By Jeremy Cook
Solar power lets us harness energy from the sun. To use this energy while the panels are not actively producing power, consider the addition of an energy storage system (ESS). An ESS—typically a battery bank with an inverter—charges via a DC- or AC-coupled solution. Both AC-coupled and DC-coupled energy storage setups have advantages and disadvantages, and energy storage isn't even the best option in every situation. We will discuss each solar scenario in this article.
First, consider the most basic solar use case: a PV (photovoltaic) array without any battery backup. In this situation, solar panels produce DC power, which is sent to an inverter that turns it into AC electricity and feeds the main electrical panel. Excess power can be sold to the utility grid via a bi-directional power meter. But these systems shut off when the grid goes down, for safety reasons.
A solar array without battery backup is comparatively easy to set up. It includes a PV array, an inverter (which interacts with the overall grid), and a PV system disconnect. The downside to this setup is that the disconnect shuts off when the grid goes down. Your system can't be used to keep the lights and critical systems on during a blackout.
AC-coupled energy storage
Battery backup lets you maintain power during a grid outage. The easiest way to install solar backup at a business or home is with an AC-coupled system. Instead of feeding the main service panel after the PV disconnect, the inverter feeds a backup loads panel. The backup panel, in turn, feeds a multimode inverter, an energy storage disconnect, and batteries to store power for later use. Finally, the system feeds the main service panel via an interactive system disconnect, which cuts off the solar/backup system in the event of an outage.
During a grid power blackout, electricity is sent from the energy storage bank through the multimode inverter to the backup loads panel, which supplies power to critical devices. Solar power can still be used in this scenario (if the sun is shining) as the panel inverter uses power from the ESS to mimic grid signals and convert DC solar power to AC. All additional solar/backup equipment is installed after the interactive system disconnect, with respect to the main service panel.
DC-coupled energy storage
In a DC-coupled setup, the PV array feeds a multimode inverter and charge controller setup through a PV disconnect. The charge controller allows DC power to pass through another disconnect to backup batteries, without any AC conversion and the corresponding efficiency losses. The inverter sends AC power—converted from the overall energy storage system—to a backup load panel. The backup panel can also send and receive power from the main service panel through an overall disconnect.
The big advantage of DC-coupled systems is that batteries are more efficiently charged by the solar array than in an AC-coupled setup.
AC-coupled vs. DC-coupled solar energy storage
From an efficiency standpoint, a DC-coupled system seems like a better choice than an AC-coupled battery storage system. An AC-coupled system has to go through three lossy conversions to produce backup solar power: PV (DC) to backup load panel (DC to AC) to energy storage (AC to DC) to backup load panel (DC to AC). DC-coupled systems only go through one DC to AC conversion: from the DC-storage system and PV array through a single inverter to the AC-backup load panel.
However, several factors may make an AC system more attractive for your use case. From a retrofit perspective, an AC-coupled ESS can be considered a "bolt-on" solution, as the existing interactive inverter, PV disconnect, and associated wiring can be left in place. DC-coupled systems require wiring and equipment changes to existing solar infrastructure. Since either backup solution needs to be individually designed, installed, and maintained by expensive (and fallible) humans , there is value in the simplicity of an AC-coupled ESS.
Additionally, consider that interactive inverters used in a PV-to-grid or immediate-use scenario tend to be more efficient than their multimode inverter counterparts. You could also debate the relative benefits of having two types of inverters in an AC-coupled system versus a single multimode inverter in a DC-coupled system. The dual-inverter AC setup provides some functionality if one goes down, but this also means multiple points of failure.
Other options: decouple solar and backup power?
System designers might also consider whether battery backup is necessary in some scenarios. Extended blackouts are rare in the U.S., and with the ability to bank energy to the overall grid, power bills after the sun goes down can be mitigated without on-site backup. Strictly from a cost standpoint, on-site storage may not be worth the extra initial capital investment. In some cases, a backup generator might be a better option than an ESS, even with panels installed for general use.
Solar energy backup conclusions
Solar power presents two main benefits: overall cost savings from reduced power utility charges, and backup power for when the grid goes down. As discussed here, AC-coupled and DC-coupled battery backup systems both have relative benefits. Given the efficiencies involved and the relative rarity of blackouts, installers might consider presenting solar power and backup capacity as two distinct challenges that may (or may not) be best solved in tandem.
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