vest to the mid level. This vest does a good job of producing heat across the upper back. During the last hour on stand, with the temps dropping into the 30s, I set the vest to High to see how it would work. The highest level of heat felt hot, so I bumped it back down to mid level. I've only used the vest for up to 3 hours, and the battery indicated a 75% charge remaining. I'm liking it so far.