What is AC or DC coupling? AC or DC coupling simply 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 used only for remote power systems and off-grid homes but over the last decade hybrid (solar and battery) inverter technology has advanced rapidly and led to the development of new AC coupled energy storage configurations. However DC coupled systems are far from dead, in fact charging a battery system using 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 using multi-mode inverter/chargers coupled with one of more common lower cost string solar 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 higher voltage (200-500V) batteries have become increasingly popular and are used with specialist hybrid inverters. More recently AC batteries have been developed by many leading solar 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 System types
1. DC coupled systems
2. AC coupled systems
3. AC Battery Systems
4. Hybrid Inverter Systems
Only DC or AC coupled systems can be used for off-grid solar installations. See details below including advantages and disadvantages each system, followed by a comparison of AC vs DC coupled for off-grid systems.
Important: This is a guide only! For less technical information see the basic guide to selecting home grid-tie or off-grid solar battery system. Solar and battery storage systems must be installed by a licensed electrical/solar professional. Solar/energy storage systems generate and store huge amounts of energy which can result in damage or serious injury if the installation does not meet all relevant regulations, standards & industry guidelines.
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 12 volt 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 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 common 150V upper voltage limitations of most DC MPPT solar controllers, only 3 panels can be linked in series which means for larger solar systems above 3kW the installation can become more complex and involves combining strings of panels in parallel with fuses.
Higher voltage MPPT solar controllers
There are several higher voltage solar controllers available; up to 250V from Victron Energy and 290V from AERL in Australia. There are also higher 600V units available from Schneider Electric but these are much more expensive and don't have multiple MPPT's like many solar string inverters. Depending on the application MPPT charge controller are a relatively cheap and secure way of ensuring batteries are charged even in the event of a AC inverter shutdown, especially in remote locations.
Many modern all-in-one hybrid inverters work much like DC coupled systems and incorporate high voltage MPPT solar controllers, plus a battery inverter/charger inside a simple plug and play unit, however they do have several limitations. This is further explained in section 4 below.
Very high efficiency - up to 99% battery charging efficiency (using MPPT)
Great low cost setup for smaller off-grid systems up to 5kW
Ideal for small auto or marine systems requiring only 1 - 2 solar panels.
Modular - Additional panels and controllers can be easily added if required.
Very efficient for powering DC appliances and loads.
If an electricity service provider restricts or limits the capacity of grid-tie solar allowed (ie. 5kW max), additional solar may be added by DC coupling a battery system.
More complex to setup systems above 4kW as often multiple strings are required in parallel, plus string fusing.
Can become expensive for systems above 5kW as multiple higher voltage solar charge controllers are required.
Slightly lower efficiency if powering large AC loads during the day due to the conversion from DC(PV) to DC(batt) to AC.
Some solar controllers are not compatible with modern lithium battery systems such as the LG Chem RESU or BYD B-Box.
Recommended DC Coupled Systems
Victron Energy Multiplus inverter/chargers and solar charge controllers,
Outback Power Radian, FXR Inverters & FLEXmax MPPT solar controllers
Schneider Conext XW+ with Conext MPPT controllers (See video review)
2. AC Coupled systems
AC coupled systems use a string solar inverter coupled with an advanced multi-mode inverter or inverter/charger to manage the battery. Although relatively simple to setup and very powerful, they are slightly less efficient (90-94%) at charging a battery compared to DC coupled systems (98%). However these systems are very efficient at powering high AC loads during the day and some can 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 string solar inverters operate with high DC voltages (600V or higher), very large systems can be installed. AC coupling is also well suited to medium to large 3-phase commercial systems.
Higher efficiency when used to power AC appliances during the day such as air-conditioning, pool pumps, and hot water systems, (up to 96%).
Generally lower installation cost for larger systems above 4kW.
Can use multiple string solar inverters in multiple locations (AC coupled micro-grids)
Most string solar inverters above 3kW have dual MPPT inputs, so strings of panels can be installed at different orientations and tilt angles.
Advanced AC coupled systems can use a combination of AC and DC coupling (Note: this is not possible 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.
Recommended AC Coupled systems
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.
