Mekanism only has a few options for power generation.
1.)
Heat Generators: These runs on standard minecraft fuel. They also produces anemic amount of energy for each block of lava adjacent to it. (Max. five sides of it since you need to run cable out of it from one side.)
2.)
Bio-Generators: These runs on solid biofuel which are easily obtained via crushing biomaterials in the Crusher. And if I am not mistaken, Wheat are the best since they gives you four biofuel per wheat compared to other forms of plant matter, which only returns one to two biofuel per item.
3.)
Solar Generator and
Advanced Solar Generator: Craft it. Drop it under the sun. Connect cables to it's output. Profit.
4.)
Wind Turbine: Like the Solar Generators, except that they will work day and night. Higher you place them, more energy it will generate per tick.
5.) Last, but not least. The
Gas Burning Generator: It was the go-to power gen for Mekanism back in the days but due to it's efficiency, it was nerfed. The nerf wasn't too bad though, it only moved it from mid game to late game option which is still worthwhile to use if you aren't using other mods for energy generation.