Here's the best I can find, from Eikka's comment on Phys.org:
"Carbon-14 has a mean decay energy of 49 keV or 7.85e-15 Joules and activity of 165e+9 bq/g which gives you a power output of 0.0013 Watts per gram.
So a gram-sized lump of carbon-14 - about half a teaspoon - assuming perfect conversion, will produce 1.3 Milliwatts, or about 1/20th of what it takes to light up a common red indicator LED."
"Carbon-14 has a mean decay energy of 49 keV or 7.85e-15 Joules and activity of 165e+9 bq/g which gives you a power output of 0.0013 Watts per gram.
So a gram-sized lump of carbon-14 - about half a teaspoon - assuming perfect conversion, will produce 1.3 Milliwatts, or about 1/20th of what it takes to light up a common red indicator LED."
http://phys.org/news/2016-11-diamond-age-power-nuclear-batte...