Everything is Interconnected

The 2023 National Electric Code kicks off its interconnection discussion with clear examples of AC and DC interconnection examples. This elicits a popular debate over whether AC or DC coupling is better, as well as a more recent debate as to whether listed DC coupled systems must co-list the AC inverter. The DC interconnected example reflects hybrid inverters, even detailing the possibility of having advanced grid controls built directly into the inverter. But the graphic implies the UL9540-2020 Edition definition of a DC ESS (which includes the inverter) as opposed to the UL9540 2016 and 2023 Editions which may list battery systems without any inverter co-listing. That is a subject better explored in a conversation about UL9540, but it’s worth mentioning that DC ESS with inverter agnostic compatibility is once again allowed, after being previously disallowed in the prior code cycle. So there’s still a lot going with the definitions of interconnection. “Stand alone mode” is the term for off-grid. “Grid Interactive mode” is the term for grid-tied. And an inverter which can do both is a “multi-modal” inverter.

NEC 705’s AC interconnection example is similar to traditional solar battery systems, where solar is on its own inverter, and the batteries are on their own inverter, like early models of the Tesla Powerwall. Essentially, AC-coupled is when you have a lot of inverters wired in parallel to each other on the AC side. Often, that might consist of “multi-modal” battery inverters which and “grid interactive” solar inverters which follow along whatever power source is forming the grid.

The debate as to which architecture is better is overblown. Both are adequate for grid-tie backup. There are slight differences, which become more important for advanced design such as off-grid or very large applications. It’s good ot know AC versus DC coupling advantages and disadvantages. AC coupling might be less elegant and controllable, but for commercial rooftops, it is very practical due to availability of built-to-purpose inverters like the inexpensive SMA Sunny Tripower. My design preference is for solar DC coupling with hybrid inverters, but hybrid inverters are best located near either the batteries or points of interconnection. If the solar array is far away from that point, it can make since to combine all the solar circuitry together on the roof with a dedicated solar inverter and then AC-couple to the hybrid inverter GEN port, such as found on the Sol-Ark 30K-208V or 60K-480V hybrid inverters.

Now, if that were a solar carport, where the interconnection, solar array, and batteries are all located in the parking lot, DC coupling is ideal. If anything, it keeps the GEN port available on hybrid inverters for use with other items, such as convenient EV charging interconnection (as makes sense in a solar carport). And for residential application, I prefer DC coupling simply because it is less complicated to reduce the number of power sources within a system, which as an increasingly important consideration if you want to incorporate other sources like gas generators or multi-modal EV charging. But if the solar array already exists, or if the installer is wed to using microinverters on the rooftop, then AC-coupling gets the job done too.

DC coupling has a variety of advantages, but the most basic is that it runs at higher voltage than AC building electricity. The higher the voltage, the lower the amperage for the same amount of power. And the diameter of that copper power cable has more to do with amperage than voltage. So for example where the solar array is far away from the inverter, such as a ground mount or far array roof mount, it makes sense to stay with DC coupling. Also, pragmatically, a hybrid inverter has DC-coupled solar ports. It’s a shame not to use them. There’s nothing more simple than taking the DC strings directly from the solar array and landing them directly onto the inverter, even with a long distance in between. in and out of the home and under… Through some roads, and the rest being direct burial cable.

The fact of the matter is that any site with multiple inverters uses AC-coupling. There is a debate as to whether or not it is better to use one big inverter verses a few smaller inverters, if not dozens or hundreds. From a cost and simplicity perspective, one big thing usually wins the debate compared to hundreds of small things. This is why I like the Sol-Ark whole home backup hybrid inverter specifically. At 15kW AC output, it is the largest residential inverter on the market in terms of AC power output, which results in the most simple system architecture. It also manages the grid interconnection internally, meaning no additional equipment is necessary to have full access to grid power, while providing power to all home circuits during an outage. Sol-Arks competitors offer inverters between 5kW and 12kW, meaning multiple ESS have to be installed, as well as an energy management panel, to have similar or better functionality. Line item by line item, an individual part of a smaller AC ESS might be less expensive than a SolArk. But a the bird’s eye view of the project will show that designing and installing around bigger building blocks will always be cost-effective, and large form factor hybrid inverter architecture simplifies things further by combining battery, grid, and interconnection scope.

