MW extension is completed, the site's storage capacity will be 266 MWh. Reserve services and frequency response National Grid is responsible for contracting short term generating provision to cover demand prediction errors and sudden failures at power stations. This covers a few hours of operation giving time for market contracts to be established to cover longer term balancing. Frequency-response reserves act to keep the system's AC frequency within ±1% of 50 Hz, except in exceptional circumstances. These are used on a second by second basis to either lower the demand or to provide extra generation. Reserve services are a group of services each acting within different response times: Fast Reserve: rapid delivery (within two minutes) of increased generation or reduced demand, sustainable for a minimum of 15 minutes. Fast Start: generation units that start from a standstill and deliver power within five minutes automatically, or within seven minutes of a manual instruction, with generation maintained for a minimum of four hours. Demand Management: reduction in demand of at least 25 MW from large power users, for at least an hour. Short Term Operating Reserve (STOR): generation of at least 3 MW, from a single or aggregation of sites, within four hours of instruction and maintained for at least two hours. BM Start-Up: mainstream major generation units maintained in either an energy readiness or hot standby state. These reserves are sized according to three factors: The largest credible single generation failure event, which is currently either Sizewell B nuclear power station (1,260 MW) or one cable of the HVDC Cross-Channel interconnector (1,000 MW) The general anticipated availability of all generation plants Anticipated demand prediction errors Control of the grid The English and Welsh parts of the National Grid are controlled from the National Grid Control Centre which is located in St Catherine's Lodge, Sindlesham, Wokingham in Berkshire. It is sometimes described as being a 'secret' location. the system is under consistent cyber attack. Although the transmission network in Scotland is owned by separate companies – SP Transmission plc (part of Scottish Power) in the south, and Scottish Hydro Electric Transmission plc (part of Scottish and Southern Electricity Networks) in the north – overall control rests with National Grid Electricity System Operator. Transmission costs The costs of operating the National Grid System are recouped by National Grid Electricity System Operator (NGESO) through levying of Transmission Network Use of System (TNUoS) charges on