Initially, guerillaClock was to incorporate a solar panel for it's primary power source.  As such it would need a backup battery, for cloudy days, so the case was designed around a 6600 mAh LiPo battery that was on hand.  

To determine the power draw, a USB Charge Doctor was put in line and it showed voltage and amperage usage.

Calculating off the peak amperage, which interestingly is when the display is static, we can determine the approximate uptime.

Using some simplistic napkin-math, the LiPo's 6600mAh capacity / peak 360mA draw, we can anticipate the guerillaClock staying online for over 18 hours.  

When not displaying a message the guerillaClock draws far less power.  So, uptime could be improved by increasing the interval between arrival time calculations and display or by adding a physical button for on-demand only inquiries which could be considerably less frequent.