This Wiring Setup Could Save You Thousands on a Solar and Battery System - CNET

Batteries aren't one-size-fits-all, and neither is the way they're wired to your house. Here's what you should know about AC- and DC-coupling.

When we think about solar panels and batteries, we don't really give much thought to all the wiring and other little boxes on the wall.  Solar Panel Inverter

This Wiring Setup Could Save You Thousands on a Solar and Battery System - CNET

The way your solar panels and battery connect to each other and to your house can be a major factor in equipment costs, installation costs and the efficiency of your battery. When you get a solar battery, there are two different ways it can be connected to your solar panels and your house: AC-coupled or DC-coupled. 

AC-coupled systems have two inverters and are typically retrofitted to existing solar panel systems. If you don't already have solar yet, you could potentially save thousands of dollars on a new solar plus storage system by going with a DC-coupled setup, which only involves one inverter and is usually more efficient. 

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Here's what you need to know about how AC- and DC-coupling works.

First, let's define what "AC" and "DC" mean, and why it matters.  

AC and DC describe the flow of two different types of electrical currents. DC is direct current; electricity that flows in one direction. AC is short for alternating current and can change direction. In this case, all you really need to know is that your solar panels generate DC electricity and your battery stores DC electricity, but your home appliances (and the power grid) run on AC electricity. 

"The grid is AC, solar panels generate DC, and batteries charge and discharge in DC," Steve Lysenko, a senior engineer with the North Carolina Clean Energy Technology Center, told CNET. "That's how it is. There's no change in that."

Here's where inverters come in: Their main job is to convert the DC electricity generated by your solar panels into AC electricity that can power your home. 

Depending on what kind of battery you have and how you want it wired up, you might need another type of inverter added to the mix. But we'll talk more about that below.

At its core, the difference between AC- and DC-coupling is the way your battery is wired between the solar panels on your roof and your home's electrical system, as well as how many AC/DC conversions it takes to get the job done. 

An AC-coupled system requires three conversions to go from solar to battery storage and then to your house. This type of setup is typically better for homes that already have a solar panel system installed because the original solar inverter and wiring are already there. Rewiring to a DC-coupled setup could be costly. 

"The inverter does not have the rectifying abilities or the electronics in there to be tied in with battery storage," Lysenko said. "So they can't just take batteries to their existing solar system and wire it up."

For DC-coupled systems, it's a bit easier. Energy only needs to be converted one time (DC to AC) to go from your solar panels or battery to your home. DC-coupled setups are generally encouraged for homes that don't already have solar installed. There's less wiring involved and you'll likely save a few thousand dollars because you won't have to buy another inverter. Since there's only one energy conversion happening instead of three, DC-coupled systems are usually much more efficient than AC-coupled systems, but there are exceptions.

"Is there an AC to DC conversion before charging the batteries? If there is, then it's an AC-coupled system," Lysenko said. "If not, it's a DC-coupled system."

Here's how both types of coupling setups work.

For this type of setup, you'll need two inverters: your main solar panel system's inverter (or a bunch of microinverters) and your battery's multimode inverter, which is a type of inverter that's capable of converting DC to AC and AC to DC. 

In an AC-coupled system, DC electricity generated by your solar panels goes through your system's main solar inverter and then gets converted to AC electricity. The freshly converted AC electricity can either go straight to your house or detour and head for battery storage. 

There's a slight roadblock. Your battery only stores DC electricity, not AC. This is where your battery's inverter comes in. The battery inverter converts the AC electricity into DC electricity and stores it. When your home needs to draw electricity from the battery, the stored DC electricity gets converted one more time into AC electricity, and is then sent to power your home. 

"When you use that battery energy in your home or sell it back to the grid, you'd convert it to AC because that's what your appliances are expecting," Larry Pileggi, Coraluppi Head and Tanoto professor of electrical and computer engineering at Carnegie Mellon University, told CNET.

DC-coupled setups don't require an additional inverter for the battery. You'll just need your solar panel system's main inverter (or a bunch of microinverters). 

In a DC-coupled system, DC electricity generated by your solar panels either makes an instant detour into battery storage, with no conversion necessary, or it goes through your main inverter, converted to AC and then straight to your house. When you want to pull energy from your battery, the DC energy exits the battery and goes through the main inverter where it's converted into AC for your house. 

Almost every battery system on the market has a round-trip efficiency rating. That rating essentially tells you how much of your energy makes it into battery storage without getting lost during the travel process. DC-coupled systems typically have a higher round-trip efficiency rating, around 94-95%, while AC-coupled systems usually have a round-trip efficiency of about 90%. 

"When the battery has a built-in inverter, that means it's an AC-coupled system," Chris Thompson, vice president of product and technical marketing at SolarEdge, told CNET. "And what that basically means is inherently the efficiency and the losses go much higher when you have an AC coupled system." 

The main reason why the efficiencies are so different between these two setups has to do with the number of times energy gets converted between DC and AC, particularly when it goes from solar to storage and then to your house. In a DC-coupled system, your electricity only gets converted once during this process. In an AC-coupled system, there are three conversions involved. The more times you convert that energy, the more of it you might lose along the way, usually to heat.

"We call it the triple conversion loss," Thompson said.

DC-coupled systems can also help eliminate energy losses from inverter clipping. Clipping happens when your solar panels are producing more electricity than your inverter can handle. Your inverter converts that energy that it can and "clips" the rest of it away. That clipped energy is wasted.

In an AC-coupled system, all of the electricity generated by your solar panels goes through an inverter first before it goes to your house or to your battery. In a DC-coupled system, the energy can go to the battery first before going through an inverter. This can give that extra "clipped" energy a place to go. 

"When you DC-couple something, you can put a big PV array onto a smaller solar inverter because that PV, being DC, can go into the battery," Thompson said. "It doesn't get limited by the inverter."

AC-coupled systems are usually about 5% less efficient than DC-coupled systems, but the efficiency losses you experience in an AC-coupled system may not matter.

"When you're talking about system efficiency, it depends on how you're using your PV system," Lysenko said.

If you plan on using most of your solar energy to power your house, as opposed to sending it to storage, then you likely won't see as much of an efficiency loss with an AC-coupled system. DC solar that goes straight to power your home only needs to be converted to AC once before it reaches your house. When it's coming from storage, it'll be converted three times before it reaches your house. If you really only want to use your battery as a backup for when the power goes out, or if you don't plan on using your battery very much in general, then AC-coupled efficiency losses won't affect you nearly as much as if you were charging and using your battery every day.

If you have big plans for your battery and intend to use it pretty frequently, you're much better off with a DC-coupled setup. This way, the DC electricity from your solar panels can just go right on into the battery with no conversion necessary. Your stored energy only needs to be converted once (to AC) before it reaches your house. If your solar panels' main job is to charge up your battery, you should seriously consider a DC-coupled system.

"With solar, that's going to generate DC voltage and current, and then that's what you would want to use to charge your storage," Pileggi said. "DC is definitely the way to go for charging the battery."

A lot of the costs related to AC- and DC-coupling boils down to whether or not you already have solar on your roof before you add a battery. An AC-coupled system tends to be more budget-friendly if you already have solar panels installed on your roof. This is because your solar system's existing inverter is already wired a certain way and it's generally easier to install an AC-coupled system to an existing solar system. 

If you don't have solar panels and are looking to install a new solar panel system for your home, then a DC-coupled system will likely be cheaper since it will save you the cost of buying multiple inverters.

"If you're putting together a completely new system, you don't have solar on your house, you don't have anything, DC is going to be cheaper," Lysenko said. "If you have an existing system, AC is going to be cheaper. It's going to be easier to design because you don't have to rewire the panels going to the inverter. You can kind of keep that portion of it and just add on to it."

If you already have solar on your roof, Lysenko suggests looking into the lifespan of your current inverter. Most string inverters last somewhere between 10 - 15 years and up to 25 years for microinverters. If you think your current inverter might kick the bucket soon, then DC-coupling your battery might be more beneficial in the long run since your current inverter might need replacing anyway. There's also the cost of the actual wiring to consider. Lysenko says the wiring for AC-coupled systems tends to be cheaper than wiring for DC-coupled setups.

"A lot of this depends on the layout," Lysenko said. "But in general, despite the individual wiring and cabling costs, if it's a new system, DC is going to be cheaper, [if it's] retro, AC."

The right coupling setup for your home depends on your situation. If you already have solar on your roof and you want to add a battery, then an AC-coupled system might be easier if you're on a budget. Your solar panel system and inverter are already wired a certain way and rewiring for a DC-coupled system might cost more in installation labor. Yes, AC-coupled systems are less efficient than DC-coupled systems, but the percentage loss is relatively small.

But ultimately, there tend to be more benefits with a DC-coupled system. There are fewer boxes on the wall and less wiring, your battery is more efficient and you won't have to buy another inverter for the battery (if your battery doesn't already come with one). Fewer components need to communicate with one other, meaning less room for something to go wrong.

If you are thinking about going solar and installing battery storage, a DC-coupled setup is almost certainly the way to go.

A professional installer can also help you determine which type of coupling setup is best for your home and your budget. Make sure to shop around for multiple quotes from installers in your area. This could save you thousands of dollars in the long run. 

Hybrid inverters, also known as multimode inverters, are capable of converting DC to AC and AC to DC. A normal grid-tied inverter only converts DC to AC.

"They still call it an inverter, but it does both functions," Lysenko said.

Your inverter's main job is to convert the DC electricity from your solar panels and convert it into AC electricity that your home can use. The inverter is also in charge of monitoring your solar panel system, as well as its connection to the grid. Lysenko says the inverter can optimize power production, sense electrical faults and grid outages and track your solar system's energy production.

AC/DC energy describes the way two different types of electricity flow. DC electricity flows in one direction. While AC electricity changes directions. 

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This Wiring Setup Could Save You Thousands on a Solar and Battery System - CNET

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