Did some experiments. 17x17 Fillerfarm (Forestry Logger cutting+Buildcraft Filler planting trees) produce 1.4 wood per second. Converted to scaffolds, it's 700 EU/t (no typo).
With full 21x21 farm (logger work radius) this will be 1000 eu/t. And it's just from wood, saplings can give some more (20% more I think), but not sure they worth the hassle and gold for electric generators. It's probably better to use saplings energy for BC/Forestry needs,Costs are: 4 diamonds for logger and one diamond tube, some gold for tubes, 800 of iron for 100 generators.
Alternatively, two thirds of wood can be converted to slabs, and "burned" in recyclers. Then Fillerfarm will be able to produce one UU per 25 seconds all by its own.
You're going to need a crapload of generators to keep up with that demand of energy from something that outputs 10 eu/t, not to mention a system capable of evenly distributing those over all generators. At that point, you're pretty much better off going with wind or nuclear, although if the amount of space taken up/massive amounts of iron required is ignored, this would probably work pretty effectively for starting out mass fabrication