Adding Battery Backup to an Existing Residential Solar System

Adding Battery Backup to an Existing Residential Solar System

When we first started Milestone Solar, a residential solar system that featured a battery backup option was not at all common. It was mostly a cost issue, as the batteries do add substantial cost to the system. But I think it was also true that many installers did not (and probably still do not) like the extra complexity that an integrated battery backup (bimodal) capability brings to the project, so they did not promote the capability.

We’ve always liked the bimodal technology and the capabilities it brings, and have offered it as an option to customers for years now. Our installed base of battery backup systems speaks for itself.

But there are thousands of residential solar systems that were installed without batteries. This type of system, which I call a straight production system, does a great job of producing electricity on a day-to-day basis, but when the grid is down, your solar system shuts down – by design. One of my friends, an engineer in the solar industry, calls it buyer’s remorse to discover that you now want to add battery backup to your legacy solar system.

I think it may be any number of factors, to include spreading the cost over a longer period of time, a response to some of big storms that have caused prolonged power outages for thousands of homes, or maybe a response to the various threats to the grid as discussed in the book Lights Out by Ted Koppel.

Over the past few years we have been hearing more and more about a capability to retrofit legacy solar systems with batteries using an electrical design called AC Coupling. Our standard or typical bimodal battery backup system uses DC coupling and features the array, charge controller, batteries, inverter and critical loads subpanels. Everything on the input side of the inverter is DC.

As you can see on the graphic at the top from Enphase Energy, the main components in this AC Coupling design/ retrofit are the battery bank, a compatible inverter/charger and a critical loads subpanel. On a day-to-day basis the solar array and, in this example, micro inverters, are sending AC power to the critical loads panel. Any excess is sent on to the new inverter/charger to be routed to the main panel for use in the house, or sent back to the grid for credit via the bidirectional meter. But when the grid is down, the inverter/charger begins supplying power from the battery bank, and after a short pause, the micro inverters will see a 240 VAC connection and will once again begin producing electricity.


One of the keys to this process is to have a fully compatible inverter/charger that is monitoring the state of the battery bank to insure that the batteries are protected from overcharging. Most use a process called “frequency shifting” to take the AC connection to the solar system out of spec, shutting down the array inverter(s) when the batteries are at a certain state of charge. Some companies are also recommending an additional inline relay to further protect the batteries from overcharging – an option worth looking into as well.

The obvious question that comes up now is, should we now abandon DC coupling for this AC-coupled configuration? In my opinion, if you are starting from the beginning, the DC coupled system design offers significant advantages, like highly efficient MPPT charge controllers with a tapered charge cycle that can be “tuned” to your individual system and also provides great battery protection.

It probably goes without saying that this is not a good do-it-yourself project for the average homeowner. But there is now more than enough of an installed base to consider AC coupling a viable and fully supported option for the many customers with legacy solar systems who would like to add batteries for when the grid is down.