You could try running powercfg /energy which also generates a report to examine: <some directory>\energy-report.html. There's a section in the report (about halfway through) describing the battery capacity. On my SP2 the figures in the energy-report were somewhat different than the battery-report.html had shown.
The /energy report takes over a minute to generate, but the results are quite informative.
Same thing happening to me... The capacity is not displayed in the battery report but only in the more complete energy report. I have a 256 GB SP2 with 4300U CPU. I also think my battery life is quite terrible, max 4h or 4h30 while running Office, some PDF, browsing over Wi-Fi - a far cry compared to some reporting 7 or 8h of battery life
I have run powercfg /batteryreport on my brand new Asus ux31a laptop, it does not show any figures for mWh either. Could this have something to do with European win 8.1 installations vs US installed versions? I cannot get the batteryreport to show any figures of mWh on any computer.
Ok well, if you really want to know that info.
I am working on a Surface Pro 2 tweak tool (not decided on the program name). One of the things it has it shows the Surface Pro battery information in details, as you can see from this screen shot.
It's the only thing that this software does at the moment. (yay?!)
You can download this very early in dev. version here: http://www.helpweaver.com/tweak.zip (Extract all files anywhere, and run)
Hope this helps.