As homeowners seek to reduce energy costs and increase sustainability, residential energy storage systems (RESS) have gained popularity. These systems allow homeowners to store energy for later use, providing numerous benefits. Residential energy storage systems enable homeowners to store energy generated from renewable sources, like solar panels, for later use. This technology empowers people to manage their energy needs more efficiently and reduces dependence on the grid.
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Residential energy storage systems consist of batteries that store electricity generated from renewable sources, such as solar panels. Homeowners can use this stored energy during peak demand times or when the sun isn’t shining. The most common types of batteries used in RESS include lithium-ion, lead-acid, and flow batteries. Each type has its own advantages and disadvantages, so homeowners should evaluate their specific needs before making a choice.
By storing energy during off-peak hours when electricity rates are lower, homeowners can reduce their overall energy costs. They can use this stored energy during peak hours when rates are higher, leading to significant savings on utility bills.
RESS allows homeowners to become less reliant on the grid. During power outages or emergencies, stored energy can power essential appliances, providing peace of mind and security.
By integrating renewable energy sources with storage systems, homeowners can significantly reduce their carbon footprint. Using stored solar energy decreases reliance on fossil fuels and promotes a cleaner environment.
Homeowners with energy storage systems can contribute to grid stability. During peak demand, they can discharge stored energy back to the grid, helping to balance supply and demand.
Residential energy storage systems (RESS) work by storing electricity generated from renewable sources, such as solar panels, or from the grid during off-peak hours.
Solar Panels
Many homeowners install solar panels to generate electricity from sunlight. During sunny days, these panels convert sunlight into electricity.
Grid Electricity
Homeowners can also draw electricity from the grid during off-peak hours when rates are lower.
Batteries
The generated or purchased electricity is stored in batteries. The most common types of batteries used in residential systems are lithium-ion batteries, which are known for their efficiency and longevity.
Charge Controller
A charge controller manages the flow of electricity to and from the batteries, ensuring they charge properly and preventing overcharging or deep discharging.
Inverter
An inverter converts the stored DC (direct current) electricity from the batteries into AC (alternating current) electricity, which is used by most household appliances.
Energy Management System
Many RESS come with an energy management system that monitors energy production, storage levels, and consumption. This system helps homeowners optimize their energy use and decide when to draw from the batteries or the grid.
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During Peak Hours
Homeowners can use the stored energy during peak demand times when electricity rates are higher, reducing their reliance on the grid and saving money.
During Outages
In the event of a power outage, the stored energy can power essential appliances, providing backup power and enhancing energy independence.
Net Metering
Some systems allow homeowners to sell excess energy back to the grid through net metering. When the solar panels generate more electricity than the home needs, the surplus can be sent back to the grid, and homeowners receive credits on their utility bills.
Grid Support
During peak demand periods, homeowners with RESS can discharge stored energy back to the grid, helping to stabilize the grid and support overall energy demand.
A communication system allows different components of the RESS to exchange information effectively. This includes monitoring data on energy production, consumption, and battery status, enabling homeowners to manage their systems remotely via apps or web interfaces.
Installations of energy storage systems (ESS) are rapidly increasing across the country, especially for residential dwellings. In my dealings with plan reviews and inspections for ESS, I’m often asked by individuals if there’s any provisions of the code that would allow an energy storage system to be installed within the habitable or living spaces of a home. The focus of this article will be to try and answer that question by explaining some key requirements of codes and standards related to ESS installations within the habitable spaces of a dwelling.
Let’s begin with the National Electrical Code (NEC). When searching through the NEC, one comes to realize that the NEC itself does not get into specifics whether an ESS can be installed within habitable spaces of a dwelling. However, the NEC does require that the system must be installed per the manufacturer’s instructions and per the listing of the equipment [NEC 110.3(B)]. As for the requirement that ESS shall be listed, that can be found under NEC 706.5, but the specific standard that ESS must be listed to is not specified. The reason the specific standard is not noted in the NEC text is because of the NEC Style Manual, which requires that listing standards are to be referenced in the informative Annex A. In Annex A of the NEC, you will find that UL is noted as the standard for energy storage systems. The UL standard is also shown under informational note #2 at NEC 706.1. This author acknowledges that annexes and informational notes in the NEC are not enforceable and are for informational purposes only. But it’s important to point out that UL is indeed the standard for the listing of ESS, which is clarified in other codes published by the International Code Council, one of which is the International Residential Code (which will be explained later in this article).
