Glass bottles give you 1000 EU at 2eu/t per filled glass bottle.
Glass bottles can be made from cobblestone at a 1 to 1 ratio, so this mean you can make 1000 EU from cobblestone in a watermill.
Out of curiosity, I also decided to gather up some info on how much it costs energy-wise to make the bottle, assuming you did rotary macerator -> Induction furnace -> auto-crafting table with redstone engine(s) -> deployer to make them;
The macerator can vary a lot since it can accept overclockers, and according to to its thread on the IC2 forums it apparently becomes ever more efficient per block at higher speeds. According to this; https://docs.google.com/spreadsheet/ccc?key=0Al9yx7D_ef7vdEUtSzd0Ymh5VlBxdkR3VWZBaDBFNUE#gid=0, no overclocks costs 288 eu per block, which I'll just assume is what most people would do.
Induction furnace costs 208 per operation, and is capable of smelting two items per operation, so if you can fill both slots that will be 104 eu per cobble.
These are the only steps in the chain which cost energy, assuming you don't use Gregtech to automate, which means a grand total of 392 - 496 eu per bottle to make, for a total gain of 608 - 504 per cobblestone.
Meaning that each watermill will basically make 1.216 - 1.008 eu/t after costs are applied.
Considering this is essentially free power anywhere, and that watermills are only 4 iron, 1/2 a battery, and either 2 aluminum (with gregtech) or some wood, (without gregtech) I'd say that's pretty good.
Glass bottles can be made from cobblestone at a 1 to 1 ratio, so this mean you can make 1000 EU from cobblestone in a watermill.
Out of curiosity, I also decided to gather up some info on how much it costs energy-wise to make the bottle, assuming you did rotary macerator -> Induction furnace -> auto-crafting table with redstone engine(s) -> deployer to make them;
The macerator can vary a lot since it can accept overclockers, and according to to its thread on the IC2 forums it apparently becomes ever more efficient per block at higher speeds. According to this; https://docs.google.com/spreadsheet/ccc?key=0Al9yx7D_ef7vdEUtSzd0Ymh5VlBxdkR3VWZBaDBFNUE#gid=0, no overclocks costs 288 eu per block, which I'll just assume is what most people would do.
Induction furnace costs 208 per operation, and is capable of smelting two items per operation, so if you can fill both slots that will be 104 eu per cobble.
These are the only steps in the chain which cost energy, assuming you don't use Gregtech to automate, which means a grand total of 392 - 496 eu per bottle to make, for a total gain of 608 - 504 per cobblestone.
Meaning that each watermill will basically make 1.216 - 1.008 eu/t after costs are applied.
Considering this is essentially free power anywhere, and that watermills are only 4 iron, 1/2 a battery, and either 2 aluminum (with gregtech) or some wood, (without gregtech) I'd say that's pretty good.