The term “whole home backup” really comes from the generator industry. “Whole Home” generators have been sized between 20kW and 24kW traditionally. Now remember that a typical residential electric service is 200 amps at 240 volts, in other words, 48 kilowatts. So a “whole home” backup generator is not actually capable of running every circuit in the house simultaneously, and neither are typical whole home backup ESS.

The question is, where do you interconnect a home hybrid inverter? Do you do interconnect like traditional solar installations, on the load-side breaker at the bottom of the service panel? Or do you intercept the conductors between the meter and the main breaker? For “whole home” hybrid inverters, the common path is to do a supply-side connection, between the utility meter and the main service panel. This is a very meaty connection. Fundamentally, the conductors between the meter and the main service panel are rated for the service connection. If the home has 200 amp electric service, the service conductors are rated for at least rated for 200 amps. A 200A system could be interconnected and feed the service. The home doesn’t care if the 200A come from the grid or home.

Regarding 200A connections, evaluate the inverter terminations closely. It’s common for smaller hybrid inverters to include breaker switches for grid, load, and generator connections, but the larger the inverter model, the less switches are included. This is because the breakers themselves get bigger. A 200A breaker will not fit on the load side of a service panel, and for the same reason, there are limitations to what can be fit into inverter wiring boxes. Furthermore, NEC requires a readily accessible AC ESS outside the home, and it gets expensive to purchase redundant high amperage disconnects. So the Sol-Ark 15K only has a load and battery disconnects. Likewise the 30K and 60K commercial models do not include any breaker disconnects. Whereas the Sol-Ark 12K has breakers for the load, grid, and generator, which are sized similar to breakers found on the load-side of a residential service panel.

It’s obvious that a breaker can be used to interconnect an device to a service panel. But how does a supply-side interconnection work? Supply side taps which intercept the conductors between the meter and the main service panel typically require a site power down. The electric meter is pulled, and the taps are installed. the site, you have to pull the meter, and then taps are installed, providing an additional interconnection point. The tap has an amperage rating as well. But from the electric service panel point of view, it doesn’t matter how large the source is, the panel will only be fed 200A from the main breaker, because if more power is drawn, it will trip the main breaker. So supply-side connections can feed feed as much as 200 amps through the meter into the panel. This is precisely what the “grid pass through” capability of a whole home hybrid inverter does. Although the Sol-Ark 15K can only pass through 180A continuously, most homes don’t get anywhere near that. If they do, they’re already upgrading to 400 amp services. Sol-Ark increases its pass-through amperage cumulatively, such that a minimum “whole home backup” system design of 400A service would require at least two Sol-Ark 15Ks, perhaps double lugged at the external ESS disconnect, and then landed onto a 400A Main Lug Only panel, making effective use of the inverter harder.

Proper load side interconnections used to be a very important subject in solar design, with the famous 120% Rule, which in short, allows up to a 40A source breaker on the load side of a 200 amp service panel. Those rules have gotten much more flexible, as we’ll learn in a few slides. Despite being designed for high amperage, supply side connections, whole home hybrid inverters can be interconnected to the load side of the service panel in a few cases. It’s less of a question as to what is possible, and more of a question of “why would you?”, but there are a few good answers. At the very least, some service panels, like certain meter-based combo panels, force the designer’s hand and require a load-side connection.

