Power Sources

Fusion reactors - these are industrial scale fusion reactors that supply energy to cities, planets and starships. Commercial fusion reactors have been available since the end of the 21C. Over time they have been made smaller and able to be fitted to spacecraft, first the fusion drive ships that opened up the solar system, then later the Jumpships which brought us the galaxy.
Portable Fusion Cores (PFC) - these are medium sized power cores, that are small enough to supply energy to meka, large vehicles and spacecraft. Containment had always been the biggest problem, and it wasn't until the 23C that PFC became practical. At first they weighed hundreds of tons, but now a military grade core, such as powers a small meka, can be as small as 15 tons. Most PFC run for at least 12 months before needing replacement, those in spacecraft are designed to last much longer.
Fuel Cells - rechargeable fuel cells are the most commonly used mobile power source. Small nanoengineered units provide power to small devices, while medium size units power cars and large, industrial scale cells can power towns and factories. Fuel cells require a primary power source, such as a fusion plant, PFC or a renewable source.  ,
Renewables - solar, wind and geothermal power are often used as primary power sources. Solar panels are very effective while spacecraft operate in system. 
Fossil Fuels - fossil fuels are only used on 'low tech' planets, or in some instances where some quick and dirty terraforming is needed.
Fission Plants - fission reactors are generally regarded as an environmental risk, though there are some situations where they are still used.