UL 1741

UL 1741 is a safety and performance standard published by UL (Underwriters Laboratories) that covers inverters, converters, charge controllers, and interconnection system equipment (ISE) intended for use with distributed energy resources (DERs), including solar photovoltaic, wind, fuel cells, and battery energy storage systems. It defines the requirements for safe grid connection and establishes the test framework used to demonstrate compliance with IEEE 1547 interconnection requirements in the United States.

Published by: UL (Underwriters Laboratories), also designated ANSI/UL 1741

Current edition: 3rd edition, September 2021 (ANSI/UL 1741-2021), with Supplement SB for IEEE 1547-2018 compliance

Replaces: UL 1741:2010 (2nd edition); Supplement SA first published 2016

Primary market: United States (required for utility grid interconnection approval)

Scope

UL 1741 applies to equipment for use in stand-alone or grid-interactive (utility-interactive) DER systems. Covered equipment includes:

  • Inverters and power converters for solar PV, wind, and other generation sources
  • Charge controllers for battery energy storage systems (BESS)
  • Bidirectional inverter-chargers for hybrid and storage systems
  • Grid-interactive controllers and power conditioning equipment

Exclusions: UL 1741 does not cover the battery cells or modules themselves (covered by UL 1973 for stationary storage, or UL 1642 for lithium cells), nor the overall energy storage system enclosure (covered by UL 9540). It does not apply to equipment operating on DC bus systems above 1,500 V DC or to utility-scale generation on the transmission grid.

Key Requirements and Test Methods

Core UL 1741 Requirements

  • Anti-islanding protection – inverter must detect and disconnect from the grid within specified time limits when the utility supply is lost, preventing energising of a de-energised grid segment
  • Over/under voltage protection – disconnection thresholds and trip times as defined in IEEE 1547
  • Over/under frequency protection – disconnection thresholds and trip times as defined in IEEE 1547
  • DC injection limits – maximum DC current injection into the AC grid to prevent transformer saturation
  • Ground fault protection – detection and disconnection for ground faults on the DC side
  • Power quality – harmonic distortion limits (THD), power factor requirements
  • Surge withstand capability – immunity to voltage surges and transients on the AC grid
  • Enclosure and dielectric withstand testing for electrical safety of the physical product

Supplement SA – California Rule 21

UL 1741 Supplement A (SA) was developed to support California’s Rule 21 smart inverter requirements. It extends the base standard with advanced grid support functions:

  • Voltage ride-through (VRT) – inverter must remain connected and ride through specified voltage disturbances rather than disconnecting
  • Frequency ride-through (FRT) – similar ride-through capability for frequency disturbances
  • Volt-VAR control – reactive power adjustment as a function of grid voltage to provide voltage support
  • Frequency-watt control – active power reduction as a function of over-frequency to support grid stability
  • Required for grid interconnection in California; uses California-specific grid parameters

Supplement SB – IEEE 1547-2018

UL 1741 Supplement B (SB) provides the test framework for compliance with IEEE 1547-2018, the national standard for interconnection and interoperability of DERs. SB is recognised by most US states that have adopted IEEE 1547-2018 (the majority of states from approximately 2020 onwards):

  • Ride-through requirements aligned with IEEE 1547-2018 Category I, II, or III performance categories
  • Reactive power capability – Q(V) and Q(P) modes, power factor set-point control
  • Active power curtailment – watt-watt and watt-VAR control modes
  • Many modern inverters carry dual SA+SB listing for hardware compatible with both California and national requirements

Certification and Regional Requirements

  • Most US utilities require UL 1741-listed inverters for grid interconnection approval. Supplement requirements (SA vs SB) vary by state and utility tariff
  • California (CPUC Rule 21): UL 1741 SA (or dual SA/SB) required
  • Most other US states (post IEEE 1547-2018 adoption): UL 1741 SB required
  • Hawaii (Rule 14H): Has its own requirements distinct from SA and SB
  • For a complete BESS installation: batteries must meet UL 1973, the power conversion system must meet UL 1741, and the integrated system must meet UL 9540
  • Canada: CSA C22.2 No. 107.1 is the parallel Canadian standard; many manufacturers pursue combined UL/CSA (cULus) listing

Related Standards

  • UL 1973 – Standard for batteries for use in stationary, vehicle auxiliary power, and light electric rail applications (covers the battery system in a BESS)
  • UL 9540A – Test method for evaluating thermal runaway fire propagation in BESS
  • NFPA 855 – Standard for the installation of stationary energy storage systems (references UL 1741 for power conversion equipment)
  • UL 1642 – Standard for lithium batteries (cell-level safety, often required alongside UL 1741 certification)
  • IEEE 1547 – Standard for Interconnection and Interoperability of Distributed Energy Resources with Associated Electric Power Systems Interfaces (UL 1741 is the primary test vehicle for IEEE 1547 compliance)

Disclaimer: This is not a definitive guide, just an indication and it is important that you always go back to the source for legislation.

References

  1. UL 1741:2021 – Standard for Inverters, Converters, Controllers and Interconnection System Equipment for Use With Distributed Energy Resources, UL Standards
  2. ANSI/UL 1741 Ed.3-2021 – Standard for Inverters, Converters, Controllers and Interconnection System Equipment, ANSI Webstore
  3. UL 1741 SA vs. UL 1741 SB: Inverter Certification Guide, SurgePV
  4. UL 1741 Standard Explained for Inverters, EcoFlow

See also: Grid Battery Energy Storage Systems, IEC 62477-1