With increasing deregulation of the European electricity market, the quality of supply is becoming an issue of growing importance. Grid-connected electricity storage systems (ESSs) can enhance the quality of supply by: (i) shortening black-out periods; (ii) shifting excess energy for use during periods of high demand; (iii) sustaining the grid for better power quality. These problems are being addressed by using technologies such as power electronics and ICT. But storage systems offer a cheap and efficient solution to such concerns. ESSs can also power high-value, ancillary services. This paper analyses the new potential markets for storage systems in the context of distributed energy resources with a high penetration of renewable energies in the electricity networks. While lead-acid batteries are the most used technology in all types of stationary applications, many different storage technologies are claimed to fulfil the technical requirements of the above applications, in particular the emerging ones. Therefore, a comparison is made of lead-acid technology and its competitors in terms of technical and economic considerations. (c) 2004 Elsevier B.V. All rights reserved.