Adding a solar battery as a retrofit installation

The market for home energy storage options like the Tesla Powerwall has taken off in recent years, and costs are falling quickly. Many homeowners and businesses are considering adding a battery backup to their solar panel system.

The benefits of a home battery can be significant, especially if you have solar: you can use more solar energy onsite or save it to use as backup power if the grid goes down. If you are on time-of-use (TOU) rates for your electricity or pay monthly demand charges, you can even save money by using power from your battery when rates are high.

The difficulty associated with adding a battery depends on whether your solar panel system was designed to add energy storage later on.

If you have a so-called "storage ready" system, you already have an inverter that can easily integrate a battery into your solar panel setup. In this situation, a battery is relatively simple to install, and installation won't require much additional equipment.

If your solar panel system wasn't originally designed with the ability to add storage later, the installation would be slightly more complicated. You have two options in this scenario: an AC-coupled solution or an inverter replacement.

AC coupled solution

If you choose an alternating current (AC) coupled solution, your battery will be installed with a separate inverter integrated into your home's energy system.

If you install an AC-coupled solution, you can keep your solar system's existing inverter, saving money upfront. You can also be more flexible regarding the size of your battery relative to your solar panel system. However, choosing an AC-coupled solution does result in some efficiency loss over time.

Most buildings use AC power, but your solar panel system produces electricity in direct current (DC) power. When your solar panel inverter converts the DC power to useful AC power, a small amount of electricity is lost in the conversion process. There are additional conversion steps if you install an AC-coupled battery backup system. As a result, AC-coupled systems are less efficient than systems with a single inverter.

Inverter replacement/DC coupled solution

The alternative to an AC-coupled solution is to replace your existing solar inverter with one that works with a battery. String inverters need to be replaced every ten years or so, so if you have a solar panel system that's at least five years old, you may want to swap your existing inverter for an all-in-one solar and storage inverter option.

The primary benefit of a DC-coupled solution is that it will improve your efficiency by reducing the energy you lose during current conversion. This option will be more expensive upfront. Replacing your inverter to accommodate a battery can result in additional costs associated with system redesign and rewiring. However, the exact costs will always depend on your current system and the installer you're working with.

Check out this blog to learn more about the differences between AC and DC-coupled batteries.

While some exceptions exist, most solar batteries designed for small-scale use are compatible with existing solar panel systems. The battery you choose for your home or business depends on your reason for installing an energy storage system.

Most batteries cannot take your home fully off the grid to the point where you sever your connection with your utility – they don't have enough storage capacity to power your home for days on end in the event of an extended bout of cloudy, rainy, or snowy weather. If you want to go off-grid with solar, you'll need much more storage capacity than most common lithium-ion batteries can provide. One solution? Stack multiple batteries together to make a larger battery bank.

If your primary goal is to ensure a few hours of backup power during a power outage, a larger battery like the Sonnen eco will best serve your needs. However, if your utility has TOU rates or demand charges, and your goal is to reduce grid electricity consumption when rates are high, you can choose a smaller battery like the Enphase Encharge.

Every energy storage project is different. If you want to add a backup battery to your existing solar panel system, share the specifics of your system, your electricity use, and your storage objectives with your installer to determine what kind of system is the best fit for your home.

Adding a battery to your solar panel system is a relatively simple process in terms of installation. You can think of installing a battery as more of an electrical project than a roofing project like solar panels.

When your solar panel system was installed, your installer likely brought in an installation crew to work on your roof. By comparison, batteries are installed on the ground, usually in a garage. With fewer people needed to complete the installation and simpler logistics, you're likely looking at a smaller installation charge than your panels.

Assuming that your solar panel system is in good working order and there are no significant upgrades, the labor cost to add a battery shouldn't be more than a thousand dollars (of course, this depends on the hourly cost of an electrician in your area). If you are also replacing your inverter as part of the installation, the job will take longer and cost more.

Energy storage is growing in popularity but isn't as common today as solar. Many, but not all, solar installers will install batteries. If you contact your installer and are told that they don't offer energy storage, you have a few options:

Ask your installer for a recommendation.

Chances are good that you aren't the first customer who has asked your installer about storage. They are the first place you should go to find out about other companies that offer storage options in your area.

Look for referrals from friends, family, or neighbors.

Referrals are one of the most common ways that property owners find an installer for a solar panel system. If you know anybody else who has installed solar (or, even better, has installed a battery), ask them for their recommendation. Even if your installer doesn't offer energy storage, somebody else's installer will!

Find an installer through battery manufacturers' networks

If you know what type of solar battery you want, you can often go directly to the manufacturer and ask them to help you find an installer. Most battery manufacturers for home storage systems have their own network of installers that are trained to work with their equipment.

EnergySage is the nation's online solar marketplace. When you sign up for a free account, we connect you with solar companies in your area who compete for your business with custom solar quotes tailored to your needs. Over 10 million people visit EnergySage annually to learn about, shop for, and invest in solar. Sign up today to see how much solar can save you.