Solar and other purely renewable power is generally a late-game option if you're entirely working within IndustrialCraft. You can make water mills as soon as you get an industrial Electrolizer, but the return on investment is incredibly low. You can get Blutricity solar and wind into MJs with a bluctric engine, then to EUs through an electric engine, but while the yield is somewhat greater, it's costly to implement on a large scale. Replacing your smelter with a Blutricty furnace and wind power is relatively simple, and can save EU in the long run, although the startup costs are pretty high unless you have a flax farm.
Nuclear power is the most fire-and-forget after that point, especially with GregTech. The basic investment is pretty significant -- three stacks of copper and two stacks of iron, as well as a small amount of lapis lazuli and redstone -- but a zero-chamber reactor can provide 20- or 30 eu/t for hours, and be easily and readily expanded in the future. It also requires relatively little infrastructure for its most basic versions, primarily a compressor and industrial centrifuge. GregTech's thorium actually
makes the thing easier once you have an Industrial Grinder and a Rock Cutter, as thorium will burn for tens of hours without needing to be replaced, and only require a small amount of coal or monazit to replace. Be careful with planning, however, as the variable output a reactor can produce is a good way to end up with burnt wire or an exploded BatBox if you underestimate current.
Thermal power is another option. Bluctric thermopiles are completely hands-free if set up properly, but again the conversion to EU is very messy and yield isn't great. Geothermal and especially Thermal Generators are a lot better a choice -- especially the GregTech Thermal Generator, which returns the tin cells that make the geothermal generator so expensive, and it sounds like you already have. Do not, however, gather the stuff manually: it'll drive you batty, make a mess of lava flows, and likely waste materials you'd otherwise need. Instead, a BuildCraft pump, five redstone engines, a couple levers, and a tank will pretty consistently feed you all the materials you need until getting to a more permanent power supply.
Biomass and biofuel are a less obvious choice, since the fuel chain is fairly long and many of the options are terrible (never use a refinery : stills are almost always cheaper, and give better return to boot) and requires BuildCraft power until the end-game Distillation Tower. Burning biofuel in a diesel generator doesn't give great returns, only something like 30k EU per unit, but production can be automated fairly easily with Thaumcraft Golems (or the less balanced Steve's Cart farms).
Alternatively, if you collect a lot of Netherrack, using biofuel in a combustion engine (or other BuildCraft power) to run a magma crucible array to a Thermal Generator can return stupid amounts of power, but the infrastructure, need to maintain BuildCraft engines, and netherrack requirements are not great (and this may be a balance issue that won't last forever).
