Tesla Motors, designer and builder of high-end electric cars, recently reported its development of batteries that can store electric energy for later use. The idea is that a place with a solar electric installation could store energy generated but not immediately used. The batteries could also be interconnected with the local utility grid, and they could be used for standby power during an outage.

Called Powerwall, it’s a rechargeable lithium-ion battery that mounts on a wall. Tesla expects to begin making 7-kilowatt-hour and 10-kilowatt-hour batteries for homes, pointing out that people can buy more than one, if needed. Larger units for utilities are also in Tesla’s plans.

The National Rural Electric Cooperative Association recently looked into the Tesla batteries and concluded that “the systems appear to be a significant step forward for the home and business battery market. The 10-kwh-DC and 7-kh-DC Powerwall batteries are technically intriguing, particularly the thin form factor and much higher-than-typical battery voltage, which offers increased efficiencies.”

The proposed pricing was lower than expected, said NRECA. Reported prices for the residential batteries: $3,500 for the 10-kwh and $3,000 for the 7-kwh, not including installation and additional equipment that is required. They have a 10-year warranty.

Skeptics said the systems have limited applicability and noted they are far more expensive than standby residential generators.