AC or DC coupling refers to the way solar panels are coupled or linked to a battery system.
The Solar battery evolution
Simple DC coupled solar battery systems were once only used for remote power systems and off-grid homes but over the last decade hybrid (solar and battery) inverter technology has advanced rapidly and lead to the development of new AC coupled energy storage configurations. However DC coupled systems are far from dead, in fact charging a battery directly using traditional DC-DC charge controllers or modern DC coupled hybrid inverters is still the most efficient method available.
As off-grid systems became larger and more advanced AC coupled systems evolved as the preferred configuration due to the use of low cost, easy to install string solar inverters. Most modern AC coupled systems now use advanced bi-directional multi-mode inverters coupled with one or more compatible solar string inverters.
Over recent years battery technology has improved significantly with many new lithium battery types emerging as manufacturers explore different ways to add or couple batteries to new or existing solar systems. The original Tesla Powerwall was the first 'high voltage' DC battery system and since then high voltage (200-500V) batteries have become increasingly popular. More recently AC batteries have been developed by many leading manufacturers including Tesla, Sonnen and Enphase.
With the many complex varieties of battery storage systems now available, here we explain the advantages and disadvantages of each type.
The 4 main solar battery Systems
DC coupled systems
AC coupled systems
Hybrid Inverter Systems
Off-grid - AC vs DC coupled?
Why would you use an AC coupled 'off-grid' system rather than DC coupled? One reason is most modern solar grid-tie systems use solar inverters to convert solar DC power directly to AC power which can then be used immediately by most modern appliances. In addition the installation cost of string solar inverters is lower for larger systems as they operate at much higher DC voltages (200-600V) which means solar panels can be installed in long strings making it far easier and simpler to install. AC coupled systems are also more efficient during the day when there is high AC power demand such as air-conditioning systems, modern kitchen appliances and pool pumps.
However as explained below DC coupled systems are extremely cost effective for smaller systems and still advancing to becoming more competitive for a wide range of applications.
Below are the full details of the advantages and disadvantages of each type...
1. DC Coupled systems
DC coupled systems have been used for decades in off-grid solar installations and small capacity automotive/boating power systems. DC coupled systems use solar charge controllers (also known as solar regulators) to charge the battery directly from solar, plus a battery inverter to supply AC power to appliances.
For micro systems such as those used in caravans/boats or huts, the PWM type solar controllers are very low cost way to use 1 or 2 solar panels to charge a 12V battery. PWM (pulse width modulation) controllers come in many different sizes and cost as little as $40 for a small 10A version.
For larger systems MPPT type solar charge controllers are up to 30% more efficient and available in a range of sizes up to 100A. Unlike the simple PWM controllers, MPPT systems can operate at much higher string voltages, typically up to 150 Volts DC. However this is still relatively low compared to grid-tie solar string inverters which operate 300-600V.
Due to the 150V voltage limitation of most DC coupled MPPT controllers typically only 3 panels can be linked in series which means for larger solar systems above 2kW the installation can be much more complex and involves combining strings of panels in parallel with fuses. There are higher voltage solar controllers available; up to 250V from Victron Energy and even 600V units, but they are much more expensive and don't have multiple MPPT's like many solar string inverters. Therefore for larger solar installations above 2kW it is usually more cost effective to use an AC coupled system.
Hybrid solar inverters
Many modern all-in-one hybrid inverters are in effect advanced DC coupled systems which incorporate high voltage MPPT solar controllers and a battery inverter/charger inside a simple plug and play unit. However these are a different type of system and are explained in the hybrid section below.
- Very high efficiency - up to 99% battery charging efficiency (using MPPT)
- Great low cost setup for smaller off-grid systems up to 2-3kW
- Ideal for small auto or marine systems requiring only 1 - 2 solar panels.
- Modular - Additional panels and controllers can be easily added if you require more solar panels.
- Very efficient for running DC appliances and loads.
- If your electricity service provider restricts the size of solar allowed to be installed (ie. 5kW max), additional solar can be added using DC coupling.
- More complex to setup systems above 1-2kW as multiple strings are required in parallel, plus string fusing.
- Much more expensive to setup for systems above 5kW as multiple higher voltage solar charge controllers are required.
- Lower efficiency when powering large AC loads during the day due to the conversion from DC-DC-AC.
- Many solar controllers are not compatible with modern lithium batteries battery management systems (BMS).
