The Bio Generator is used to convert Biomass or Biofuel into EU. It Produces 8,000 EU at 8 EU/t when running on Biomass or 32,000 EU at 16 EU/t when running on Biofuel (per bucket). It stores 10 buckets worth of fuel and can store up to 30,000 EU of excess energy.
It's worth noting that while a Biogas Engine takes 10,000 ticks, or 8 minutes and 20 seconds, to use a bucket of Biomass, the Bio Generator will use up the same amount of fuel in just 50 seconds. Similarly, a Combustion Engine takes 40,000 ticks or 33 minutes 20 seconds to use a bucket of Biofuel, while a Bio Generator burns it in 1 minute 40 seconds. Thus, if you have a setup that uses Biomass or Biofuel to power engines, and you connect a Bio Generator to the same fuel pipe, you should expect it to starve your engines.
Even when using cobblestone on the Magma Crucible one biomass bucket can provide 62,500 EU and a biofuel bucket provide 250,000 EU. Using Netherrack on the Magma Crucible will net twice the amount of EU.