This village in the U.K. is the unlikely testing ground for a new technology to store and use local power for longer, bypassing the grid and cutting energy bills. This month dozens of homes in Oxspring will have a home battery installed to see if they can make solar power more valuable to homeowners and less painful for grid managers. The batteries are smaller than the high-profile Powerwalls introduced by Tesla last year. The project is a response to what until recently would have seemed an improbable challenge for the U.K.: too much solar power. There are now 875,000 homes with solar photovoltaic panels, and that is beginning to pose issues for network operators.