Energy storage systems (ESS) might all look the same in product photos, but there are many points of differentiation. What power, capacity, system smarts actually sit under those enclosures? And how many of those components actually comprise each system? The number of options – from specialized component providers to all-encompassing ESS + smart circuit functionality – is unwieldy and often hard to pin down. The 2025 Solar Builder Energy Storage System Buyer’s Guide is here to cut through the noise.
This ESS Buyer’s Guide is a comprehensive list of what each brand is offering in the residential and C&I space heading into 2025. We sent a questionnaire to every manufacturer to ascertain their top product and what components are included. Is it a hybrid inverter with a roster of battery partners? Is the battery included? What about the energy management or smart circuits?
Below are all of the answers we received. They are listed in alphabetical order. To improve readability a bit we divided them into groups:
- Hybrid inverters and smart panels
- Full ESS w/ battery
- Batteries and BMS
- C&I size systems
A big focus this year was also UL 9540 certification. We asked if each product was listed to UL 9540, and for a link to the certification documentation. Not many provided the link, but some did – look for that in the tables.
What is UL 9540?
As part of our 2025 Energy Storage System Buyer’s Guide, we asked manufacturers to explain 9540A testing, and what installers should keep in mind when installing ESS and batteries listed to UL 9540. The UL 9540 certification focuses on the safety and performance of energy storage systems (ESS) and their components. The UL 9540 certification and 9540A test results are very often used in conjunction to show the safety and efficacy of battery storage. UL 9540A testing evaluates the risk of fire propagation in energy storage systems by determining if a thermal event in one battery module can spread to others.
Test results “guide the need for safety measures like fire barriers, ventilation systems to handle gases or heat, fire suppression systems for larger installations, and recommended spacing between battery units or nearby structures to reduce fire risk,” says Jon Proffitt, Head of Business Development, Home Solutions SMA America.
“Some ESS systems have location restrictions, requiring outdoor installation or fire-rated enclosures. In some cases, batteries must also be protected from direct impact with the use of specific mounting or enclosures. In other cases, systems may include monitoring or shutdown mitigation such as self-activated fire extinguishers, and installers must follow installation and commissioning guidelines.”
“Under UL 9540, Ed 3, the 9540A testing is a requirement, along with more stringent testing for communicating batteries and seismic,” says Sequoya Cross, VP Energy Storage, Briggs & Stratton Energy Solutions. “All batteries that want to achieve UL 9540 will need to complete the module level testing. Unit level testing is an additional step to advise on spacing and install location.”
“The intent of this test is to prove that within a system as installed there is no risk of propagation from battery to battery, and that the surface temperature of what the unit is installed on does not exceed 175°F,” Cross says. “Additionally, there cannot be any explosion hazards or flaming beyond the outer dimensions of the BESS unit. If the unit meets all of these requirements, no further testing is required.”
Beyond cell, module and unit level tests, manufacturers can conduct system level testing that incorporates fire suppression mechanisms to prevent flaming outside of the test area and explosion risks. Once the tests are concluded a report is generated on the results and delivered to the manufacturer and NRTL. These tests are evaluated for the UL 9540 certification.
Why UL 9540 matters
“UL listing simplifies several steps in the process. It tells installers the system meets a minimum standard in the industry, streamlines the project acceptance process, and eliminates the need for field testing of components,” says Rich Law, Sr. Engineer – ESS Technical Solutions (CCI), Sungrow. “When the latest UL9540 standards and UL9540A test results are conducted by a certified third-party tester and successfully passed, it provides the installer confidence the system was designed by the OEM to meet quality and safety standards widely utilized by the energy storage industry.”
What should installers keep in mind when looking at UL 9540 certification and 9540A testing results?
Eli Chu, Product Manager from Pytes Energy says installers should focus on:
Gas Release Data: Review any information on hazardous gas emissions during a thermal runaway event. Understand the gases that may be released and their potential hazards. Ensure the installation design includes adequate ventilation to mitigate the risk of gas emissions and/or buildup.
Minimum Spacing: Review specific recommendations for minimum clearances required between ESS units and walls or between multiple ESS units to prevent fire spreading. Installers should ensure these recommendations are followed to mitigate the potential for thermal runaway propagation within a system.
The NFPA 855 gives the standard as 3ft in separation distance, but if the battery is properly tested, the manufacturer can specify shorter distances.
“If a battery doesn’t need less than 3 feet of separation between each 20 kWh unit, or is installed outdoors, the additional testing is compulsory,” Cross says. “However, most battery companies want to go through this extra step to provide the most flexibility of their products for installers.”
The certification is also inclusive of the installation manual and requires that installers follow the instructions in order to achieve a UL 9540 certified installation. “Additionally, AHJ’s may have different requirements based on their building and fire codes that may limit or allow different installations beyond what UL 9540 and the install manual state due to building density, construction and fire risk,” Cross says.
AC and DC ESS: Under UL 9540, Ed. 3 there is also modification for battery systems to have AC or DC ESS certification. An AC ESS is inclusive of named/specific inverters. For DC ESS, the battery does not rely on the inverter to perform safety functions under UL 9540A, and that it can stand alone to perform all of the requirements. This allows the DC ESS to be installed with any inverter that has met its own safety certifications such as UL 1741.”
Regarding installation guidance for UL listed batteries, Jane Curtis, CEO of Renon Power says there may be the following differences:
- Detailed Installation Instructions: UL certified batteries typically come with detailed installation instructions, including the necessary tools and steps. These instructions help ensure that each step of the installation process meets safety and performance standards.
- Specific Environmental Requirements: There may be specific requirements for the installation environment, such as temperature range, humidity control, and fire protection measures.
- Testing and Verification Steps: Specific testing and verification may be required after installation to ensure the safety and functionality of the system.
- Maintenance Guidelines: Detailed maintenance and inspection guidelines help installers and users maintain the long-term safety and reliability of the system.
Discerning between UL listed 9540 batteries and unlisted, illegal batteries, is of utmost importance in the market in 2025. If you want to know why we focused so much on 9540, be sure to listen to our recent Solar Builder Buzz podcast.
