Case Study

Relay solution for Rule 21 solar + storage interconnection

Client Type
Project developers
Project Type
Commercial scale, Industrial scale, Solar + storage
California, PG&E, SCE, SDG&E

California Net Energy Metering (NEM) laws and Rule 21 interconnection process forbid battery storage from exporting power back to the grid. For sites that have existing solar, utilities require a verifiable method to ensure NEM integrity.


HRE developed a novel monitoring and relaying scheme to prevent battery export. This solution requires multiple real-time data inputs feeding otherwise “off the shelf” relays.


The HRE relay solution is a cost effective way to enable interconnection of new energy storage systems for customers that also want solar. Dozens of solar + storage systems have received permission to operate from California utilities thanks to the HRE design.