on 1 April and running to 31 March. Supply companies have until the 31 August following the period to submit sufficient ROCs to cover their obligation, or to submit sufficient payment to the buy-out fund to cover the shortfall. The cost of ROCs is effectively paid by electricity consumers of supply companies that fail to present sufficient ROCs, whilst reducing the cost to consumers of supply companies who submit large numbers of ROCs, assuming that all costs and savings are passed on to consumers. Percentages and prices by year Sources: Certificates A ROC is the green certificate issued for eligible renewable electricity generated within the United Kingdom and supplied to customers in the United Kingdom by a licensed supplier. ROCs are issued by Ofgem to accredited renewable generators (or in the case of generating stations subject to a NFFO (non-fossil fuels obligation), Scottish Renewables Obligation or Northern Ireland NFFO contract, to the nominated electricity supplier). It is worth noting that the Scottish Renewables Obligation was superseded by the Renewables Obligation (Scotland) in 2002. The default is that one ROC is issued for each megawatt-hour (MWh) of eligible renewable output. Some technologies get more, some less. For instance, offshore wind installations receive 2 ROCs per MWh; onshore wind installations receive 0.9 ROCs per MWh and sewage gas-fired plants receive half a ROC per MWh. ROCs are issued into the ROC Register and so are electronic certificates. Normally, a renewable generator will transfer the related ROCs through Ofgem's electronic registry when it sells power to an electricity supplier. The legislation The Utilities Act 2000 gives the Secretary of State the power to require electricity suppliers to supply a certain proportion of their total sales in the United Kingdom from electricity generated from renewable sources. A Renewables Obligation Order is issued annually detailing the precise level of the obligation for the coming year-long period of obligation and the level of the buy-out price. The Renewables Obligation (England and Wales) was introduced by the Department of Trade and Industry, the Renewables Obligation (Scotland) was introduced by the Scottish Executives and the Northern Ireland Renewables Obligation was introduced by the Department of Enterprise Trade and Investment (DETINI). The Orders were subject to review in 2005/06 and new Orders came into effect on 1 April 2006. The relevant pieces of legislation for the period April 2006 – March 2007 are: The Renewables Obligation Order 2006 (Statutory