AC coupled battery inverters*
A more recent trend is to use a ‘retrofit’ AC coupling inverter to create a AC battery system. These systems use a specialised AC coupled battery inverter such as the SMA sunny boy storage together with a common DC battery such as the popular LG chem RESU.
Easy retrofit - can be added to homes with an existing solar installation
Economical way to add energy storage.
Generally simple to install.
Modular system to allow expansion.
Lower efficiency due to conversion (DC - AC - DC) - approx 90%
Some AC batteries cannot function as a back-up supply (Enphase)
Not designed for off-grid installations.
Recommended AC Battery Systems
4. Hybrid Inverter systems
Hybrid systems are essentially an on-grid DC coupled battery system using one of the many hybrid or multi-mode inverters available. Modern transformerless all-in-one hybrid inverters incorporate high voltage MPPT controller/s and battery inverter/chargers inside a common unit. The first generation hybrid inverters were compatible with 48V lead-acid or lithium battery systems, however over recent years higher voltage (400V+) hybrid systems have become popular.
High Voltage or Low Voltage? The new generation 'high voltage' batteries operate in the range of 300-500V DC (400V nominal) 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. This can be confusing but the high voltage (400V) batteries are actually classified as Low Voltage or LV.
The new generation higher voltage (400V) batteries and compatible hybrid inverters use lithium battery systems operating between 200-500V DC, rather than 48V. Higher voltage batteries can be configured in two different ways:
DC coupled between the solar array and inverter.
DC coupled with a compatible hybrid inverter, (as shown below).
Since most solar arrays operate at high voltages around 300-600V, high voltage batteries use efficient DC-DC converters with very low losses. The first generation Tesla Powerwall was the first 400V 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 400V battery systems available being compatible with many hybrid inverters including SolarEdge Storedge, SMA sunny boy storage and Solax X-hybrid Gen 3.
Economical and simple to install
Compact, modular battery options
Smaller cable size and low losses using high voltage (400V battery systems)
Can be retrofitted to 'some' existing solar installations.
High efficiency battery charging - approx 95%
Growing number of hybrid inverters becoming available
Some systems cannot function as a back-up supply
Many systems with back-up have a 3 second delay during a blackout
Generally not suitable for off-grid installations due to transformerless hybrid inverters with low surge rating and no generator controls.
Recommended Hybrid systems
AC vs DC coupled for off-grid systems
Why would you use an AC coupled 'off-grid' system rather than DC coupled?
AC coupled systems use modern efficient solar inverters to convert solar DC power directly to AC which can then be used immediately by most appliances during the day which is very efficient, especially when powering high loads such as air-conditioning systems, modern kitchen appliances and water/pool pumps. In addition the installation cost of string solar inverters is much lower for larger systems above 5kW as they are available in much higher capacities, up to 10kWp single phase. The longer strings and dual MPPT inputs of solar inverters also make installation easier and simpler for larger systems.
If an AC coupled system is used for off-grid installation the solar inverter/s must be compatible with the main (multi-mode) battery inverter/charger to enable charge control. This is required so the solar generation can be 'managed' or ramped up and down to ensure safe and accurate battery charging. There are several management systems used by different manufacturers such as frequency ramping or direct communication - refer to manufacturers specifications.
As explained previously DC coupled systems are extremely cost effective for small to medium size systems. Another advantage of DC coupled systems is solar controllers are very flexible and scalable which means additional panels can be easily added if required using relatively low cost DC solar controllers.
Combining AC and DC coupling
Most modern interactive inverter/chargers and such as Selectronic's SP PRO, SMA sunny island and Victron multiplus can function in both AC and DC coupled configurations which offer the best of both worlds and provides DC back-up battery charging in the event of an AC shutdown.
If there is a shutdown in a remote location due to low battery voltage or low SOC the DC coupled solar charge controllers will continue to function (without AC operation) and charge the battery system which preserves the battery life and can restart the system if configured correctly. This creates a fail safe setup unlike a pure AC coupled system which are unable to be automatically or remotely restarted unless a back-up generator/source is functioning.
This is a guide only. All solar and energy storage systems must be installed by a licensed electrical/solar installer. Solar & battery systems generate huge amounts of energy which can result in damage or serious injury if the installation does not meet all relevant regulations, standards & industry guidelines.
Best off-grid solar systems review