National Grid need to access to sources of extra power in the form of either generation or demand reduction, to be able to deal with unforeseen demand increase and/or generation unavailability. These additional power sources which are available to National Grid are referred to as Reserve and comprise synchronised and non-synchronised sources. Different sources would require different timescale of getting ready to delivery the services. In this section Reserve Services are listed on an increasing timescale of getting in the state of readiness as illustrated below.