The Smart Electric Power Alliance (SEPA) has published its survey of US utilities, and has some promising findings for smart grid advocates. Grid-scale battery storage capacity has grown 68% in the past year, up to 1.3 GWh, with longer-duration batteries becoming more prevalent. Residential storage was the strongest performer, growing some 317%. The longer duration units make it important to clarify that there are two ways of measuring battery capacity – how much energy it can hold (in watts), and how long you can run it for (in watt-hours). Watts measure power, while watt-hours measure energy. Essentially comparable to the difference between speed and distance, with batteries, watt-hours are telling you how long you can run the battery for. This…