Question: Do you think we will be able to transfer mass into energy on scale so we can only use mass for energy?

Oli Weber answered on 5 Jan 2017:

Nuclear energy converts mass into energy, either by breaking up heavy atoms (fission) or fusing light ones together (fusion). There’s enough radioactive fission material on Earth (elements like uranium) to generate as much energy as we currently use for over 1000 years, though whether it’s the cheapest or the safest way to generate electricity is another matter.

If fusion eventually gets going, especially fusing deuterium with deuterium, we’ll have enough fuel for much, much more energy than we currently use, for over 1 million years!