AC or DC coupling refers to the way in which solar panels are coupled or linked to a battery system.
Solar battery systems were traditionally only used in off-grid or remote locations but during the last decade the technology has advanced rapidly with many different battery types and configuration options now available. Previously almost all battery systems were DC coupled, which charge a battery directly from solar using simple DC charge controllers. For a long time this was the only way to charge a battery from solar; as panels produce DC power and batteries store DC this is still one of the most efficient methods of charging a battery from solar.
As solar systems became larger and more advanced AC coupled systems became the preferred configuration as they are compatible with most common solar inverters which is more cost effective. These use advanced multi-mode inverters coupled with a solar inverter.
Over recent years battery and inverter technology has improved significantly with many new 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 HV batteries have become increasingly popular. More recently AC battery systems 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 solar battery system types
DC coupled system
AC coupled system
HV DC batteries
AC vs DC coupled
Why would you use an AC coupled system rather than DC coupled? 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 appliances. The cost of solar inverters is much lower for larger systems as they can handle high DC voltages (up to 600V), this means solar panels can be installed in long strings (in series) which is far easier and simpler to install. AC coupled systems are also more efficient during day when there is high AC demand such as air-conditioning systems, modern appliances and pool pumps.
Below is the full breakdown of advantages and disadvantages of each type...
1. DC Coupled system
DC coupled systems use solar charge controllers (also known as solar regulators) to charge the battery system directly from a solar array. The very simple PWM type Solar controllers are a low cost, effective way to charge a small battery and come in many different sizes, costing as little as $50 for a small 10A version.
The larger more efficient MPPT type solar charge controllers are much more powerful (up to 80A) but are still limited as most cannot operate above 150 Volts DC. Due to the voltage limitation usually only 3 panels can be linked in series which means for larger solar systems above 1kW it is slightly more complex and involves combining strings of panels in parallel. There are higher voltage solar controllers, up to 250V from Victron Energy and even 600V units available but they are generally very expensive and don't have multiple (MPPT) inputs. At this size it is usually more cost effective to use an AC coupled system.
Many modern all-in-one solar hybrid inverters are in effect DC coupled systems with integrated solar controllers and inverters housed inside a common enclosure.
- 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.
- Scalable - Additional panels and controllers can be easily added if you require more solar panels.
- Very efficient for running DC appliances and loads.
- If a electricity service provider restricts the size of solar allowed to be installed (ie. 5kW max), additional solar can then be added using DC coupling.
- Much more complex to setup larger systems above 3kW as multiple strings are required in parallel, plus string fusing.
- 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 system: Victron Energy Multiplus inverters and Victron Bluesolar charge controllers
2. AC Coupled system
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 (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)
- 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 concept and simply combine a lithium battery, battery management system (BMS) and battery inverter/charger in one simple unit. At this stage they are only available for grid-connected systems and are not designed to function completely off-grid. The most well known AC battery is the Tesla Powerwall 2. Leading micro inverter company Enphase Energy also manufacturer a small compact AC battery system for home use. These systems are generally simple to install, modular and are one of the most economical choices for storing solar energy for later use. Note: some AC battery systems provide limited or have no back-up power capability.
- Very economical battery system.
- Generally simple to install.
- 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 90%
- Some AC batteries cannot function as a back-up supply (Enphase)
- Cannot function in completely off-grid installations.
Recommended systems: Sonnon Flat, Tesla Powerwall 2, Enphase.
4. HV DC Batteries
HV or High Voltage DC batteries are a new battery type available which combine lithium batteries in a 300-400V DC system rather than the traditional 48V systems. HV batteries can be configured in two different ways:
- Connected between the solar panels and inverter (in-string) as shown.
- Connected directly to a compatible HV hybrid inverter.
Since most solar arrays operate at high voltages around (300-600V) HV batteries use DC-DC converters which can be more efficient as the current is much lower. The first generation Tesla Powerwall was the first HV battery available and was mated to the popular SolarEdge Storedge hybrid inverter. The new LG chem RESU HV battery range is now one of the more popular battery systems available and is compatible with SolarEdge, SMA, Solax and Imeon storage inverters*
*Hybrid Inverters connected to HV batteries must be compatible (and communicate) with the battery in order to charge and discharge energy when required.
One way to explain this type of system is the battery is charged during the day and once night falls it discharges energy at the same voltage as a solar array so the solar (hybrid) inverter still generates power as if the sun is still shining.
- 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%
- Compatible with several new HV hybrid Inverters.
- Some HV DC in-string batteries cannot function as a back-up supply
- Not suitable for off-grid installations.