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 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 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 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 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 System types
1. DC coupled systems
2. AC coupled systems
3. AC Batteries
4. Hybrid Inverter Systems
See full details below including advantages and disadvantages of the four system types followed by a comparison of AC vs DC coupled for off-grid.
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 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 290V from Aerl in Australia. There are also much higher 600V units but these 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 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. This is explained in detail in section 4. Hybrid Inverter systems, below.
- Very high efficiency - up to 99% battery charging efficiency (using MPPT)
- Great low cost setup for smaller off-grid systems up to 3kW
- 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 3kW as usually multiple strings are required in parallel, plus string fusing.
- Can be much higher cost for systems above 5kW as multiple higher voltage solar charge controllers are required.
- Lower efficiency (approx 90%) when powering large AC loads during the day due to the conversion from DC-DC-AC.
- Some solar controllers are not compatible with modern lithium 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 powerful 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 or pool pumps, approx 95%.
- Generally lower installation cost for larger systems above 3kW.
- Can use multiple solar inverters in multiple locations (AC coupled micro-grids)
- Most string solar inverters above 3kW have dual MPPT inputs so 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 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.
Recommended systems: Selectronic SP PRO inverters and Fronius solar inverters, SMA Sunny island (SI) and SMA sunny boy inverters (SMA SI not accredited for grid-connect in Aus), Victron Energy Quattro & Fronius inverters.
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.
- Easy retrofit - can be added to homes with an existing solar installation
- Modular system which can be expanded.
- Lower efficiency due to conversion (DC - AC - DC) - approx 90%
- Some AC batteries cannot function as a back-up supply (Enphase)
- Not designed to function in off-grid installations.
4. Hybrid Inverter systems
Hybrid inverter systems are basically grid-tie DC coupled systems built around the many modern hybrid or multi-mode inverters. The common 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 in the last few years high voltage (400V) battery systems have evolved and are now available from several leading manufacturers.
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 high voltage (400V) batteries and compatible hybrid inverters use lithium battery systems operating at 200-500V DC rather than the traditional 48V. 400V batteries are typically 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, high voltage batteries use efficient DC-DC converters with very little voltage drop. 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.
Another way to explain how hybrid LV 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 losses size due to high voltage (HV 400V battery systems)
- Can be added or retrofitted to 'some' existing solar installations.
- High efficiency battery charging - approx 95%
- Growing number of compatible hybrid inverters available
- Some LV DC in-string batteries cannot function as a back-up supply
- Generally not suitable for off-grid installations due to transformerless hybrid inverters with low surge rating and no backup gen-set controls.
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.
Note when using an AC coupled configuration for off-grid systems the solar inverter/s must be compatible with the interactive 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 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.