Retrofit batteries: How to add storage to your solar panel system

You installed your rooftop solar system a few years ago and are enjoying your disappearing electric bills. But your neighbor just installed a new solar-plus-storage system that couples rooftop solar with a battery storage system. And now you've got a serious case of solar FOMO.  Is it too late to add a battery backup? Did you miss out?

The good news is that it’s entirely possible to add battery storage to an existing solar panel setup. So-called “storage ready” systems are already equipped with an inverter that can easily direct excess power into a battery. But even if your system wasn’t designed with storage in mind, you still have options. 

Let’s explore how easy it is to add a battery to your existing solar setup and what options you have based on your current equipment. Plus, we’ll break down costs and provide tips for finding the right installer if your current one doesn’t install batteries.

You can integrate storage into practically any solar panel system, but some setups are easier than others. The sticking point lies with the type of inverter your system has. 

Best case scenario: You have a storage-ready solar panel system

If your system was designed with storage in mind, or you already have a hybrid inverter (which can manage energy from both solar panels and a battery), adding a battery is relatively easy. In this scenario, a battery storage installation requires minimal additional equipment.

If this isn’t the case for you, don’t worry. While the process might be more complex, adding a battery is still very feasible. You have two primary options: an AC-coupled solution or an inverter replacement.

AC-coupled solution: Easier and cheaper, but less efficient

An AC-coupled retrofit involves installing a separate inverter for your battery, allowing you to keep your existing solar inverter. Without the need to redesign or rewire your solar panel system, this option is typically more affordable upfront. 

However, efficiency losses occur because electricity is sent through two inverters (one for solar and one for storage). Bottom line: AC-coupled systems are easier and less expensive to install but less efficient than DC-coupled systems.

DC-coupled solution: Higher efficiency but more expensive

A DC-coupled retrofit involves replacing your existing inverter with a hybrid inverter that works with both solar panels and batteries. 

The biggest benefit of a DC-coupled system is the improved efficiency. However, it’ll cost you.  Replacing your inverter to accommodate a battery can require a system redesign and rewiring, which can result in additional costs on top of the new inverter itself. 

A DC-coupled system could be a financially smart choice if you're nearing the time for an inverter replacement. Since string inverters typically last around 10 years, upgrading to a hybrid inverter for both solar and storage might make sense if your inverter is already five or more years old.

Most solar batteries designed for small-scale use are compatible with existing solar panel systems. The best battery for your retrofit installation really comes down to your unique needs and reasons for installing an energy storage system. 

Popular batteries on EnergySage

Batteries for time-of-use rates or partial-home backup power

A single 10 kWh battery can serve multiple purposes, from providing backup power during outages to helping homeowners avoid costly demand charges.

For those in areas with time-of-use (TOU) rates or demand charges, energy storage allows you to use stored energy during peak hours, reducing reliance on the grid and lowering electricity costs. 

If minimizing your demand during peak hours or providing backup power to your home during grid outages is your primary goal, you can opt for a smaller battery, such as the VillaGrid, Powerwall 3, or Schneider Electric Boost.

Batteries for whole-home backup power 

To keep your entire home powered during an outage, you’ll need a larger capacity battery or multiple batteries working together. The modular design of a battery like HomeGrid’s Stack’d Series is ideal for expanding capacity by stacking units, allowing you to create a battery bank tailored to your needs.

Batteries for off-grid use

Most batteries aren’t designed to fully disconnect your home from the grid. They generally lack the storage capacity to power your home through extended periods of cloudy, rainy, or snowy weather. If your goal is to go off-grid with solar, you’ll need significantly more storage than most common batteries can offer.

On average, the cost of installing a battery storage system is around $9,000 after federal tax credits. However, the final price will vary based on the brand of battery and your location.

Tesla offers the cheapest price per kilowatt-hour on the EnergySage Marketplace, whereas Generac is the most expensive brand. Likewise, Californians tend to pay less for battery installations, while Oregonians pay the most. 

Ultimately, every energy storage project is unique. The best way to understand your options and their costs is by working with a qualified installer. Share the specifics of your system, electricity usage, and storage goals to determine the best solution for your home.