One method to circumvent the 120% rule is to list the inverter as a power control system, capable of monitoring the busbar and adjusting the inverter output to ensure the busbar is not overloaded. This UL1741CRD listing allows any sized of listed-PCS inverter to be connected to the load side of a service panel. The historic concern was that the grid could supply 200A of power to the busbar and the ESS could supply, say, 100A of power to the busbar, and that the busbar on a 200A service panel is rated for 200A. Count up the breakers on your electric service panel and they likely add up to around that much, and if all those devices were used simultaneously, it could cause the service panel to overheat, creating a fire hazard. But the UL 1741CRD listing, which is a special PCS listing beyond standard UL1741, which indicates additional monitoring and control capability, will restrict the power supplied to the busbar to within its limits, in this case, 200A. Both the SolArk 12K and the SolArk 15K are PCS rated, and so can be load-side connected to the busbar.

The term PCS is slightly problematic. If the term PCS sounds familiar, PCS inverters are already used in industry to indicate very large commercial battery inverters, which have slightly different architecture than residential inverters. While a residential hybrid inverter may be a listed PCS, it is not quite the same PCS as how the term “power control system” is used in commercial energy storage. Also, there are products which can monitor and control busbar voltage that are not inverter, such as Span and Lumin smart panels. This National Electric Code busbar interconnection exemption to the 120% rule has now relabeled PCS as EMS, or “energy management system”, and so the UL1741CRD listing confers PCS status to the inverter, which is a type of EMS, for the purposes of exempting from the 120% rule.

Despite losing whole home backup capability, here are some values in keeping the interconnection on the load-side, despite it being less common. While losing whole home backup capability, connecting to the load side of the service panel means the electricians don’t need the power company to visit site to power down the electric service, so the installation is faster. Secondly, there might be an impossibility of a supply-side connection, such as certain meter-based all-in-one panels. Third, an existing 200A service might be overloaded to the point a 400A service upgrade is required to add new devices like EV chargers. Instead, adding a battery inverter to power the device prevents overloading the panel, saving the cost of the upgrade.

The Sol-Ark 12K makes for a very easy load-side interconnection, because it includes breakers for the load, grid, and gen ports. Why might a Sol-Ark 15K be used instead of a Sol-Ark 12K? To start out with, the 12K is unfortunately named, only having 9kW of AC output capacity. At the time of its release, it seemed innovative to add an additional 3kW of DC side charging power, such that under the right circumstance, it can process the same amount of power as a 12 kilowatt inverter. But ultimately, AC power is more important than DC power, and the inverter is only 9kW AC. So the 15KW, even with providing external disconnects, is often cost-effective still, because of the additional 6kW of AC output capability. That’s the difference between running large HVAC systems in parallel with other loads, or the ability to do laundry and and cook dinner at the same time.

At any rate, when performing a load-side connections, the AHJ may require use of Sol-Ark’s user lockout feature to restrict access to the inverter settings, in the instances where the inverter capability exceeds the interconnected power run between the inverter unit and the main service panel. While this run is also protect by in-line overcurrent protection at the breaker, its not a good practice to rely upon safety systems to govern normal system operation. Locking the user out of the settings changes, and that’s done with SolArk by two ways. One, you can lock them out of the LCD screen locally as the last part of system commissioning. After checking the locking setting on the LCD screen, it will require a support call to Sol-Ark to get the unlock code. The second step in locking out the user from settings adjustments is to designate them as a viewer rather than manager within the user profiles of the online monitoring system.

Now that supply and load side connections have been discussed, it’s time to point out some additional flexibility found on most hybrid inverters, which is an additional GEN Port. The key takeaway here is this the GEN port is another tap point of connection that can be used to land a generator onto the system, but it can also be used for other devices, such as AC coupled solar, an additional electrical sub-panel like an outdoor panel, or an electric vehicle charger. Regardless of how many inverters are installed, this GEN port can only be programmed for one functionality.

So that Gen Port is incredibly flexible. On the 15K model, it is large enough for 24kW generators, although output will be throttled down to 19.2W. The Gen Port is rated to 100 amps max, but is software constrained to 80 amp continuous. On the 12K model, the GEN port is half the size, and so can accommodate smaller portable generators or EV chargers.

