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ESS storage … per the GBA post “Storing Residential Solar Power”

sabotcat | Posted in Building Code Questions on

I just posted this under the post…but thought it might get a quicker response over here…

This is a great overview of the code. And it’s amazing how little of this seems to be known or adopted at least where I am. But I have a couple of questions…first the specific “mark” you mention on 9450A rated/tested ESS systems…where where you find that on a spec sheet? Finding 9450A ratings is easy enough, but I’m not familiar with the “Mark.” Mostly I find that with some pushing with the JHA you can get them to let you put the batteries closer together.

But most interesting is the idea that you could get a code approved system with car batteries so long as the dedicated solar shed was 5 feet from the house. There’s a link to an article on reusing car batteries…but no reference to the IRC. Can anyone point me toward the documentation I’d use when proposing such a system? I’ve been running an off-grid 2.75 kWh system with 10.4 kWh or used Tesla Batteries (5.2×2 for 48 v) for years now with an excellent external BMS and no problems. But it’s not permitted. It’s exciting to think I could to something similar and make it street legal.


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  1. Expert Member
    DCcontrarian | | #1

    The article is here:

    I'll just say it: storing power on site is a bad idea.

  2. Expert Member
    BILL WICHERS | | #2

    "Car batteries" are not appropriate here. You would, at least, need "deep cycle" batteries, which are a better fit for this kind of applications. Car batteries are designed to handle very high, brief, discharges (for starting a car), then get immediately recharged. They have relatively thin plates as part of their design, and they are not suited to extended periods of discharge. Deep cycle batteries have much thicker plates, and are better suited to extended periods of discharge such as an off-grid setup.

    I would recommend a different battery though: a "front access terminal" telecom battery. These are MUCH more compact, and better suited to the application here. I just quoted some of these into a project for one of my contractors, and these batteries have a 20 year rated life (which probably means you'll actually get about 15 years of service). Typical deep cycle batteries are usually 5 year rated batteries. You do pay more for the front access telecom batteries, but they are cheaper over time.

    All that fun stuff said, I would avoid doing on-site power storage unless you have no choice. Batteries of any cell chemistry are a risk, and they are a maintenance item too (i.e. you have to service them periodically). You are much better off doing a grid tied system instead, and not using batteries at all. If you're in an area with net-metering, you can, in effect, use the grid as your "battery" if you're trying for net zero. My own preference is to put in systems for "peak shave" instead, which means you size the system to offset the full load of your home during the period the solar panels are producing, and use grid power the rest of the day. Peak shave systems usually pay off in 5 years or less, and you can use this type of system just fine without net metering rates. Batteries are not necassary.


    1. sabotcat | | #3

      Bill and DC Contrarian,
      Huh, I think these are both reasoned analyses, but I'm not sure I agree given current technologies. I have found after 5 years of service, that my Tesla car batteries are phenomenally reliable. It's true the 18650 cells are designed to deliver max amps (for cars) but in an off-grid application, they are barely taxed...and work flawlessly without maintenance!

      It's true that like firearms, pharmaceuticals and power tools, Batteries are not without their risks. But as with any potentially dangerous need only take measures to work within safe parameters.

      Battery storage is going to be a fact of life in the future. And huge strides have been made in the last few years. And while the Lithium Ion technology that we have in our old tesla batteries is not perfect, as we "skim" a bit of power off the batteries each day (and refill it quickly at .25 C the next morning) we have reliable, off-grid power. And a robust BMS system, consistently monitored insures that if anything were to happen that might encourage poor performance in the battery system, it shuts down!

      So my question remains, according to the article in Fine Homebuilding, there is a code provision for reusing these batteries...and I'm still curious if someone can point me toward it.

      But thank you both for your points are good. But my experience has been different. Finally, it's like all building codes and cautions. The rules exist to prevent reckless builders from creating dangerous structures. And here, with the batteries, the rules are designed to allow for the least attention to detail. But if one pays a little attention, inexpensive, reused car batteries and actually provide years of safe, reliable power.

    2. Expert Member
      Akos | | #4

      I think the intent is to re-use end of life EV battery packs. My guess this would mean using the complete battery pack as is inside its OEM enclose and using the original controls. I can't see this happening without the car manufacture's support.

      Once you take a module out of a pack, you are on your own and well outside of any code.

