If you’re thinking about adding solar to your home, or if you already have solar, it is worth considering whether you should include or add battery storage or battery backup to your system. First, why is it a good idea to add storage or battery backup to your solar PV system?
Combat time-of-use rates
Many utilities are transitioning their rate schedule to time of use (TOU) rates instead of tiered rates -- this means that using electricity at times during the day when demand is high (and therefore more expensive) will result in a higher monthly electricity bill. You could get around this by avoiding electricity use during peak-demand hours, but running your washer/dryer early in the morning or late at night can be inconvenient, to say the least.
Instead, you could avoid these inconveniences by generating your own electricity and storing it for use at times of peak demand. You can generate electricity for your home by installing a solar PV system and you can store any excess power into batteries, which you can then use when it is the most cost-efficient for you.
Protect your home from power outages
The grid can go down for many reasons, whether it is due to downed power lines, maintenance, or simply a preventive measure by the utility. When this happens and whether it lasts for just a few hours or up to several days, it can be a huge inconvenience for you and your family, and oftentimes dangerous.
Adding a battery storage system to your home would help you protect your home and family from power outages. In the event the grid goes down, you can use the stored electricity in batteries to run critical loads in your home, or if your budget allows, you can choose to run all of your electricity loads.
AC vs DC coupled systems
When it comes to storing energy into batteries, you have two popular choices -- you can either do a DC-coupled system or an AC-coupled system. So what is the difference between these options, and which one is better for you?
Essentially, whether a system is DC-coupled or AC-coupled comes down to the way electricity is transferred from a solar panel array into a battery bank. See below for the advantages and disadvantages of each:
More efficient than AC coupled systems
Easy to expand if needed
Low-cost for small systems
Great for battery backup to protect from blackouts
Difficult to install and is costly for larger systems
Not ideal for retrofitting existing solar installations with battery backup and storage
Some may not work with available rapid shutdown devices or DC optimizers
DC-coupled systems are those that store DC power directly from solar panels into a battery using a charge controller. Charge controllers, as the name implies, regulate when and how much electricity is transferred to the batteries -- they monitor the state of charge of the batteries and protect them from overcharging or discharging to unsafe levels. When the batteries are full, the charge controller stops sending power to the batteries and then sends it to an inverter, which inverts it from DC to AC power to run the loads in your home or in the critical load panel.
If the grid goes down and the solar panels are not producing enough to meet demand, a battery inverter or inverter charger can draw DC power from the batteries and inverts it to AC to power your home just the same as if it were coming from your solar panels. Also, if your solar array is not producing enough to charge your batteries, the inverter can draw power from the grid to charge the batteries and keep them at the ready for when they are needed.
You can find several options for DC-coupled systems from Outback Power and Schneider or Sol-Ark. The easier option to install would be a Sol-Ark inverter, which can work with rapid shutdown devices or optimizers from Tigo such as the Tigo TS4-A-F and TS4-A-O. Disclaimer: As of 2/11, this combination of Sol-Ark inverters + Tigo MLPE devices have not received UL listing.
Easier to install than DC-coupled systems
Lower cost for larger systems
Works with rapid shutdown devices
Great for retrofitting existing solar installations with battery backup and storage
Less efficient than DC-coupled systems
Only make sense for larger systems, ~5kW and up
In an AC-coupled system, the electricity generated by the solar panels gets transferred directly into the inverter, not into the batteries. The inverter inverts DC power coming from the panels and sends AC power to a battery inverter or hybrid inverter. This allows the AC power to go through to power your home loads or convert it back into DC power to store into batteries for later use, depending on your preferences and needs.
If the battery inverter or hybrid inverter needs to draw from the batteries at any time, it will have to convert the stored battery power from DC back to AC. This is a major drawback, as doing this back-and-forth conversion multiple times results in lower system efficiency overall.
It is worth mentioning that new AC batteries are now being offered from Enphase which can mitigate this inefficiency, although they come at a higher cost than traditional DC batteries. Some equipment options for AC-coupled systems include SMA Storage and SMA Sunny Island battery inverters or the Sol-Ark 8K and Sol-Ark 12K hybrid inverters. Lastly, some battery inverters or hybrid inverters such as the Sunny Island and the Sol-Ark inverters have an additional AC input that allows you to connect a generator to supplement your battery backup system.
Retrofitting an existing solar PV system with battery backup
If you have an existing solar system and want to add battery backup, you can retrofit it into an AC coupled battery backup system by adding a battery inverter such as the SMA Sunny Boy Sunny Island or a hybrid inverter such as the Sol-Ark 8K or Sol-Ark 12K and DC batteries, all without having to rewire the DC side of your installation (the equipment before the solar inverter). This solution is especially convenient because battery or hybrid inverters can generate their own grid signal which allows traditional string inverters, DC optimizers or microinverters to continue to operate even when there is a power blackout.
Please contact the RENVU support team if you have questions about DC or AC-coupled battery backup systems.