Recommended systems: Victron Energy Multiplus inverters and Victron Bluesolar charge controllers, Selectrconic SP PRO multi-mode inverter and Schneider Conext XW+ with MPPT controllers (See video review here)
2. AC Coupled systems
AC coupled systems use a common solar inverter coupled to a multi-mode inverter or inverter/charger to charge the battery. Although simple to setup and very powerfull they are slightly less efficient at charging than DC coupled systems (90-94%). However these systems are very efficient for powering AC loads during the day and are able to be expanded with multiple solar inverters to form micro-grids.
Most modern off-grid homes use AC coupled systems due to the advanced multi-mode inverter/chargers, generator controls and energy management features. Also since solar inverters operate with high DC voltages (up to 600V or higher), very large systems can be installed. AC coupling is also well suited to medium-large 3-phase commercial systems.
- Higher efficiency when used to power AC appliances during the day such as air-conditioning.
- Lower installation cost for larger systems above 3kW.
- Can use multiple solar inverters in multiple locations (AC coupled micro-grids)
- Most solar inverters above 3kW have dual inputs (2 x MPPT) so solar panels can be setup in different orientations and tilt angles.
- AC coupled systems can use a combination of AC and DC coupling (Note: this can be problematic with some lithium batteries)
- Slightly lower efficiency when charging a battery system - approx 92%
- Quality Solar inverters can be expensive for small systems.
- Lower efficiency when powering direct DC loads during the day.
3. AC Batteries
AC batteries are a new evolution in battery storage for grid connected homes which allow batteries to be easily AC coupled to your new or existing solar installation. AC batteries consist of lithium battery cells, a battery management system (BMS) and inverter/charger all in one compact unit.
These systems combine a DC battery with an AC battery Inverter but are only designed for grid-connected systems as the inverters are not powerful enough to run most homes completely off-grid. The most well known AC battery is the Tesla Powerwall 2 along with the SonnenBatterie which is more common in Europe and Australia. Leading micro inverter company Enphase Energy also manufacturer a very compact AC battery system for home use. These systems are generally simple to install, modular and one of the most economical choices for storing solar energy for later use.
Note: AC battery systems generally provide limited or no back-up power capability in the event of blackout.
- Very economical battery system.
- Generally simple to install.
- Retrofit - can be added to homes with an existing solar installation
- Modular system which can be easily expanded
- Lower efficiency due to conversion (DC - AC - DC) - approx 89%
- Some AC batteries cannot function as a back-up supply (Enphase)
- No designed to function in off-grid installations.
4. Hybrid Inverter systems
Hybrid inverter systems are basically grid-tied DC coupled systems built around the many modern hybrid (multi-mode) inverters. These all-in-one inverters incorporate high voltage MPPT solar controller/s and a battery inverter/charger built inside an common unit. The first generation hybrid inverters mostly operated with 48V lead-acid or lithium battery systems but over time high voltage (400V) battery systems have evolved and are becoming more popular.
High Voltage or Low Voltage? The new generation 'high voltage' batteries operate in the range of 200-500V DC as opposed to the traditional 48V battery systems. However the current electrical standards classify anything which operates from 120 - 1,500V DC as Low voltage or LV. This can be confusing but the high voltage batteries are actually classified as LV.
The new generation high voltage (LV) batteries and compatible hybrid inverters use lithium battery cells in a 200-500V DC system rather than the traditional 48V systems. Hybrid LV batteries can be configured in two different ways:
- Between the solar panels and hybrid inverter (as shown).
- Connected directly to a compatible hybrid inverter.
Since most solar arrays operate at high voltages around 300-600V, the high voltage batteries use DC-DC converters which operate more efficiently as the current is much lower which in-turn reduces voltage drop and heat during conversion. The first generation Tesla Powerwall was the first high voltage battery available and was mated to the popular SolarEdge Storedge hybrid inverter.
The new LG chem RESUH battery range is now one of the most popular LV battery systems available being compatible with many hybrid inverters including SolarEdge Storedge, SMA sunny boy storage and Solax X-hybrid*
One way to explain how hybrid inverters function is described in the diagram below. The battery operates at the same or similar voltage to the solar array and is charged during the day directly from solar. Once night falls the battery discharges energy at the same voltage as a solar array so the hybrid inverter continues to operate as if solar energy was still available.
- Economical and simple to install
- Compact and modular
- Small cable size and low cable losses size due to high voltage.
- Can be added to 'some' existing solar installations.
- Good efficiency - approx 94%
- Growing number of compatible hybrid inverters available
- Some LV DC in-string batteries cannot function as a back-up supply
- Not suitable for off-grid installations.