ROCs or Renewable Obligation Certificates, are certificates given to renewable energy generators who have had their systems certified by Ofgem . The generator receives one Renewable Obligation Certificate for each Mega Watt Hour generated.
Electricity Suppliers in the UK are required to produce a certain percentage of their electricity from renewable sources. This is their Renewable Obligation. If they fail to generate this electricity themselves, they can either pay a fixed amount into a 'buy out fund' which is shared among renewable energy generators, or purchase Renewable Obligation Certificates on the open market.