Roof top solar power

The issue is not the engineering, it is the materials.

True, but irrelevant.

To restore the part you snipped

"I love the idea that it's possible (if not practical) to build a little fusion power source - about the size of a car battery ? - which could power a house for millennia ...."

The "size of a car battery" is the problem. One of the things you are up against is the strength of interatomic bonds. The other is getting your nucleons close enough together to overcome the barrier.

Stars work by being big enough for gravity to provide the containment. Tokamaks and the like work by having big enough magnets. Laser confinement works by having very powerful, and hence large, lasers. Similarly accelerators.

Now, I *might* be wrong, it might, some day, be possible to miniaturise these, or other processes to get you "battery size". But personally, I would bet it will never be possible to engineer materials to be strong enough to provide containment together with the other functions at "battery" scale. Fission is of course a completely different case. You can get nuclear weapons down to close to this scale, and even fission reactors below the half metre scale.

AIUI it's quite a way from being able to light up.

You're only going to get a 'post hoc' answer, anyway.

None, of course.

In particular the electrical repulsion between protons.

And stars are big. Ours is a million miles across. Even then they have to heat up to 15 million degrees for stellar nucleosynthesis to start.

newshound might like to look at: