I think you guys are talking past each other. You're both right.

Up-time is meaningless if almost no energy is being produced.

But I think @zuspotirko 's point is that, even if the energy output is tiny, if it can be sustained 24/7 we have unlimited energy. This is true no matter how small the output number is.

It's like the concept of limits in calculus. When something is approaching infinity, it may as well just be considered infinity for all intents and purposes.