Now, the Gen Port is bidirectional, and one way to think about the Gen Port is that it is purely an additional tap that you can put high amperage devices onto, that is a sheddable relay. And so it’s intended purpose is to disconnect if the total system is overloaded, and also act as a load shed for when the batteries are at low states of charge.

If you can think of no other use for the GEN port, then don’t forget the smart load function, which can provide battery state-of-charge load control to any device or subpanel. In principle, I’m skeptical of load control. Imagine managing your electricity by standing in front of a service panel, periodically turning breakers on and off. It does not make sense much sense for most home devices, which could otherwise be managed through manually such as recognizing a power outage and then adjusting the thermostat setting, remembering not to do laundry and cook at the same time. But there are some energy management advantages of moving all miscellaneous loads to a subpanel which sheds during low battery states of charge. This technique can significantly extend the backup capability of a home, and because it only requires a generic subpanel, it is much less costly than the installation of a traditional smart panel.

At the very least, using the GEN port for an electric tank water heater is almost like gaining a free hot water battery, because it can heat hot water with solar production. When the smart load state-of-charge controller is combined with the inverter time-of-use controller, this functions as a load shed, only enabling hot water during the day, when solar is likely available. And both inverters and tank water heaters are commonly located in shared spaces like garages or utility closets, so this is a relatively easy scope addition to a residential project.

What I like most about hybrid inverter GEN ports is it provides a flexible architecture. If something was missed in the design process, whether it be an existing solar array, or the need for upgraded electric service such as when adding an EV charger, or both indoor and outdoor service panel, it provides a solution for the challenge at hand. So the Sol-Ark 15K deserves credit for being flexible enough to accommodate an incredibly wide range of jobsites and applications, helping installers resolve otherwise costly design mistakes or additions, as well as simplify inventory management.

What about a vehicle-to-load export power from EV? Can the EV output look like a generator input to the Sol-Ark GEN input, in addition to charging the vehicle? The short answer is yes, but no specific multi-directional EV charger settings have been programmed into the inverter. The ability to add dozens of kwh of storage to the home during a grid outage is revolutionary to ESS design.

But while it’s relatively easy to charge your electric vehicle during a grid outage, being able to discharge in concert with a manufacturer agnostic platform is still in its infancy. If the ability to discharge the electric vehicle charger is desired, I recommend thinking of the EV charger as a battery inverter. Is it capable of discharging to the grid? Is it capable of forming its own grid? If capable of forming its own grid, is it programmed to synchronize to the ESS instead? What is the relative side of the charger compared to the load? Uni-directional EV charging is very simple, but bidirectional EV charging still requires expertise. Keep thinking creatively about incorporating EV charging into solar and battery design. In the future, you might even incorporate batteries and solar into the EV charger system, rather than the other way around. For example, the Sol-Ark 8k-1p is not capable of whole home back on its own, but can charge an EV. Together, any V2H whole home backup system could act as the grid to the 8k-1p, providing a way to backup the home and charge the car, erstwhile prioritizing lowest possible project cost.

One important item to note, is that when installing with multiple inverters, it’s very easy to imagine the GEN port on each inverter managing different features, like using one port for a generator connection and another for a smart load panel. But unfortunately this is a case of imagination exceeding reality. Balanced against all-expansiveness is reliability – there are communication and monitoring errors that become more complicated with inverters wire in parallel as part of a single system, and Sol-Ark errs on the side of reliable, restricting the GEN port to a single purpose. If multiple GEN port features are desired, then some of those devices will need to be interconnected on the LOAD side, such as a Span or Lumin smart panel.

Let’s talk about transfer switch capabilities and options. When a whole home ESS fails, the concern becomes whether or not the building has any power source at all. Any ESS architecture, hybrid or otherwise, has some microgrid interconnect device between the home and the grid, so what happens if that device fails? Keep in mind this is a small fraction of possible inverter failures – in other words, there are many cases where an inverter can fail and keep the building powered with grid or generator power. Nonetheless, its good, but not mandatory, to have a backup plan for when the backup plan is in servicing. There’s different ways to do this, with a variety of cost options.

