Should I Add a Battery to My ADU Solar System, or Is That a Waste of Money?

By Buz Artiano, BuildX | May 2026

Watch: Solar Truth Bombs, Battery Myths & The Real Cost of "Free" Power w/ Brett Duguay | BuildX Podcast #02

You've decided that you're going to build an ADU and you're going to install solar panels on it. But somebody mentioned batteries, so you looked into it. The internet says you need a Tesla Powerwall, but then your neighbor or someone you know says that batteries are a waste of money. The solar company told you it was going to be $15,000 to $20,000 for a system that might not even run your air conditioning. At this point you can't figure out whether you're leaving money on the table or whether you're about to throw a bunch of money away.

My name is Buz Artiano, and I started BuildX. Since then we've built dozens of ADUs all over Massachusetts, but most of our builds have been around the South Shore and Plymouth County areas. We handle design, permitting, and construction ourselves. Every project has to have an electrical and energy strategy, which means we have to deal with the state's energy code (which is constantly changing), septic constraints, lot coverage limits, and zoning requirements before a single solar panel ever gets mounted. We work with licensed installers on every project that includes solar, and we've seen what batteries can and can't do on a small structure.

The honest answer is that batteries are not a waste of money, but they're not what most people think they are, either. In this article we're going to talk about if a battery system can really power an ADU, when it makes financial sense, and when skipping it is actually the better move. We're not trying to sell you on solar, we are just sharing our experience with you and things we've noticed.

Is a Battery Worth the Extra Cost on an ADU Solar System?

For most ADU owners, a battery is a great addition, but not really required. Only 5 to 10 percent of residential solar installations include a battery. That number is growing, but it tells you that most solar systems work perfectly well without one.

The value of a battery depends on three things. One, how often you lose power, two, whether your utility offers a program like Connected Solutions that pays you for stored energy, and three, whether a battery solves a technical problem with grid interconnection. If any of those things apply to your situation, a battery is absolutely worth the investment. If none of those apply, then your money is better spent on a larger solar array or just kept for other things.

The cost of a battery system for an ADU is usually between $15,000 and $20,000 installed. Mass Save offers a 0 percent interest loan for battery installations, which can really help with upfront costs. But before you commit, you need to understand exactly what a battery can and can't do.

What Can a Battery Actually Power on a 900-Square-Foot ADU?

This is where the conversation gets real, and where most solar companies oversell. A battery system is sized based on how much electric load there is, not on your bill. That means the installer looks at what you want to run during a power outage and builds the battery capacity that will run just those appliances.

On a typical ADU, a single battery can handle lights, electrical outlets, a refrigerator, bathroom exhaust fans, and basic electronics. But it will not run your air conditioning, a hot tub, an electric dryer, or multiple high-draw appliances at the same time. As one installer we work with said, you're not going to run your jacuzzi on these batteries.

We see this disconnect on nearly every ADU project where a homeowner asks about batteries. They picture a perfect backup system where life continues exactly the same when there's a power outage. That's not how it works. We size the electrical system for the ADU at 120 amps total with everything running, including the heat pump, electric stove, dryer, and all outlets. A battery absolutely cannot give you 120 amps. When we talk to people about batteries, we start with a list of what you actually want to keep running when the power goes out. That question alone usually changes expectations. Most families realize they need lights, the fridge, and charging capability. Once you have the right expectations, the battery becomes a practical decision instead of a purchase that you don't really understand.

The sizing process involves evaluating where you're at on the grid, how often you lose power, how long that usually lasts, and what appliances you have to have when there's no power. The installer figures out the total wattage for a day and factors in what the industry calls "days of autonomy", which just means how many consecutive days you could lose power. The answer to that will determine whether you need one battery or multiple units.

What If I Want Full Backup Power, Including Air Conditioning?

If a family member living in the ADU needs air conditioning during outages, whether for comfort or for a medical condition, there is a combined strategy that works. Pair a battery with a backup generator. The battery handles the lighter loads (lights, fridge, outlets, fans), and the generator takes care of things like air conditioning, electric dryer, hot tub, etc.

Builders call this "energy resiliency". The battery provides instant, silent backup for just the basics, and the generator handles the heavy lifting. The two systems work perfectly together when you use the right type of generator and transfer switch.

The honest reality is that today's battery technology isn't good enough to run a full household. Your energy goals are important, and future-proofing your ADU's electrical service is smart. But for right now, in order to run a whole household when the power goes out, you have to have some form of generator backup. We're not trying to sell you a generator, it's just the reality of where the technology is currently at.

