If you have NEI-IC2 Burn plugin or lately just Mistaqurs NEI plugin it shows eu value for every fuel that can be burned in a IC2 generator...and it shows 50000 eu/lava....and I'm pretty sure he get's his data from source code.
Yes, in a generator. Not a Geothermal generator.
The geothermal generator (reading from 1.106 source here) will produce 20 EU and consume 1 fuel every tick. It gains 1000 fuel from one bucket of lava (or lava cell), which, through basic algebra, tells us it will produce a total of 20,000 EUs out of 1,000 millibuckets of lava.
As for lava's value in the generator, again reading from 1.106 source, the generator will *refuse* to accept lava buckets. They are, in fact, explicitly exempted from burning in the basic generator because the geogen exists. Lava cells do not need to be explicitly mentioned because they do not have a fuel value, and what Mistaqur's NEI plugin really shows is the item's fuel value multiplied by 10 (or whatever the value of energyGeneratorBase is in your IC2.cfg, if non-default).