This summer, electric car manufacturer Tesla Motors formed Tesla Energy (TE) and began selling lithium-ion battery packs that homeowners can hang on their basement walls. These units enable 24-hour use of solar energy or storage of low-cost nighttime (base load) electricity, for use during peak demand periods. In this series, we explore the concept of stationary battery packs as they might be used in buildings, factories and even electric grids. In Part 1 we explain how these work, how they arose out of solar energy and electric vehicle (EV) markets, who will buy them, and why they are getting cheaper. In Part 2, we consider how this development could affect the gas industry going forward in terms of prices and competition for demand growth.