Two documents published by Underwriter Laboratories (UL) will be discussed here. One is UL , which is the UL Standard for Safety for Energy Storage Systems and Equipment. As stated before, UL is the standard for which ESS is required to be listed and labeled. The other document we’ll discuss is UL A, which is the Test Method for Evaluating Thermal Runaway Fire Propagation in Battery Energy Storage Systems. UL A includes testing provisions for determining if a battery technology has the capability to go into thermal runaway and, if so, what fire and explosion hazards are associated with the energy storage system during a thermal runaway event. It should be noted here that the thermal runaway testing of UL A is often referred to as “large-scale fire testing” per the International Fire Code and International Residential Code, published by the International Code Council.
When it comes to where an ESS can be installed in a residential dwelling, the UL Standard has some key requirements that apply. UL , Section 23.2.2 of Edition #2, requires that electrochemical ESS that is intended for use in the living or habitable space of a residential dwelling is required to meet the performance requirements of the Cell Level Test of UL A and be marked per UL Section 41.3(n). In the UL Standard (again, Edition #2), under Section 41.3(n) it’s noted that systems complying with Section 23.3.3 must be marked “Suitable For Use in Residential Dwelling Units Where Permitted.”
This particular wording has been shown to be confusing for authorities having jurisdiction (AHJs) who sometimes misinterpret the requirement, meaning that the only way an ESS could be installed at a residential dwelling would be if the system had the marking with that specific wording. However, it was never intended for the “Suitable For Use in Residential Dwelling Units Where Permitted” marking to be required in order for an ESS to be installed anywhere at a residence; it was only intended to be required when an ESS would be installed in the habitable or living space of a dwelling. In Edition #3 of UL , the requirement for marking [moved to Section 45.3(e)] was updated for clarity to specify “Suitable for Use In Residential Habitable Spaces.” Again, this marking would indicate that the system has been able to meet the performance Cell Level Test requirements of UL A [see also Section 26.2.2 of Edition #3 of UL ].
It must also be pointed out that just because UL A may be indicated or shown on a manufacturer specification sheet, that does not mean that the Cell Level Test provisions of UL A have been met. In most cases, if UL A is shown on manufacturer specification sheets, it usually means that the system has met the performance requirements of the Unit Level Test, not the Cell Level Test. It’s beneficial for a system to be able to meet the Unit Level Test provisions of UL A, but unless the system has met the Cell Level Test provisions, the system is not allowed to be installed in the habitable or living space of a dwelling.
In the edition of the International Residential Code (IRC), published by the International Code Council, Section R327.2 requires that “stationary storage battery systems” be listed and labeled in accordance with UL . As discussed already in this article, UL has provisions that potentially allow ESS to be in a habitable dwelling space if certain requirements are met. But Section R327.3 of the IRC clearly specifies that ESS shall not be installed within the habitable space of a dwelling and does not give any alternatives otherwise.
Now, jumping to the edition of the IRC, for equipment listing, Section R328.2 requires that ESS be listed and labeled in accordance with UL . Section R328.4 also lists locations where ESS is permitted at a dwelling. The first allowable locations noted are detached garages and accessory structures. The second is the attached garages, which are properly separated from the living space of the home with gypsum wallboard meeting the requirements of R302.6. Third is the exterior location of the walls of the home, where the ESS is located not less than three feet from doors or windows that open directly into the dwelling. And lastly, noted as potentially acceptable locations for ESS are enclosed utility closets, basements, and storage or utility spaces – as long as such spaces meet certain requirements for wall and ceiling coverings and such room or space does not open directly into a bedroom. So, there are options for where an ESS can be installed at a dwelling other than habitable spaces. However, out of the areas listed under Section R328.4, of particular interest, and often a point of confusion, is the word “basements.” At a quick glance, it may seem acceptable to install an ESS within the basement of a dwelling. However, it’s important to remember that the requirements of UL must still be met for the system. Once again, unless an ESS has been tested having met the Cell Level Testing criteria of UL A and is marked “Suitable for Use In Residential Habitable Spaces,” the system is not permitted to be installed within living or habitable space, even if the space is a basement.
At the time of writing this article, and to this author’s knowledge, no energy storage systems that have been tested thus far have been able to meet the performance Cell Level Test requirements of UL A, even though this author has come across a couple of ESS manufacturers who have tried to claim their systems meet the Cell Level Testing criteria. What this means is there does not appear to be any ESS currently on the market that is allowed to be installed in the habitable or living space of a dwelling, even if a manufacturer claims otherwise. If an AHJ happens to come across a system where the manufacturer is claiming their system can be located in the habitable space of a dwelling, it’s strongly recommended that the AHJ ask the manufacturer for the actual Cell-Level Testing report (produced by the laboratory which performed the UL A testing) to verify if the system really has been able to meet the Cell-Level Test provision of UL A.
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