Can a Battery Actually Make Me Money?

Yes. Massachusetts has a utility program called Connected Solutions. If you allow the utility to dispatch your battery during high-demand hours, they pay you for that. The utility sends a signal to your battery, your battery discharges stored power back to the grid, and you get paid for the electricity your batteries provide.

This is part of a broader approach called distributed generation. Instead of relying on a single substation to power entire neighborhoods, the utility company pulls power from onsite battery systems people have in order to manage peak demand. Your ADU battery becomes a tiny power plant that earns income when the demand for power is higher than usual.

Mass Save also offers a 0 percent interest loan just for solar battery installations. Between the Connected Solutions payments and the interest-free financing, the payback period on a battery is pretty short. You can also install a battery without solar panels if backup power or Connected Solutions income is what you're looking to do.

Do I Need a Battery to Get My Solar System Connected to the Grid?

Sometimes, yes. People don't really talk about this a lot, and it has nothing to do with backup power.

When you apply for solar interconnection with your power company, they evaluate whether the local transformer can handle the additional power your system will feed back to it. If the transformer is at its max, the power company can actually deny the interconnection or require you to pay to upgrade the whole transformer. One installer we work with told us about a job where someone wanted to expand a 15 kW system by 3 kW. The power company gave them two options. They either had to pay $4,500 for a review (just the fee to determine upgrade costs), then potentially $10,000 to $20,000 for the actual transformer upgrade. Or add a battery and curtail the extra power so you don't overload the transformer.

Curtailment means that you're only allowed to feed a certain amount of power back to the grid. When your panels produce more energy than you're allowed to feed back, the extra just goes to charging the battery instead of feeding back to the transformer. When you're producing less than your max allowed energy, the battery discharges and sends stored power to the grid. This makes the interconnection process simple without having to upgrade the transformer.

For ADU projects where the main home already has solar, this is more common than most people think. The existing system may have already consumed most of the transformer's capacity, and adding ADU solar on top pushes it over the limit. In that situation, your only viable option is a battery system.

Which Battery Brand Should I Choose for My ADU?

The two most common battery brands in Massachusetts are Tesla Powerwall and Enphase. Both are lithium-based systems with about the same capacity and output. Both are now rated for outdoor installation, which is really important for ADUs because inside space is limited. You can mount them on the exterior wall where they stay out of sight without sacrificing square footage for them inside.

The main differences between the two brands are price and customer service. Tesla batteries are competitively priced and are expected to be a major market disruptor. The tradeoff is that Tesla's customer service has a reputation for being difficult to reach when you need help. Enphase batteries cost more but they have much more responsive technical support. If your installer runs into a diagnostic issue, Enphase support helps troubleshoot right there on the spot.

For an ADU where the homeowner may be an aging parent or a family member who is not technically inclined, that technical support is important. A lower price doesn't do you any good if the system goes offline and nobody answers the phone to help you fix it. We recommend discussing both options with your installer and weighing cost savings against the peace of mind that comes with reliable support.

Is a Battery the Wrong Choice for Some ADU Projects?

Yes. As we have touched on already, a battery is not the right thing for every ADU. If you live in an area with reliable power and almost never have outages, having a battery won't really do much for you. If the power company in your area doesn't participate in Connected Solutions or a similar demand-response program, then you have no way to generate income from your battery. And if your solar interconnection is really basic and doesn't need a transformer upgrade, you don't really need to spend the extra money on a battery system.

In those situations, the $15,000 to $20,000 you would spend on a battery is better off being used elsewhere in your project, whether that means upgrading finishes, adding more solar panels, or just holding on to it for unexpected site work. Solar panels paired with net metering still offset your electricity costs over 12 months without a battery. You bank credits during high-production months (April through October) and draw them down during winter. The system works.

We are always upfront about this because the decision should be driven by your specific situation, not by a sales quota. If a battery makes sense for your ADU, we will tell you. If it doesn't, we will tell you that too.

What Does Your ADU Energy Strategy Actually Need?

A battery is not a waste of money when it solves a real problem such as backup power for a family member who needs it, income through Connected Solutions, or a grid interconnection problem. It's a waste of money when it is purchased based on marketing hype and unrealistic expectations about what it can power. The right decision starts with understanding your property, your grid situation, and your family's actual energy priorities.