Renewable electricity generation is planned to increase dramatically over the remainder of the decade. If the UK government's commitment to the EU to provide 20% of all energy by 2020 is to be met, the contribution to electricity generation from renewable sources could be as high as 35% by this time. This is higher than can be absorbed by the network without substantial changes in network operation and the network operators are understandably concerned. Energy storage is often viewed as the solution to many of the operational difficulties posed by time varying renewable sources of generation but it is difficult to justify the costs of additional energy storage purely in the context of electricity system operation. At the same time, the pressure to reduce vehicle emissions in urban areas is expected to drive up the use of electric vehicles. Public transport operators like First and Allied Vehicles Ltd are looking seriously at substantial electric vehicle purchases. This opens up the possibility of using the vehicle batteries as energy storage for the network. Battery charging that responds to real time electricity cost, a form of responsive demand side, could greatly ease the operational challenges of integrating high penetrations of renewable generation. And it could even be possible to briefly discharge otherwise charging batteries to me short term demands for active power and also perhaps local voltage support. The aim of this study is to quantify the potential for such an approach, whist at the same time meeting the requirements of the vehicle fleet operators. Additional costs of the grid interface over and above conventional charging systems will be assessed and compared with a valuation of the system benefits. Market arrangements that could facilitate the take up and exploitation of the approach will also be explored, the over aim being to assess the market potential of this innovative approach to energy storage provision.