Neat show! I'd first heard of it a few years ago... even did some research to see what it would take to possibly make one of my own. Actually 'seems' relatively simple outside cutting thousands of disks for the pile. Threw it in the 'one of these days' stack.
They mention the battery runs at 2,000 volts, 1 nanoamp. So, 176 years x 365 days x 24 hours is a little over 1.5 million hours . 1 nA x 1.5 million hours (much easier in scientific notation of 1x10^-9 x 1.5x10^6 = 1.5x10^3) = 1.5milliamp-hours delivered so far... if early morning math holds out.
So the whole apparatus is most notable in the miserly use of power and incredible long-term stability of the materials, not so much for battery storage capacity!