Fusion

Although still too volatile for certain uses, fusion reactor technology and room-temperature ("cold fusion") superconductors are increasingly popular for large-scale energy production.

It does not scale down particularly well... small-scale fusion reactors can be built, but tend not to be cost-effective. Consequently, energy storage and transmission tends to be the major bottleneck for low-infrastructure areas and small-scale uses (like powered armor, for example).

Also, fear of blowups is relatively common, so many parts of the world prefer to stick to solar, wind, and hydroelectrical systems.