Choosing between AC and DC battery storage

No, this isn’t a blog post about great rock ‘n’ roll bands but as long as we’ve got your attention … Whether you already have a solar energy system or are getting ready to install one, a crucial decision you should be making is about energy storage. While it’s great to have a solar energy system to power your home or business, installing a battery – or multiple batteries chained together – enables you to enjoy secure, reliable, renewable energy even after the sun goes down or during a power outage. But choosing between AC and DC battery storage can be confusing or even stressful for people already overwhelmed by financial and technological considerations; we’ll try and make this easy and painless.

Solar panels produce “direct current,” or DC power; this is the kind of energy that batteries hold in storage. However, “alternating current,” or AC electricity, is the kind used on the grid and in most households. Nearly all solar installations must include an inverter in order to convert the DC electricity generated by solar panels into the AC electricity required to power your appliances and devices (the exception being when you are powering a DC device, like a water pump or street light). There are generally three different kinds of grid tie inverters: Traditional/String inverters, Micro-inverters, and DC-Optimized inverters. But that’s a topic for another day …

Whether you are installing the solar energy system and batteries together, or adding the battery (or batteries) at a later time, you’ll need to make a decision on whether to use AC-coupled or DC-coupled storage. The simplest explanation is that a DC battery will use the same solar inverter as your rooftop array to convert its stored DC electricity into AC power for your home or business, whereas an AC-coupled battery will require its own inverter to convert the stored DC electricity into AC. It may seem as though a DC-coupled battery would be easier and less costly but, as with most things, it’s actually not that simple.

Let’s go a little deeper and look at how the coupling options work as well as the advantages and disadvantages of each option.

DC-Coupled Systems

For decades, DC-coupled systems have been used to help homeowners go off the grid as well as for automotive and boating power systems. They are most common when installing the solar and the battery (or batteries) simultaneously – this enables the solar system and the battery to share one inverter to convert the DC power to AC electricity, rather than necessitating the installation of two separate inverters for both the solar array and the battery.

DC-coupled batteries have the advantage of being very high efficiency when in off grid mode (but less efficient than grid tie inverters when feeding power to the grid), and cost less for smaller off-grid systems. They are efficient for powering DC appliances and loads, and are ideal for very small auto or marine systems. However, DC coupling can become much more complex and more expensive when setting up larger off-grid systems, are typically lower efficiency than grid tie inverters, and can require significantly more and larger array wiring for a given system size.

AC-Coupled Systems

AC-coupled systems are made up of a solar array and a battery system that are independent of each other and require two inverters: one for the solar panels and another for the battery bank. If you already have a solar array, installing an AC-coupled system will most likely enable you to keep your existing solar inverter and wiring; a second battery inverter will need to be added.

AC coupling has numerous advantages, including its efficiency for powering AC loads during the day (since grid tie inverters are more efficient), its ease in retrofitting batteries onto pre-existing solar PV systems, plus it generally costs less to install for larger systems. However, they are slightly less efficient at charging than DC coupled systems, the inverters can be more expensive for small systems, and the efficiency is lower for powering direct DC loads.

AC Batteries and Hybrid Inverter Systems

In case you were wondering, there are other options, such as AC batteries and hybrid inverter systems.

AC batteries, such as the Tesla Powerwall 2, integrate the battery and battery inverter/charger in one compact (and beautiful) unit. AC batteries are relatively economical and can be easily AC coupled to an existing or new solar installation. While some AC battery systems, like the new Enphase battery system, have limited or no backup power capability if the grid goes down (instead they are designed to arbitrage time of use rate structures), others – like the Tesla Powerwall 2 – are fully capable of handling even large off-grid and backup power applications. Tesla also has its Powerpack solution for even larger commercial and utility-scale storage and backup power.

Which Option is Right for You?

Now that we have discussed the options, understanding how you will use the solar and battery system will help you determine which option is better for you. 

If you:

Are grid connected but need backup power for emergencies: In this case, you will be connected to the grid 99 percent of the time over a period of many years. Because of this, the improved efficiency of a grid tie inverter over a battery inverter will win out, with more energy produced in grid tie mode and the slight inefficiency in battery charging only taking place very rarely. Thus, an AC-coupled solution is most likely your best bet.

Want solar now and may or may not want batteries in the future: In this case, a standard grid tie inverter is your best bet and you can always install an AC-coupled battery like the Tesla Powerwall in the future, confident that your investment in the grid tie system will not be wasted.

Are always off grid or rarely grid tied and need power: A DC-coupled battery system is most likely your best bet, since you have no need for a grid tie inverter and you must size your system to meet all of your expected electrical loads, not just for emergency purposes.

Need to power DC loads: In this case, you may not need an inverter at all but instead simply a charge controller as well as a DC to DC converter (in case you need to support 6V, 12V, 24V and/or 48V loads simultaneously).

We know this is a lot to take in but the good news is that we LOVE talking about this stuff and we’re always thrilled to help our customers find the system that makes the most sense – based on budget, technological requirements and power goals – for them. Don’t hesitate to give us a call with any questions!

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