In FTB, it's easy to use Watermills as a cheap source of EU early game. You can use the Aqueous Accumulator from Thermal Expansion in order to produce infinite water from 2 water source blocks placed on it's side. You then set up pneumatic tubes throughout the watermill farm, making a loop such that one end of the pneumatic network connects to the retriever, and the other end on a filter. Set the filter up so that it requests empty buckets from the tube network and feeds them into the Aqueous Accumulator, and the output feeding into a Filter requesting Water Buckets. Since you said you already have a lot of Tin, you can use the alternate bucket recipe to craft buckets out of Tin in the same pattern as using Iron, and make each watermill output 1EU/t instead of 0,25EU/t. You might want to use buildcraft tanks as a water "buffer" because a single accumulator might not be able to keep up with a lot of Watermills running simultaneously. All of this uses only Tin, Copper & Redstone for the most part.
You said you have 20 watermills, so that's 20EU/t. Converted with an unupgraded Electrical Engine, you'd be pulling in about 7MJ/t for your farm. You'll spend about the same amount of Tin to set up this system that you would for adding more Watermills to the system, but you'll get far better returns out of your investment. 3 tin per bucket, per Watermill, plus 1 for the filter and 1 for the retriever, means 66 tin, or just over a stack of Tin. Plus whatever Tin you'll have to use to make Brass for the Pneumatic tubes, 2 Gold for the Filter, 2 ender pearl for the retriever (which you can get out of 4 iron using Minium Stone). A bit more resources in the way of setting up the production line for this, but nothing really too fancy.
Once you're ready to be weaned out of your Watermills, you can disassemble the pneumatic tube network and rework it into something else. For example, the Filter/Retriever set up in conjunction with a Buildcraft Pump in the Nether and an Ender Chest can be used to pump infinite lava to a network of Geothermals.
That being said...
Watermills are some of the most godawful EU generation engines in the game. And they cost 4,5 iron a piece to make. If you use Gregtech, i'd just recycle all 20 back into 10 generators and quest for rubber and copper while making do with burning wood/coal in order to power stuff, until you can turn them into Solars or Geothermals. Seriously, they're bad. The only thing worse are Windmills. Windmills are bad unless you're playing in Skyblock or have a floating castle or something, in which case they are more or less OK for early game power generation. Energy loss over the distances required for placing a Windmill where it will generate useful power (147 blocks according to wiki), is too high. To go from 147 to 64, you'd be looking at 39 tin cable for 1EU loss, then batbox, plus another 8EU loss over the remaining 43 copper cables. Just to break even, you'd need fiveish Windmills.
What i usually do is i skip the whole early game EU generation, instead going for Thermal Expansion & Forestry ones, getting a Quarry set up, and then get the ball rolling on resources and jump straight for Magma Crucibles into Geothermals for 1MJ to 1EU conversion ratio, or Nether pump for even better "convertion" ratios.
If you're looking for best MJ production in early game, Thermal Expansion/Railcraft is the way to go. Railcraft also has the benefit of having the best late game MJ generation as well, with Steam Boilers. There is no best early game EU production. It only gets good after Geothermals, unfortunately...
