Tree farm + generators, or steam boilers.
Or a glass-powered watermill array;
Basically, you macerate/pulverize cobblestone into sand, then smelt the sand into glass, which gets auto-crafted into glass bottles. You then either use a liquid transposer with an aqueous accumulator (less laggy, small energy cost) or deploy these on a pool of water. (more laggy, no energy cost) Insert these into the bottom of a watermill array (routers are amazing for this) and you will now generate 1000 EU per cobblestone, at 2eu/t per watermill.
You could also use recyclers to power generators or steam boilers, and routers are again amazing for doing this on a large scale, but doing this on any meaningful scale is known to cause a good amount of lag.