I was reading up about how the caloric burn that a body experiences is far too nuanced for any device to be able to accurately measure it... mostly meaning that every body is different and they are doing the best they can with a one size fits all sort of thing. The only thing that they could do to accurately tell anyone would be to hook them up to very complicated and expensive machinery and then it wouldn't be around your wrist either, it would be a vest sort of thing that follows bloodflow and BPM and body heat and all sorts of stuff that a tiny lithium USB battery couldn't possibly power.