DC solar battery storage systems have higher efficiency, so they may be the best option if you’re installing a solar PV system with energy storage. However, AC solar battery storage systems are easier to install and more compatible with existing PV systems. So, if you already have solar energy and want to add battery storage, AC coupling may be best for you.
Battery systems are an excellent addition to solar PV systems for maximising self-consumption. Instead of always exporting excess solar power to the grid, you can store it in batteries for use in off-peak periods.
When it comes to including batteries in solar PV systems, an important consideration is whether you should go for AC coupled batteries or DC coupled batteries.
In this article, we’ll explain these two “solar plus storage system” architectures in order to help you decide what’s best for you.
At the end of this article, you’ll know:
- The difference between AC coupled battery systems and DC-coupled battery systems
- Examples of AC coupled batteries and DC=coupled batteries in the market
- The top solar inverters that are compatible with AC coupled batteries as well as those compatible with DC batteries
- The advantages and disadvantages of an AC coupled system and a DC-coupled system
- When AC coupled systems are best as well as when DC-coupled systems are best
Let’s get started with a brief introduction of AC power and DC power.
AC Power Versus DC Power
There are two forms of electricity – alternating current (AC) and direct current (DC). The two forms simply describe the direction of the flow of electric current.
What is AC Power?
AC, which is short for alternating current, refers to current flowing in alternating directions. It is the standard electricity that comes from power outlets in the home.
In AC power, electric charge (current) changes direction periodically. The flow of the current alternates between an upward (positive) direction and a downward (negative) direction. This creates a number of waveforms, with the most common being the sinusoidal or sine wave.
Because the current flow changes direction periodically, the voltage also reverses along the wire.
An alternator produces AC power by spinning a loop of wire in a magnetic field, in accordance with Faraday’s Law of electromagnetic induction. As the wire spins, it enters a different magnetic polarity periodically, causing the current and voltage to alternate on the wire.
What is DC Power?
DC stands for direct current, and it refers to a linear electric current. Unlike AC power, where the flow of current oscillates upwards and downwards, in DC power, the flow of current is unidirectional.
Because the current flow is linear, DC power produces consistent voltage. This makes DC have a wide range of electronics applications.
DC is mainly produced by sources like solar panels and batteries. It can also be produced from AC power using rectifiers that convert AC to DC.
What are the benefits of AC Power vs DC Power?
The world’s first power system, which was built in Godalming (England) in 1881, produced AC electricity. However, supply was intermittent, and in 1882, another electric power system was built in New York that produced electricity in DC form.
Thereafter, the AC or DC war started. This was a fierce debate over which form of electricity should be adopted as the standard for supplying homes.
Benefits of AC Power
AC electricity won the “AC vs DC war” and was adopted as standard because of its many advantages, which include:
- AC is better for transmission over long-distances
The main advantage of AC over DC is that it is better for transmission over long distances, and this is possible because it is easier to transform its voltage.
Long-distance transmission requires high voltage so that efficiency losses are minimised. And AC power is easily transformed to the high voltages required for such transmissions. Usually, the power is generated at low voltage and stepped up for long-distance transmission. At the distribution end, step down transformers is then used to reduce the voltage.
- It is easier to generate AC power
It is easier to build AC generators than it is to build DC generators. This is because you will not need components like brushes and commutators which are needed by DC generators in creating electricity.
- It is cheaper to generate and transmit AC power
The absence of brushes and commutators means that AC generators have fewer components than DC generators, and this makes them relatively less expensive.
Another thing that makes AC power cheaper is its ease of transmission over long distances. You can simply haul AC power to vast distances using miles of wires and pylons. But to supply DC electricity to a wide area, power plants producing DC will need to be located relatively closer to each other.
Benefits of DC power
Though AC power is the current standard for supplying homes, DC electricity is threatening a comeback. Let’s see why.
- DC power can be used for special applications
There are cases when AC power transmission is not practical such as undersea power transmission. Unlike AC lines, DC lines do not have a capacitive effect under saltwater. So, they are the only option for under-sea power transmission.
- Suitable for HVDC transmission
With HVDC (high voltage direct current) transmission, direct electricity transmission over long distances is now possible. HVDC transmission is even more efficient because HVDC lines have less efficiency losses than AC lines. However, HVDC systems are significantly costlier than AC systems.
