Keep in mind the engines are upgraded with iron tube which means 5 eu/t for 2 mj/t.
Lava cells claim to make 50,000 EU (which is what I'm using.. not pumping direct lava) however IC2 wiki states 20,000 EU. I'll do the math for both.
That means 1 bucket/cell produces 50k EU, thus 20k MJ
OR...