Consumers use appliances at different times of the day, based on their specific household and livelihood needs, rather than limited by the availability of power. In the case of distributed solar PV mini-grids, for example, users need lighting precisely when sunlight is not available. Similarly, refrigerators need to run continuously through the night and day. This makes bulk electrical energy storage services essential for renewable energy mini-grids in off-grid locations, where sources are intermittent, and backup power from main grids is not an option. Lithium ion batteries present a promising alternative; however, even with rapidly declining costs, they are presently too expensive to push mass adoption in mini-grids.Other rechargeable battery technologies are not yet available at commercial scale, at the right level of cost and performance. Beyond low cost, a suitable technology will also need to be robust, durable, compact, easy to transport and easy to install. It should also be modular and configurable, with the ability to scale up or down based on usage levels. Operation and maintenance should require minimal technical expertise, and limited additional infrastructure.