So this summer, the 45-seat eatery is using six free plug-in batteries provided by David Energy, a startup power supplier. "This all becomes enabled because batteries have gotten much cheaper through time," said Allison Weis, global head of storage at Wood Mackenzie. Electricity prices rose 6.1% in New York City last year, squeezing small businesses with notoriously tight margins. The restaurant's power bill is one of its biggest expenses. "The cost of a battery is nothing compared to how much money you can make on the price arbitrage," he added.