The most expensive and professional option is a manual bypass switch to reroute grid power to the home. While expensive, keep in mind an external ESS grid disconnect is already a project requirement. Landing additional, fused conductors onto the bypass meets all disconnect requirements and provides full bypass power to the building. For example, crimp-on fuses are useful here, as most of these manual throw switches are not fused throw switches. There is also wisdom in adding a second inverter for redundancy, if the budget allows and power demands are justifiable.

That said, for projects on a budget, a smaller option may be sufficient for service power, to power essential loads. This less expensive option is not intended for whole building temporary power, but is sufficient to provide a small amount of power when the system is being serviced. This solution is a generator interlock switch. And be careful implementing these as some jurisdictions require the service panel be intended for a generator interock switch with pre-drilled holes for mounting, because as you can see from this photo, otherwise the service panel must be modified to accomodate the security plate added to the panel. The concern being that drilling holes in service panel can violate its UL listing. It is strongly recommended when retrofitting to confirm service panel brand and model compatibility. Eaton panels are coming with pre-punched zones for Eaton-supplied interlocks.

What this Generator Interlock Switch does is lock the main breaker coming from the ESS out of the system, providing a a grid bypass the size of whatever load-side breaker is installed at the top of the busbar. Assuming the ESS has an external disconnect, double lug taps could be installed to connect both the 200 amp ESS grid input, as well as the smaller interlock redirect, let’s call it 50A. At the very least, that would allow the homeowner to keep all their refrigerator, kitchen, garage, well pump, and mini-split running while the inverter is being replaced. At the very least that can keep the system owners in good spirits. The generator interlock solution isn’t for everyone. Many installers will find it too cheap. Many installers, the customers would just prefer a 200 amp throw switch, or something even more automatic than that. But knowing the option exists can be a way to differentiate yourself from being the lowest bid installer.

We’ve covered whole home backup in great detail, so let’s talking about some not-whole-home interconnection options, which might be very lightweight or middle-of-the-road regarding budget and capabilities. Sol-Ark has a Lite option, the 8K-1P is that is pure 240V AC, rather than other Sol-Ark inverters producing 120V/240V split phase service. Because of this, the Sol-Ark Lite can’t be used for home backup power. Technically it could still backup 240V circuits like a dryer, water heater, or EV charger. Which going back to the EV charger discussion earlier, could provide some interesting vehicle-to-home compatibility. The great thing about the 8K-1P is its price. The cost of that 120/240V transformer has to come from somewhere. So the Sol-Ark Lite option is the cheapest entry point to Sol-Ark ESS ownership.

I’m super excited about this inverter, because I rent, so I can’t put solar on my roof. But I can interconnect a battery system to my service panel and use it to lower my time-of-use electric rate.

Another great use of the Sol-Ark Lite is to expand existing Sol-Ark systems. It’s not a good idea to parallel batteries which have different usage amounts, not to mention models or chemistries, and sometimes you want to expand system power or storage capability without paying for all of that expansion upfront. In those cases, the 240V inverter AC-coupled into the system is run in demand-management mode, only providing power when the system goes above a certain power threshold. This can significantly extend the life of an older battery bank that can only contribute power at lower amperage rates. The weaknesses to this technique is online monitoring, as the independent systems would essentially be monitored independently. But whether its small time-of-us metering where backup isn’t important, or to add additional power or energy to an existing system, the 8K-1p is definitely worth thinking about.

