Wasn’t this an experiment in grade school general science, (back in the day), involving batteries and water? ;-)
Sort of. But that was a very inefficient reaction, and it used electricity, this one uses sunlight directly, just like plants do.
And plants are, for the most part, where fossil fuel comes from. Energy from fossil fuel is stored sunlight. Thus it's really nuclear energy.
But still, this is some time off, how long is hard to know, even if it's everything it's cracked up to be, we'll still need oil and other fossil fuels to feed the petrochemical industry.
How else will they make the stock for my next several firearms?