AC vs DC coupled battery storage for the home: buyer's tips
Home batteries can be charged with DC power direct from solar panels, or with AC power. Here's a guide to the pros and cons of both setups.
Due to prices that continue to fall, new incentives from the Inflation Reduction Act, and power outages that are becoming routine in places like California and Texas, storage batteries are increasingly popular with homeowners.
Homeowners can use batteries to keep the lights on during a blackout, store their excess solar electricity for later use, or to save money by lowering their reliance on the grid during utility peak hours.
Consumers now have several battery manufacturers to choose from. Price, storage capacity, and availability are some of the key factors to consider. Another is whether to choose an AC-coupled or DC-coupled battery.
This is a more technical detail, and it’s often confusing for consumers. There are pros and cons to both types, and this guide will try to give you a simple explanation.
How storage batteries work
All batteries operate on direct current (DC) power. This means that they receive electricity and release electricity as DC current.
However, your home operates on alternating current (AC) power. To work with your home, electricity that is moved into and out of a storage battery must be converted between AC and DC.
This is true for both AC- and DC-coupled batteries. The difference between the two types of batteries is when the conversion process takes places.
AC coupled batteries explained
An AC-coupled battery is typically connected to your home at the electrical panel, which means that it receives AC power from the panel when it needs charging, and sends AC power into the panel when it’s discharging.
Usually, an AC-coupled battery will incorporate an inverter and charge controller into the unit. The inverter performs AC-to-DC conversion when charging the battery, and the opposite when sending electricity into your home. A charge controller makes sure the battery is charged quickly and safely.
This all-in-one design of the inverter, charge controller, and battery enables AC-coupled batteries to have a clean design. A popular example of this is the Tesla Powerwall.
DC-coupled batteries explained
With DC-coupled batteries, the process is simpler. Electricity flows from the solar panels into the battery. Because solar panels generate direct current, no conversion is necessary. A charge controller is still required to regulate the flow of electricity, but unlike an AC-coupled system, there’s no inverter between the power source and the battery.
When discharging electricity to supply your home, an inverter is needed to convert the DC power into AC. The inverter is often a separate unit, as is the case with the Generac PWRcell battery.
Pros and cons of AC- and DC-coupled batteries
There are good reasons why you might choose either an AC- or DC-coupled battery. Here’s a list of advantages for each type of system:
Advantages of AC-coupled batteries
- Can be used in standalone installations without solar panels
- Can be easily retrofitted into an existing solar installation
- Usually is an easier installation
Advantages of DC-coupled batteries
- Higher overall efficiency
- Can be charged from the grid or a standalone generator with a multimode/hybrid inverter
- Appropriate for off-grid, non-utility connected systems
Multimode inverters in DC-coupled systems
The illustration above shows a typical configuration with a DC-coupled battery, where the battery is charged from solar power only. However, an alternative configuration where the battery is charged using AC power is also possible.
This requires an inverter that can work in both directions, converting DC-to-AC power when discharging electricity from the battery or converting AC-to-DC power when charging the battery with AC power.
These inverters are known as hybrid or multimode inverters because unlike conventional solar inverters that only perform DC-to-AC conversion, hybrid inverters can operate in both directions. You would choose this type of inverter if you want the option to charge the battery from either grid power or a standalone generator.
How much more efficient is a DC-coupled battery?
With an AC-coupled battery, there are three conversions: DC-to-AC from your solar panels into your electrical panel, followed by AC-to-DC to charge the battery, then finally another DC-to-AC conversion when you discharge your battery to power your home.
Unless you’re charging the battery using a hybrid inverter, a DC-coupled battery only does a single conversion from DC-to-AC power. This means fewer conversion losses and higher overall efficiency.
How much more efficient? The SolarEdge Home Battery is a popular DC-coupled battery on the market. It claims 94.5% round-trip efficiency. Another battery is the Generac PWRcell, which lists a slightly better round-trip efficiency of 96.5%.
Meanwhile, the Tesla Powerwall claims 92% round-trip efficiency and the Enphase IQ Battery lists 89% efficiency. Both are AC-coupled batteries, so a reduced efficiency compared to DC-coupled options is expected.
List of popular home storage batteries
Consumers have many options when it comes to home storage batteries, and that’s not including DIY solutions that you can build yourself using deep cycle batteries.
Here’s are some of the popular batteries currently on the market:
- Tesla Powerwall (AC-coupled)
- Enphase IQ (AC-coupled)
- Sonnen sonnenCore and ecoLinx (AC-coupled)
- SunPower SunVault (AC-coupled)
- Generac PWRwall (DC- or AC-coupled)
- SolarEdge Home Battery (DC-coupled)
Should you choose a DC-coupled or AC-coupled battery?
As mentioned above, you can expect a little less efficiency with an AC-coupled battery. Does it matter?
For most homeowners, not really. While losing some of your solar electricity due to conversion losses is annoying, in most cases your buying decision will be swayed by other factors such as price and product availability. Batteries are popular these days, so you may find that it’s hard to find certain products such as the Tesla Powerwall.
Your decision will also be affected by your system design. For example, if you’re buying a battery without a solar array or you’re adding onto an existing system, the choice is moot: you’ll need an AC-coupled battery.
On the other hand, if you’re buying a new home solar and battery storage system, you’ll have either option. The best thing you can do is get multiple quotes so that you can compare the hardware options and prices. In this case, neither AC-coupled or DC-coupled batteries are inherently superior, so work with your installers to try to find the best options and price for you, and be sure to read our guide on home storage batteries to understand the benefits of having a battery.