The Google Glass is a mobile device designed to be worn as eyeglasses. This form factor enables new use cases, such as hands-free video chat and web search. However, its shape also hampers its potential: (1) battery size, and therefore lifetime, is limited by a need for the device to be lightweight, and (2) high-power processing leads to significant heat, which should be limited due to the compact form factor and proximity to the user's skin. We use an Explorer Edition of Glass (XE12) to study the power and thermal characteristics of optical head-mounted display devices. We share insights and implications to limit power draw to increase the safety and utility of head-mounted devices.