Solar panels are much more useful when combined with an energy storage system. When used alone, they only generate power at maximum capacity for a few hours around noon. However, when you store the energy produced by solar panels in a battery with sufficient capacity, it can become a 24/7 energy source.
A solar battery is especially useful when your electricity supplier charges peak rates at night. You can simply fill the battery with solar energy at noon and then use that electricity at night to avoid the higher kilowatt-hour prices. If your electricity tariff includes demand charges, you can also use a battery to “trim” consumption spikes and reduce your bills.
Reduce your energy bills and carbon footprint with solar panels and energy storage.
There are two main ways to connect battery systems with solar panels . You can have a DC-coupled system, where solar panels and batteries interact directly with the DC power behind the inverter. On the other hand, an AC-coupled system uses separate inverters for batteries and solar panels, and electricity travels between them as AC power. As with any engineering decision, each option has pros and cons.
Remember that all battery systems provide DC power: the difference is in how they integrate with solar panels.
DC Coupled Solar Batteries Overview
Strong point: Greater round-trip efficiency.
Weak point: More complex installation, especially in existing solar photovoltaic systems.
In a DC-coupled system, batteries are connected to solar panels on the DC side of the inverter. This means that electricity can be stored without an intermediate conversion to AC, which makes the system more efficient. A high-quality DC-coupled battery can operate above 95% efficiency under ideal conditions, while AC batteries are limited to about 90% efficiency.
At first, DC-coupled batteries were more difficult to install. They needed a dedicated charge controller to manage the electricity coming from the solar panels, which would also be connected to the inverter. However, there are already hybrid inverters where this function is integrated and can be connected to solar panels and batteries simultaneously.
The LG Química RESU is among the best DC coupled batteries on the market, and the larger version offers a storage capacity of 16 kWh. Furthermore, LG Chem RESU is compatible with hybrid inverters from major brands such as Fronius, Sungrow and SMA. The first version of the Tesla Powerwall was a DC battery, but has been discontinued – the newer Powerwall 2 and Powerwall+ are both AC batteries.
When a new solar installation uses a hybrid inverter , adding a DC-coupled battery is very simple. However, these batteries cannot be added directly to existing photovoltaic systems with traditional inverters. In this case, you must upgrade the inverter to a compatible hybrid unit before installing the battery. You can also use a separate charge controller, but simply upgrading the inverter is a simpler solution in most cases.
AC Coupled Solar Batteries Overview
Strong point: Simple installation, compatibility with existing solar photovoltaic systems.
Weak point: Lower efficiency, as there are more energy conversions involved.
AC-coupled batteries are much easier to install when you already have solar panels, as there is no need to modify your existing system. In this case, the solar panels and batteries work independently, with a separate inverter for each component. The Tesla Power Wall is the best-known battery utilizing this design, with a storage capacity of 13.5 kWh.
The price to pay for the simpler installation of AC batteries is losing some efficiency. In a DC-coupled system, electricity from solar panels reaches the batteries without being converted to AC. On the other hand, there are two energy conversions involved in charging AC batteries:
- The DC power from the solar panels is converted into AC power by the solar inverter.
- Electricity reaches the battery as AC power.
- The battery inverter converts electricity back into DC power to charge the battery.
- There is a final conversion from DC to AC when you use the energy stored in the battery, but this also happens in DC-coupled systems.
Because there are two additional energy conversions compared to a DC battery, more energy is lost in the process. However, this does not mean that AC-coupled batteries are inferior products: the inverters used in both systems have comparable efficiency, but there is only one energy conversion when using DC-coupled systems. For example, three energy conversions at 97% efficiency give a total efficiency of 91%.
The main advantages of AC batteries are their simplicity and compatibility. As there is no need to modify existing installations, they are much easier to install in buildings that already have solar panels.
Conclusion
You can use DC or AC batteries to add energy storage capacity to a solar panel system , and each option has strengths and limitations. DC batteries are superior in terms of energy efficiency, while AC batteries can be more easily integrated with existing solar panels.
Both battery configurations can be used as backup power and off-grid systems, but you need an inverter designed for independent operation. Many inverters are only designed to integrate solar panels and batteries into your electrical installation, but not for backup power and off-grid systems.
Keep in mind that DC and AC batteries can be used together. For example, you can have a hybrid inverter connected to a DC battery and also a separate AC battery.
