I was working on a Rails project and had a YouTube video playing in the background, its a 49whr battery and the CPU can easily burst above 15w and usually sustains around 7w plus backlight and all the other components of a laptop, a sustained power draw of 15w while doing development work isn't unusual.
"Battery life on the ThinkPad X13s will depend on your workload ... and while you might not get close to the 28 hours that Lenovo touts in its own testing, our testing with PCMark 10 got us an amazing 15 and a half hours..."