Beginning this year, all proposed inverter-based Distributed Energy Resources (DER) must use inverters certified under Underwriters Laboratories (UL) 1741 Supplement B (Standard for Safety Inverters, Converters, Controllers and Interconnection System Equipment for Use with Distributed Energy Resources, Supplement B and labeled as UL1741-SB) and Institute of Electrical and Electronics Engineers (IEEE) 1547-2018 (Standard for Interconnection Interoperability of Distributed Energy Resources with Associated Electric Power Systems Interfaces). 
Beginning this year, all proposed inverter-based Distributed Energy Resources (DER) must use inverters certified under Underwriters Laboratories (UL) 1741 Supplement B (Standard for Safety Inverters, Converters, Controllers and Interconnection System Equipment for Use with Distributed Energy Resources, Supplement B and labeled as UL1741-SB) and Institute of Electrical and Electronics Engineers (IEEE) 1547-2018 (Standard for Interconnection Interoperability of Distributed Energy Resources with Associated Electric Power Systems Interfaces).
The new inverter requirement will go into effect on the following schedule:
- On or after April 1, 2024: New submitted DER interconnection applications that require an engineering review must use inverters certified under UL1741-SB
- On or after July 1, 2024: All new submitted DER interconnection applications must use inverters certified under UL1741-SB.
The new inverter requirements have been adopted to support City Light’s grid modernization efforts. UL1741-SB inverters have communications functions that can provide important information to City Light. In the future, as more and more customers adopt photovoltaic technologies, the new inverters will allow City Light to understand how much energy customer solar is generating in real time. This new inverter standard supports City Lights’s progress toward a carbon-free, electrified future where customers have more choice about energy generation.
For more information, please contact joelle.hammerstad@seattle.gov. You can review the 2024 RESC on City Light’s website.