  Previous work has resulted in the development of an energy model able to
calculate wind and solar fleet efficiencies. However, for investment planning
purposes, it is necessary to calculate from the lowest economically acceptable
efficiencies how much wind and solar generation would be economically
justified. The paper explains how this objective has been achieved with arrays
(investment planning tables) created after carrying out a structured
investigation of the behaviour of the electricity system over the whole of its
operational range. The tables are then applied to National Grid prediction of
the size and composition of the system in the year 2035. A conclusion is
reached that wind and solar generation will only be able to supply about 70% of
electrical demand, the other 30% being provided by dispatchable sources of
generation, which must be sufficiently fast acting to maintain electricity
system stability, such as the use of combined cycle gas turbines. This limit on
deployment of wind and solar generation restricts their ability to decarbonise
the electricity system and is likely to lead in 2035 to a residual of 72
million tonnes per annum of carbon dioxide emissions which wind and solar
generations will be unable to address
