What source of electrical energy have scientists been unable to use?
"One of the biggest reasons why we haven't been able to harness power from fusion is that its energy requirements are unbelievably, terribly high. In order for fusion to occur, you need a temperature of at least 100,000,000 degrees Celsius. That's slightly more than 6 times the temperature of the Sun's core."
Quoted from https://www.scienceabc.com/eyeopeners/why-arent-we-using-nuclear-fusion-to-generate-power-yet.html :D