[PaulHoule]

If it is sunlight you are depriving the ecosystem of it.

It's not unthinkable that a fission or fusion economy could produce enough energy that the waste heat cooks us.

[alexvoda]

Not only would you be depriving the ecosystem of that energy, but also, you would make the planet unliveable. A lot of the energy of the sun that falls on the earth is reflected back into space. If all that energy is capture and used, eventually it will turn into heat.