      1. sabotcat | | #5

        that makes perfect sense. Thanks! I thought it fell into the "to good to be true" category. and with 300 square feet of living space, I don't think I need 80kWh! Thanks to everyone for their answers. if there's anything out there more definitive, I'd love to hear what's in the code!

        1. Expert Member
          BILL WICHERS | | #6

          Codes in recent years have actually been changed specifically to DISALLOW the use of a lot of "used" equipment. I am entirely against such changes, since I often use refurbished equipment to save my customers money, and it's a form of recycling too. I can tell you with certainty that the electrical code has NOT been modified to allow the reuse of old car battery packs, and I would be *very* surprised if that were to ever happen, since the trend has been in the opposite direction, unfortunately.

          By "car battery", I meant "lead acid" batteries. Usually that's what people think of as "car batteries". I can tell you that all rechargeable batteries have a "cycle life", which is how many charge/discharge cycles they can sustain, and that's also related to how deep the discharges are (i.e. how close you take the battery down towards 0% capacity before you recharge it). Li-ion cells tend to lose capacity over time, so your "big" battery gets "less big" as it ages, but may still have some useable capacity. I don't see any reason you couldn't eek out the last big of capacity of an old battery pack in the way you describe, but you'd be doing it with no warranty, and that brings additional risks in terms of insurance coverage. A good BMS does not guarantee zero problems, either.

          My own opinion here, and a big part of my work is designing large power systems, is that battery storage is *not* going to be a big part of the future, despite the interest you often see in it now. I'm not going to go too far off on that tangent here, but the short of it is that typical batteries are too short lives and of far too little capacity to be of much use at grid scale. There are better technologies out there that also have far less enviornmental impact, and much longer service lives.

          For some perspective, I have customers where 80kWh of capacity would only support their operation for about a minute, maybe a little less (these are multimegawatt services). For a typical home, 80kWh is probably around two days' worth of capacity. It may sound like a lot, but at grid scale, it really isn't.


          1. Expert Member
            DCcontrarian | | #7

            I agree, battery storage is too expensive and doesn't really scale. As renewables become an increasing share of the electricity mix storage is going to become important at utility scale, but there are other more practical technologies.

          2. maine_tyler | | #13

            DC and Bill,

            I can't help but ask: what storage technologies do you see being more likely than batteries? I am aware of a few other options, but none that look remarkably better in all scenarios, at the moment.

          3. Expert Member
            DCcontrarian | | #14

            On utility scale? Molten salt for solar. Pumping water back up into the reservoir for hydro.

          4. Expert Member
            BILL WICHERS | | #16

            Pumped hydroelectric storage is the best utility scale option. It has essentially no cycle life, it uses no rare earth materials (typical plants are built primarily from concrete, and water is the energy storage medium), and typical design life is many decades, just like "regular" power plants. The only real downside is that the plants need to be located at sites with the appropriate topography and geology. You can easily get gigawatt hours of energy storage with this type of plant.


          5. maine_tyler | | #18

            "The only real downside is that the plants need to be located at sites with the appropriate topography and geology."

            Right, and that can be a significant barrier. Not to mention if one is messing with natural hydrologic flows there are some serious things to consider, like how much water can be recycle vs let through, and impacts to ecology and water temps. It's far from a panacea.

            I still imagine batteries will play a significant role. Maybe not the types of batteries currently used in cars, but batteries nonetheless.

            I've not heard as much about the molten salt outside of CSP applications. Will be interesting to see where it goes with PV coupling.

  3. user-5946022 | | #8

    I took the reference to car batteries to be EV (electric vehicle) batteries.

    Agree that dedicated residential battery storage makes no economic sense for the vast majority of homes.

    HOWEVER, many of us have a $30k-$70k battery operated machine sitting (EV) in the driveway, and there needs to be a commonly accepted and code compliant method to facilitate using that battery power to run a refrigerator in the case of 2-3 day power outage. If I already paid a significant amount for one battery storage bank (in my EV) why shouldn't I be able to leverage that instead of having to buy an entire separate storage bank...

    EV (electric vehicles) are becoming far more common. Likewise, due to climate change, climate extremes and the power outages that sometimes accompany those events are also more common, driving up the need for residential backup power. Traditional backup power came from polluting, expensive and noisy natural gas or diesel generators. The vehicle many already have is perfect for supplying battery backup power.

    Hyundai & Nissan have recognized this - some of their new vehicles will have Vehicle to load (V2L) capability, allowing you to use the vehicle as a residential backup battery. The key to this is codes that require new installations to have the proper equipment in a driveway, carport or garage to facilitate this bidirectional powering, and smart panels that allow load shedding and connecting only critical loads when the backup battery is in use...