- It offers better integration with energy storage
Energy storage systems like batteries are DC-based. With AC being the standard electricity supplied to homes, the power is converted from AC to DC before it is stored in the batteries. With such conversion comes losses.
But with a DC power system, direct current goes straight to the battery without the need for any conversion, offering better integration efficiency and reduced operating losses.
- DC power is compatible with renewable energy sources
Renewable energy sources like wind and solar energy produce direct currents. If you have a grid-tied system and occasionally export excess power to the grid, you’ll need a power conversion interface.
The DC to AC conversion that is done so that you can send energy to the AC-based grid also comes with losses. However, with a DC grid, DC energy from your renewable energy source can be fed to the grid with reduced losses.
AC DC Coupled Batteries and Systems
So as to maximise self-consumption and/ or to provide backup power during outages, many people are now adding energy storage systems (batteries) to their PV solar systems.
There are two architectures for the “solar plus battery” systems – DC coupled and AC coupled. Coupling simply describes how the battery system is linked or connected to the PV system. Thus, an AC coupled system or a DC-coupled system describes where the battery system is in the PV solar system.
What is a DC Coupled Solar System?
A DC-coupled system is one with a connection that allows the DC power produced by the solar array to go directly to the battery storage without needing to go through an inverter.
DC coupling refers to a solar plus battery architecture where the solar panels produce DC which outputs directly into the energy storage (solar batteries) and the inverter. The solar inverter converts the DC electricity to usable AC which is then fed into the house to be used by AC loads. Excess power is then exported to the grid.
This means that a DC-coupled solar system requires only one DC to AC conversion.
What Batteries are DC Coupled?
DC-coupled batteries are those that can be charged by DC electricity produced and flowing directly from the solar PV array. Examples of DC batteries include:
- LG Chem RESU. LG is one of the biggest manufacturers of consumer electronics in the world. The company has a range of Residential Energy Storage Unit (RESU), which are available in two voltage options – 48V and 400V. The 40 volts range include four batteries with sizes ranging from 3.3kWh to 13.1kWh, while the 400V range includes two batteries of sizes 7kWh and 9.8kWh.
- BYD B-Box battery system. BYD is the largest manufacturer of rechargeable lithium batteries in China. The BYD B-Box battery system has a modular design that gives the flexibility of having up to 4 battery units in one pack. Each unit has a storage capacity of 2.5kWh, giving the pack a total capacity of 10kWh.
- Generac PWRcell. The PWRcell from Generac is a very powerful and scalable battery. You can configure your storage system with as few as 3 of the 3kWh batteries for a 9kWh capacity. You can use as many as 6 of the units to get 18kWh capacity.
Which Solar Inverter Work DC Coupled Batteries?
There are a number of solar inverters that are compatible with DC-linked batteries. These include:
- Fronius.
- Sungrow hybrid inverters
- SMA
- SolarEdge StorEdge inverters
- Huawei hybrid inverter
- Solax X-hybrid inverter
Pros and cons of DC Coupling Batteries
DC coupling offers the following advantages:
- It is more efficient. Since there is only one energy conversion, efficiency losses are reduced, making the system more efficient.
- It is more affordable. The one-time DC to AC energy conversion also means that a DC coupled system requires only one inverter. This reduces hardware cost, making the systems more economical.
- It allows for oversizing. DC coupling allows you to oversize your inverter so that you can harvest more power. Oversizing means installing a solar array with a capacity greater than the rated capacity of your inverter.
Generally, the capacity of a solar array (group of solar panels) matches that of the inverter. But with DC coupling, you can get more solar panels on your roof with the same inverter. The power generated over and above your inverter capacity can then be redirected to charge the battery system.
That is, you can install an 8kW PV array for a 5kW inverter. While 5kW goes into the inverter, the excess 3kW will go into charging the battery.
However, DC coupling is not without its disadvantages. These include:
- Relatively difficult to install. Initially, the DC coupled batteries were quite difficult to install because they require a charge controller that makes it possible for the energy coming from the solar panels to charge the battery. However, recent DC-linked batteries are available as hybrid batteries with the charge controller as an in-built feature.
