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AC- vs. DC-Coupled Battery Power Storage Systems: What Is the Difference?
Learn What AC & DC Coupling Mean for Your Solar Battery Storage System
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When you add battery storage to your solar system, there are two main ways to connect or couple these two sources: alternating current (AC) or direct current (DC) coupling. AC and DC are the two types of voltage used to transmit and conduct the electrical energy you use at home daily.
Solar Panel Installation
When installing a solar panel, it’s essential to ensure the system fits the operation required. With the right solar panel installation contractors, the solar panel will be designed and fitted to meet unique demands.
Since individuals using solar panels have an investment in the product, they have to get the best that meets their energy needs, whether alternating current or direct current. Both modern and traditional solar panels will always produce power in the form of DC power, which must be transformed for household use.
Solar professionals must conduct all assessments and evaluations before installation. This ensures optimum energy output.
What is AC power?
As mentioned above, AC stands for alternating current. This type of voltage is the electricity flowing first in one direction and then the other.1 It is the type of electrical current used on the grid and in most household devices and appliances.
With electricity flowing in one direction, it implies that DC electricity has to flow from the solar panel to the inverter. Within the inverter, it is transformed into AC power which becomes ready for consumer use.
In other cases, power may not be needed at the moment; hence it is moved into another inverter, where it is transformed back into direct current for storage in the battery. This DC power must pass through an inverter to make it safe for household use.
What is DC power?
DC stands for Direct Current, and it flows in one direction only.2 This is the type of electrical current generated by the solar panels on your roof and stored by your home solar battery. Flashlight batteries also produce DC current.
Since your solar system produces direct current (DC) energy, but almost all homes run on AC electricity, a solar inverter is required to convert the DC electricity into appliance-friendly alternating current to power your home.3 Unlike the AC system, the electricity generated by your DC solar panels will only be inverted once and be ready for use.
Solar inverters are included with every solar system, and there are three different kinds:4
1. Traditional Inverters
Traditional inverters, also called central inverters, are inverters that draw power through all your DC solar panels. They combine the direct current of all solar panels and convert it to alternating current energy. Usually, this kind of inverter is a single box installed right next to your home breaker box.
2. Micro Inverters
These small inverters are placed on the backside of each solar panel. Installing individual inverters allows each solar panel to perform independently by converting the DC energy of each panel into AC energy. Micro-inverters are also essential features of AC solar panels.
3. DC-Optimizers
Like micro-inverters, these devices go on the back of each panel. However, optimizers “condition” the DC energy and send the power to a central inverter instead of converting the DC energy to AC. This process results in a hybrid system combining the best of both traditional inverters and micro-inverters.
Are solar panels AC or DC?
Most people are used to DC solar panels. However, AC solar panels do exist. AC solar panels already have micro-inverters attached, so you don't have to purchase a separate inverter. Although AC solar panels are more expensive, they eliminate the voltage drop disadvantage associated with centralized inverter systems.
Is a solar battery AC or DC?
Like solar panels, batteries also produce DC power. Because the electricity in your house is AC, the batteries inside devices like your smartphone and your laptop need to use a converter to turn the AC energy from the outlet into DC power so it can be stored in their respective batteries.
Home batteries also store DC power. The main difference here is a DC battery will use the same solar inverter to convert its stored DC power into AC power. In contrast, an AC battery has a built-in inverter that directly converts its stored DC power into AC power to provide home electricity.
DC- vs. AC-Storage Architecture
Strengths and Weaknesses of DC- & AC-Coupled Battery Power Storage Systems:
1. DC-Coupled Systems
A DC-coupled system connects to the main grid supply in the same place as your solar panels. This setup requires a hybrid inverter. Your panels and your solar battery share this type of inverter. The DC system utilizes solar charge controllers or regulators To charge the battery from the solar panel and a battery inverter to transform electricity flow to AC. Since the solar panels and inverter share the grid interconnection, there is a reduction in power loss.
Strengths:
Conserves power: Since your power is only converted from AC to DC once vs. in an AC-coupled system, there is less energy loss5. When installing these solar panel systems, we are always in need of optimum power supply to our household, thus the need to avoid energy loss.
Net metering: If your DC-coupled system is below 10kW and is directly connected to the renewable generator, it will be eligible for Net Energy Metering (NEM). This will get you credits for the excess energy it produces (if available in your state)6. Homeowners looking to cut electricity costs will find this advantageous.
Simplicity: Requiring only one inverter means fewer components, which usually translates to lower costs. Also, many inverters increase the chances of technical hitches and increased repair time in case of power surges.
Advanced functions: These functions are easier to coordinate with one inverter vs. synchronizing with two different inverters.
Smart option available: There is a smart option if you are yet to start your solar journey.
Weaknesses:
More costly and complex option: The setup tends to be more complicated and expensive, particularly if you already have a PV system at home and want to retrofit a DC solar battery.
Risk of power outages: With a single inverter in the DC-coupled system, there are high chances of power outages. The entire solar power and battery capacity are lost when the inverter fails.
2. AC-Coupled Systems
An AC-coupled system uses two inverters to charge your solar battery: A conventional solar inverter in addition to a second inverter, known as a “storage inverter.” This system may be easy to set up but it offers slightly less battery power storage efficiency when charging than a DC-coupled system.7
Both the solar panels and battery module can be discharged at full power. Users can also choose to dispatch them together or independently. The battery and solar panels can also run on separate interconnections or share an interconnection with the power grid. In such a scenario, the system becomes more flexible in its operation.
Strengths:
More flexibility: Since AC-coupled systems require two or more inverters, you can adjust the location of your batteries and other equipment.
Compatible with inverters: It works with all kinds of solar inverters, even micro inverters. With these systems, you can install multiple inverters and batteries at different locations.
More cost-effective: It’s a more cost-effective option if you already have a PV system at home and want to add an AC solar battery.
Versatility: The AC system enables batteries to charge from the grid and at the solar panel. That implies that the battery will still charge from the grid regardless of solar panels not providing enough power.
Retrofitting: The AC- coupled batteries are easy to install on a pre-existing solar panel system, where other batteries and inverters can be added to increase the capacity and output of the entire power system.
Weaknesses:
Lower efficiency: Slightly lower efficiency to charge your solar battery vs. a DC-coupled system due to AC (grid) - DC (battery) - AC (home) conversion6. This is because the AC system is not designed to be utilized off-grid as they lack transformers to manage surge loads from multiple appliances.
Risk of failure: Having more than one inverter and equipment in different locations may increase your system’s points of failure, which could equal a need to purchase more replacements.
DC or AC coupling — which is better for my solar and storage needs?
Well, it depends if you already have a solar system at home or are about to switch to solar energy. But do not despair! At Sunrun, power storage batteries for home are our thing.
The AC-coupled systems are the most preferred option for larger utility-scale plants. The main reason being while AC-coupled systems are less efficient at charging batteries, they are easy to install. Most individuals will always avoid high installation prices, which are mainly associated with installing AC or DC-coupling systems.
Each solar power system is designed specifically to fit different projects and individual needs.
Work with Sunrun experts today
At Sunrun, solar power storage batteries for home are our thing. We understand everything about alternating current and direct current energy requirements. Contact our solar advisors today at 833-394-3384 to request your free, personalized quote and to learn more about battery power storage capacity and the best storage batteries on the market. Sunrun Brightbox™, our holistic solar system battery storage system, lets you protect your home from outages and lower your utility bill.
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