    The codes should already account for this, but we all know they are slow to adapt. I'm hoping this is all ancient news in five years....

    1. Expert Member
      DCcontrarian | | #9

      For the past hundred years or so most families have had a gasoline engine with generator attached parked in their driveway, which could be a great backup electricity supply. But there hasn't been any great movement to attach them to home wiring.

      1. Expert Member
        BILL WICHERS | | #10

        I would tend to agree, but with an EV, you do have a big battery already there. It's not quite the same as a gasoline powered vehicle with a relatively small "generator" (alternator) on the side of the engine.

        What I do find a bit amusing though is the idea that somehow converting the home to a smart panel, with a bunch of fancy controls, and using a "$30k-70k" EV is somehow cheaper than a generator. You can easily install a home standby generator system for well under $10k, and if you go with a manual system, under about $2k. That gives you backup power for about as long as the fuel supply holds out. An EV battery is just a battery, and it might help you for a short outage, but if you're out for days after a big storm, you'll run out of battery and be done. Using an EV in place of a generator isn't something you're going to do for economic reasons. Using an EV as backup "because it's already there" is a better idea, but not quite the same thing. Using an EV's battery as power storage for grid load leveling is a *bad* idea for many reasons, but allowing the grid to schedule the charging of that EV to help wit hload leveling is a *good* idea.

        Natural gas fueled generators running for backup purposes are responsible for a miniscule amount of emissions too -- there isn't enough of them to worry about from a pollution standpoint.


        1. kurtgranroth | | #11

          Bill, I do still view the "car-to-home" backup power route to be notably cheaper and more long-term flexible than any traditional gas generator route.

          First, I would not count the purchase of the vehicle into the equation as you presumably are never going to buy the vehicle just for use as a backup generator. Rather, you already have the vehicle and use it for backup power since it's already there.

          Then, you have the cost of the car-to-house interface. Hyundai has a 15 amp 120v adapter for $180 that can handle light loads. The YouTube channel Technology Connections recently ran an experiment to power his house using his car and he estimated a total run-time of around 5 days (albeit with a very light load). Ford has a far more robust 80A 240v adapter for its F-150 Lightning for $1,300 (and it even is included with certain models). This can power an entire house for between 3 and 10 days, depending on load. All from a vehicle that you already have and is already charged.

          Auto batteries can run out, though, and you can always go buy more gas. Yes, but if you can drive to buy more gas, then you can also drive to charge your vehicle! Both have "infinite" fuel as long as you are able to go get more and there's going to be similar constraints in both cases.

          One could argue that you could simply have more gas on hand, but gas goes bad and needs cycling to stay fresh enough to be reliably used. Presumably you will always have your EV topped off and ready to go since you charge it every night.

          I have notable reservations about the whole "vehicle-to-grid (v2g)" concept since that's far more constant use and who pays for the battery degradation in that case. Plus, it shifts responsible for the grid to the users of the grid and I have objections to that entire thought process. But as a BACKUP method, I think using the EV you already have is vastly preferable to any petroleum-fueled generator!

  4. sabotcat | | #12

    Hey all!
    I'm surprised and pleased by the spirited discussion. To my original point: for me...and I think this is a point where we simply disagree. The object for us is to build a house where ESS would work...and pencil out. Moving forward I think it will be increasingly wise to build more thoughtfully and use energy more efficiently. I think Bill mentioned that 80kWh of battery storage would only power one of his clients' projects for minutes. While I'm sure this is true, it also indicates why we seem to be at cross purposes. I apologize for not making this clear as I did in earlier posts. But we're putting together a project that should pull less that 10kWh a day (about half the average for CA). We're also in a spot that has an average of 6.5 hours of sunlight a day. So...if we indulge in other cost saving (but front end expenses) we don't need to buy electricity again. What is that worth? Someone else can do the math, but with 5000 cycles on a LiFePo battery that allows for a couple of days of autonomy if the grid goes down. I'm happy spending them money up front. My sense is that the Grid will not become more reliable and less expensive. So far, we've generally seen utilities looking to make a profit since the days of deregulation.

    But if you're building large houses and are in a fairly industrialized area...I can see the point to not bothering with batteries. But as the weather gets less predictable...and our nervous utility turns off the power when it's windy, we're going to go ahead and take the up-front hit to put in a couple of days of battery storage along with American-made solar panels that can top of batteries in a few sunny hours.