- Not flexible. A DC coupled battery is usually not compatible with an existing PV system. They’ll require you to modify the existing connections before the battery can be added. This increases labour time and cost.
What is an AC coupled system?
An AC coupled solar system is one connected in such a way that the DC power from the solar panels is converted to AC power prior to being fed into the energy storage system (battery).
All power from the module goes through the inverter and is converted to AC power which is fed into the house. The AC power that is not used by the household is then directed into the battery system.
However, since batteries store DC energy, the AC coming to the battery goes through a battery inverter for conversion to DC power. When stored energy in the battery is needed, it comes in the DC form and is converted to AC before being fed into the household.
So, there are three inversions in AC coupled systems:
- DC from solar panels is converted to AC by inverter
- AC from inverter is converted to DC before charging battery
- DC from battery is converted to AC before going into the house
What Batteries are AC coupled?
Some of the top AC coupled batteries (or AC batteries) include:
- Tesla Powerwall. The Powerwall is from powerhouse Tesla. It is available as a single unit with 13.5kW energy capacity.
- Enphase IQ Battery. Enphase currently has four models of the IQ Battery models – 3, 3T, 10, and 10T. All have multiple bi-directional microinverters for safe and reliable operation. These batteries have energy capacity ranging from 3.36kW to 10.08kW.
- SonnenBatterie Eco. The Eco comes from the German company Sonnen, which is the biggest company in Europe when it comes to residential lithium battery systems. The SonnenBatterie Eco does not only store energy, it is a smart storage system that automatically adjusts your energy usage. It comes with a modular design, allowing you to buy a single unit of 2kW and to later upgrade up to 16kW.
Which Solar Inverter Work DC Coupled Batteries?
The AC coupled battery systems are compatible with inverters from:
- SMA
- SolarEdge
- Fronius
- Enphase micro inverter
- Delta
- ABB
Pros and Cons of DC Coupling Batteries
AC coupling has a number of benefits which includes:
Retrofitting. Batteries that are AC coupled are easier to fit into an already installed PV system. They can simply be added to the existing system without having to do a complete rewiring.
Reliability. Because you have separate inverters for the panels and the battery, both systems are independent. So, a battery fault will not have a direct impact on the power generation of your solar PV system.
Flexibility in Charging. The AC coupled battery can be charged by either the solar panels or the grid power. This means that if your PV system is not producing enough energy, you can rely on grid power for charging your battery system.
However, AC coupled solar systems have a few drawbacks. These include:
Lower efficiency. Every time power is converted from one form to the other, there are losses. So, the multiple conversions required to add up, leading to slightly lower efficiency for the system.
Relatively expensive. AC coupling requires you to have a battery inverter in addition to the solar inverter. This means more components, making AC coupled systems more expensive.
Which is Best for My home?
The “solar plus storage” architecture that is best for you depends on whether or not you already have solar.
If you have already installed solar power, and simply want to add battery storage, an AC coupling system is the best for you. This is because of the flexibility of AC coupling systems. They are very easy to install and are compatible with existing PV systems.
However, if you want to install a “solar plus battery” setup at the same time, a DC coupling system may be best for you. This is because they offer slightly higher efficiency.
Our Guides to Solar Batteries and More
We have a series of unique guides to solar, solar finance, batteries and more, if you are looking to do more in-depth research into solar batteries check the below:
1. Best Solar Battery Products
The best solar battery products you need to know in 2022 are the Sonnen ECO 5, Discover 48V Lithium, Tesla Powerwall, Crown CR430 FLA, and FullRiver DC400-6 with a sealed AGM battery. These batteries are excellent for storing solar energy for later use.
2. What Are Solar Batteries? How Do They Work?
Solar panels can now produce electricity at a lower cost than traditional power stations burning coal and gas. However, fossil fuels can be used at any time, while solar panels are only produced during the day. To remove this limitation, solar panels can be equipped with battery systems, converting them into a 24/7 power source.
3. Do I Need a Solar Battery? Are They Worth It?
Solar panels can now produce electricity at a lower cost than traditional power stations burning coal and gas. However, fossil fuels can be used at any time, while solar panels are only produced during the day. To remove this limitation, solar panels can be equipped with battery systems, converting them into a 24/7 power source.
Next Steps…
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