You don't want to care about energy density (aka capacity), as that number is insane for anything nuclear.
You do want to know the internal resistance, rate of "discharge", basically watts it can produce for given weight. You can burn uranium or gasoline in an instant, but decay mode sources (RTG and this) are limited to rate of decay.
Back of envelope:
(all exponents are to power of 10, not 2).
One C-14 atom decays in 5730 years, shoots off 156476 electron volt we ideally capture, and one anti-neutrino we shield off
One electron volt is 1.6e-19 J, ie that beta decay is roughly 2.5e-14 J.
You need 10^14 atoms of c-14 to get 2.5 joule every 5730 years.
Now for watts (aka joule per second):
5730 * 360 * 86400 * 10^14 to get 2.5 watts output
5730 * 360 * 86400 * 10^14 / 2.5 =
7.1e24 C-14 atoms per watt.
Now C-12 atom supposedly weights 2e-23 grams and C-14 should be in same ballpark, meaning
35 grams of this stuff, in ideal case gives off 1watt,
Which is pretty impressive, for a battery which lasts basically forever.