Mekanism has several different types of passive energy generation, each with their own cost to energy ratio which I find to be fairly well balanced.
The solar panels, for example, are rather pricey and, of course, don't function at night or during storms.
The heat generators produce an anemic amount of energy... just barely enough to get you started.
The hydrogen generators require electrolytic seperators to run, which means if you lose all power, you will need to 'prime the pump' so to speak to get it running again.
The Windmill is good for low power in remote locations, like an MFR harvester. Obviously it does better at higher altitudes, but it's not bad so long as it is out in the open.
There's also a Bio-Reactor, which does require fuel in the form of Bio Fuel, which in this mod is a solid produced by putting most plant-like things into the Crusher. It's more of a long-term energy producer, though, outputting an okay amount of energy, but doing so for a relatively long time.
In addition, Mekanism has an interesting way of ore refining. Either you can just use the Enrichment Chamber for a doubling of ore, or you can go the whole hog for Purification Chamber -> Crusher -> Enrichment Chamber -> Smelter for a tripling. Best of all, you can make Factories which can handle multiple inputs simultaneously. So you can have up to seven different items being smelted in the same machine simultaneously. Combine that with the speed upgrades, and you can crank out some pretty insane output rates.
Also, Mekanism machines are configurable with the Configurator. So you can have any side be an input or output, so you can automate them easier.
Oh, and the Mekanism Universal Cable and Energy Cube (energy storage device) can output EU, MJ, or UE energy, depending on which one is requested on the other side. So there's no need to rely on conversion blocks, it does so internally!