    But for the purposes of my question, the answer is that no one can point to a code statement (regardless of what was written in Fine Homebuilding) that indicates that used car batteries have a place in an ESS system.

    Instead, I'll stick to 9450A tested and approved batteries that can feed back to the grid or not depending on my needs. And if they lose 20% of their storage capacity after 5000 or 10000 cycles I'll live with that. It may not pencil out yet. But it may well soon.

    1. Expert Member
      DCcontrarian | | #15

      If you have access to the grid it's hard to see battery storage ever penciling out.

      For emergency backup, we're a long way from batteries being competitive with liquid fuels.

    2. Expert Member
      BILL WICHERS | | #17

      As DC states, batteries just aren't going to pencil out. Let's assume the battery alone is $20,000, and can handle 5,000 cycles of 80kWh. Let's assume the BMS allows about 80% depth of discharge, and lets assume an overall system efficiency of around 80% (which is probably a bit optimistic). That works out to about 8 cents per kWh stored, just for the battery storage.

      Let's assume you need about 2kWh average to run your home, which is probably close, but maybe a bit light if you have high heating or cooling demands -- or if you have a lot of gizmos to run. You assume about 6.5 hours of charging, so you need to produce 24kWh in that 6.5 hour period to keep up with one day of demand, and you need to allow for the system losses for the recharge portion of the day (24 - 6.5 hours, so 17.5 hours subject to the storage system losses). That means you need to produce about 44 kWh to support the 17.5 hours when your solar system isn't producing, and you need 13kWh while the solar system is producing to support your home's load. That means you need to produce 57 kWh in the 6.5 hours your solar system is producing. Let's assume 90% efficiency in the solar system, which means the solar panels themselves need to produce 63.3 kWh in that 6.5 hour sunny period. That means you need about a 9.75kW solar system, so let's just call that 10kW. A quick Google search shows the average cost per kW for solar to be $ 2,660. That actually sounds high to me, so I'm going to knock that down to $ 2,250 per kW. Your 10kW solar system will cost about $22,250.

      If we assume the battery to last 10 years (cycle life works out to about 13.5 years, but that's probably overly optimistic in terms of other aging factors), then we can "match" the solar system to the battery and consider it to be producing over the 10 year period.

      So we now have some numbers, and the solar production works out to around 9.7 cents per kWh. Add that to the battery costs and you're at about 17.7 cents per kWh. The average cost per kWh in the US on the residential rate is 13.72 cents per kWh (Google mojo again), so your solar system carries a 29% cost premium compared to grid power.

      Some caveats to mention here:
      1- I haven't allowed for any system maintenance. It's probably optimistic to assume the solar system inverter will last for 10 years without problems. It would be better to assume one replacement cycle in that 10 year time period, which would significantly increase your effective per-kWh rate.
      2- I haven't allowed *any* excess solar capacity to deal with things like cloudy days, storms, etc., or *any* extra capacity to handle periods of higher than average load. That means that if you go over the 2kWh average per hour per day, you will NEVER be able to "catch up" and recharge the battery storage system. In a properly designed system, you need to build in excess capacity, probably at least 25% or so, ideally more.
      3- I haven't allowed for any financing costs of the system. This would be a "present worth" calculation, and would make your effective per-kWh rate higher.
      4- I haven't allowed for *any* extra power being produced to let you drive your EV!

      I wouldn't be surprised if there are some other things I haven't thought of that would negatively impact those pricing numbers. The energy storage density is another issue, and right now, diesel fuel is King here. That's really the biggest issue with ALL electric energy storage systems -- squeezing good amounts of power into reasonable spaces with reasonable weights is not easy. More R+D is needed here. I do think we'll eventually get there for transportation systems, but we're only just beginning to get close right now.

      Battery storage and solar isn't going to save you money. If you have an off-grid system with no other options, battery storage makes it practical to have modern power in a true off-grid setting, which is a Good Thing. Where battery storage really doesn't make sense is to try to go off-grid in an area where you can get a grid connection. If you want the best economics with solar, you can't really beat a peak-shave system right now, which is why I tend to advocate for the installation of such systems. Peak shave systems often can pay off within about 5 years or so if you can pair them with a "time of day" electric rate, so that's the easiest and cheapest way to save money using solar, and you also get the green benefit of offseting a chunk of your home's overall energy demand too.


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