What Is California Rule 21? Understanding Smart Inverter Requirements

California is the first state to require the use of smart inverters in solar projects. Smart inverters are mandated in California Rule 21 (or CA Rule 21), which lists communication requirements that will increase the interoperability and security of solar inverters on the connected smart energy grid.

Why Is California Rule 21 Being Mandated?

California is a solar energy leader, making significant strides toward a clean energy future, and has set a goal to achieve 100 percent clean renewable energy by 2045. The state recognizes a huge opportunity to leverage smart devices in renewable energy efforts, where advantages include increased efficiency, reliability and visibility into energy consumption. Estimates suggest that there will be more than 1.5 million connected devices by 2020. As each connected device is added to the larger energy grid, there is also new potential malicious exploit and intrusion.

California Rule 21’s smart inverter requirements were created to allow better hosting capacity with minimal infrastructure upgrade. This rule paves a new path for Distributed Energy Resource (DER) growth and decreased fossil fuel usage.

California is the first state to adopt these requirements, but if you’re a solar contractor in any state, you may want to educate yourself on these new requirements. Other states are expected to follow suit with similar requirements in the near future.

Who Is Affected by California Rule 21?

Solar installers in California are directly affected by California Rule 21, but other parties include:

  • Inverter and hardware manufacturers
  • Software and aggregator providers
  • Installers and engineering, procurement and construction (EPC) firms
  • Policy makers

California Rule 21 consists of three phases:

  • Phase 1: This phase became effective in September 2017 and states that new solar projects must use a smart inverter and comply with testing protocol UL 1741-SA.
  • Phase 2: This phase becomes effective on August 22, 2019, and establishes communication requirements for inverters. In this phase, inverters must be able to communicate effectively over the Internet using the IEEE 2030.5-2018 networking standard.
  • Phase 3: Parts of this phase will be become effective on August 22, 2019, and will require advanced inverter functions.

How Do I Become Compliant with California Rule 21?

We understand that the path toward California Rule 21 compliance can be confusing. SunSpec Alliance—an information standards and certification organization for the DER industry—is entrusted to enforce California Rule 21 communication requirements for all solar inverters and energy storage systems. Kyrio is the official SunSpec PKI provider and SunSpec Authorized Test Lab provider and is equipped to provide security and testing services to help you on your path toward compliance.

SunSpec Alliance, Kyrio and Sectigo will be hosting a series of webinars in the coming weeks to educate attendees on what is expected on the California Rule 21 roadmap. To learn more about how to get started on getting your device certified and compliant, register for the upcoming webinar now:

Register Now

Prefer to skip the webinar? Register for our onsite workshop on March 25th instead.


For manufacturers and service providers, Kyrio accelerates and deploys new network innovations into the ecosystem. Backed by the power of CableLabs, Kyrio sets technology on a path to commercialization, enabling not just today but tomorrow’s communication.