So where does this leave the Sol-Ark Essentials 12K-2p inverter? It still has its uses. The primary one is that the 12K comes with all the integrated breakers. So it comes with a breaker for load, a breaker for generator, a breaker for grid, and a breaker for the batteries. The larger Sol-Ark 15K only has a breaker for the load and a breaker for the batteries. That external grid disconnect with overcurrent protection, as well as the generator disconnect, aren’t included. Plus, a load-side connection to an essentials load panel doesn’t require having the utility out to site to pull the meter. The 12K is a very simple installation, with backup power capability, making it a good compromise between non-backup and whole home options. Another example of a unique 12K application is for small, distributed outdoor charging stations. It’s a very simple way to provide a little bit of outdoor 120/240V power that doesn’t require the architecture of the 15K.

At the commercial level, this Sol-Ark GEN port is rated for 200 amp, the same as the grid-pass through rating, whether it’s the 30K-208V or 60K-480V inverter. This is an incredibly generous for interconnection point but remember with great power comes great responsibility. Keep in mind these are the maximum amperage port ratings, and the continuous output is software constrained to 180A continuous.

But now we have also that same kind of port as our Gen Port. So on the residential line, the Gen Ports are much smaller. This can accommodate much larger power generation for EV chargers, for interconnected generators, building generators, three-phase generators, so much larger than what we’ve had in the past with our residential line.

But fundamentally, it’s still the same. You still have a load port for your main panel, and then you have a port for DC coupled solar. And then actually there’s two battery ports. And I think what’s this… Really, this is off topic, but the key design issue is these battery ports are 50 amps each.

And so if you want the full 100 amp battery power capability and you only have one battery cabinet, you need to run parallel conductors from the cabinet, or at some point have a short busbar. Or you get two battery cabinets. Or you get a battery cabinet that has two sets of parallel conductors. I mean, there’s couple ways to do that. The main panel is a 50 amp backup. If you have two inverters, that’ll get you to 100 amp backup. You can go all the way up to 10 inverters indoors, 6 inverters outdoors at the commercial level.

To close out this interconnection discussion, let’s finish with AC coupling, using some commercial project examples. DC-coupling is my preferred system architecture more many reasons, one of which being it keeps the GEN port available for other uses besides solar AC coupling. But commercial solar has a wide variety of low cost rooftop solar options, especially at 480V, such as the SMA Sunny Tricore. It’s valid to keep these UL3741 inverters on the commercial rootop (eliminating module-level rapid shutdown equipment), to utilized that GEN port on hybrid inverters to land the AC-coupled solar, and then to locate the hybrid inverter near either the battery location or point of interconnection.

There are a few project considerations when interconnecting AC-coupled solar. The first is to determine if solar is part of the backup project. If so, the solar AC output size should be less than or equal to the nameplate of the hybrid inverter, even if the GEN port can accommodate higher amperage. If it gets much larger than that, then the frequency shifting to turn off the solar array in backup mode is unreliable.

Similarly, zero export mode for AC couple isn’t possible. Again, zero export mode is caused by frequency shifting, and its not possible for such a small system to change the grid frequency if the grid is online. The grid frequency is going to win out unless there’s a power failure. So if you need zero export, or you need the solar array for backup power, keep any AC coupled solar to be smaller than the inverter nameplate for standard installations.

AC coupled solar can land in two locations on hybrid inverters, it isn’t necessary to AC couple solar onto the GEN port. It can be connected onto the LOAD port along with the rest of the service breakers, assuming it is otherwise code compliant. Frequency shifting will still work to control the solar array while in backup mode. What is gained by interconnecting solar onto the GEN port is dedicated solar monitoring within the hybrid inverter app, which is very convenient. Whereas if you connect solar on in with all of your other electrical loads, the data that’s reported through that exchange is mixed together.

Combining AC coupled solar and generators simultaneously is difficult. Not all hybrid inverters can do this well. Nor can it be done with all generators. In particular, large, clean “top shelf” generators which provide an electric signal similar to the grid are tough to implement, because if frequency shifting cannot work, the the solar array can backfeed the generator. It is possible to run AC-coupled solar and generators connected to the GRID port on Sol-Ark systems, but again, its better to avoid AC-coupled solar and generators all together if at all possible.