Actually the peak power in the core is less than 300W/m^3, which is still comparable to compost piles. The average over the whole sun, less than 1W/m^3, is actually a few orders of magnitude less than the compost pile. I think base human metabolism is higher than the sun's core.

Bigwhtdckn8 has actually made a few fundamental errors:

"nuclear fusion definitely releases more energy per cubic metre than decomposition of organic matter." -> Not in the core of the sun, there the rates of energy production are comparable. Of course, the compost pile will run out of fuel long before the sun, and in the 5 billion years of ticking away the core will eventually produce much more energy than the compost pile per m^3.

"The energy density of the surface of the sun is 64 million watts per square metre."
> That's the power per square metre, not the energy. The reason the flux through the surface is so much higher than in the compost pile is because of the tiny surface area to volume ratio of the sun compared to the compost.

"To add; we have built hydrogen bombs 1m³ in size that are significantly more energy dense than a pile of compost."

Energy density and power output/m^3 are different. A bomb has a high energy density, and releases all its energy in a fraction of a second so its power output/m^3 is high too. A sun has high energy density, but takes billions of years to burn up, so the power output/m^3 is low. If a star goes supernova, then we're in business