More electronics are being incorporated into residential rooftop solar systems all the time, mostly due to updates to the National Electrical Code (NEC). With evolving rapid shutdown requirements and a strong desire for module-level monitoring, inverter or optimizer choice becomes important when determining how a battery fits into a solar + storage system.
When microinverters are used, the battery will have to be AC-coupled to the solar system, which requires an additional inverter. If power optimizers or rapid shutdown devices are used on each module, the battery could be DC- or AC-coupled, depending on installer preference.
A review on what this all means: Energy storage systems cannot be charged on AC power, so they must be fed by DC lines in some form. If a battery can be added to a system before the inverter converts solar’s DC power to AC, then it is DC-coupled. If batteries must be added after the DC-AC conversion (like when microinverters are involved), a second inverter is required to turn the AC power back to DC to charge the batteries. This is AC-coupling because the batteries are added after the initial AC conversion.
Darfon America manufactures microinverters, hybrid inverters and battery products, so the company is well-versed in all forms of solar + storage design.
“I have had many customers use our [hybrid inverter] paired with microinverters, and the reason is always they want to monitor each module,” said Bryan Whitton, Darfon product manager. “I can appreciate that, but I always recommend [power optimizers] or RSD devices paired with our DC-coupled inverter as being less expensive and able to supply monitoring on a module level.”
DC-coupled systems are generally less expensive since fewer inverters are needed. They’re also viewed as more efficient than AC-coupled setups because power is only converted once. Each time there is a conversion from DC to AC power or vice versa, some energy is lost.
But AC-coupled systems have their benefits too. In a DC-coupled setup, if that single inverter fails, both the solar and battery power are lost at once. An AC-coupled system allows for some consumption of power if one of the inverters fail — either you lose solar production or stored power, but not both.
Panasonic, which makes solar panels and now offers the EverVolt battery, says one of the main benefits of using multiple microinverters in an AC-coupled setup is this “inverter security” concept.
“When a microinverter fails, the other microinverters can still produce power,” said Dan Glaser, Panasonic senior sales engineer. “In a DC-coupled system, when the inverter fails, the solar will be offline until the inverter is serviced.”
Many energy storage systems, including the Panasonic EverVolt and the Tesla Powerwall, now come with their own paired inverters, making AC- and even DC-coupling a little more plug-and-play.
“Our go-to is the AC-coupled Powerwall because of its reliability. We install optimizers and microinverters. They both work well with the Powerwall,” said Mike Kelley, co-owner of East Coast installer Mass Renewables.
California installer Solarponics also prefers AC-coupling to ensure entire systems aren’t brought to a halt if one component goes down.
“[With AC-coupled systems], a regular installer can go out and install one of those systems, minus some of the electrical panel work. We don’t have to retrain our workforce. It’s easier to service down the road since it’s a more modular system — plug-and-play stuff,” said Mike Wiegel, Solarponics special operations officer.
Solarponics installs batteries with over half of its solar projects each year. The company is also a Tesla Powerwall dealer, since the unit’s easy AC-coupling can work with both power optimizers and string inverters, as well as microinverter-based systems.
“We’re an early adopter to SolarEdge and we love that product, but the stuff that Enphase is coming out with really has our attention,” Wiegel said. “I would never go back to a DC-coupled system. There aren’t great rapid shutdown products out there. With Enphase storage, you know that’s all built-in — it’s nice and easy. So for rooftop, it’s definitely better to go that route.”
The highly anticipated Enphase Ensemble launch has finally hit some markets. Enphase now offers an all-inclusive residential battery backup solution that includes a lithium battery with integrated microinverters in one enclosure, in addition to the microinverters on each panel. This AC-coupled system is grid-forming (essentially establishing a solar + storage system as its own microgrid) and only works with Enphase microinverters attached at each solar panel, so module-level rapid shutdown is built in.
More manufacturers are making system choices easier on installers — either inverter companies are getting involved with batteries or batteries are partnering with inverter brands to provide full system setups that also meet NEC requirements. No more grabbing various products and then figuring out whether the battery needs to be AC- or DC-coupled; installers can decide what they want to do up-front.
Over the last say 15 years, rules, regulations and “safety” concerns are bringing “modular” additions to solar PV installations that used to be simple strings of panels wired in series/parallel to create the D.C. buss to feed the house inverter. IF one thinks about it, a lot of the “Energy Star” devices are actually 120 or 240VAC rectified to D.C. then switched to a “ramping” A.C. control or a pulsed D.C. speed control to run things like house fans in air handlers, the VFDs in A/C compressors. I’d say most of the electronics devices are using 120VAC to 24VAC then rectified to 24VDC to run most modern day TVs, computers, electronics devices. Get a D.C. standard from appliance manufacturers around 120VDC and one could go to simple string wired solar PV panels a 120VDC house battery and a much simpler system than is used now. Just sayin’ circular conversion techniques are adding energy inefficiency to (our) energy efficient devices.
John Almond says
Well said Solarman. Sometimes, the label isn’t as good as what it seems especially with the way devices are modified.
It’s going to be interesting how “Energy Star” starts to change the way their data is shown on certain appliances and products.