Where Does the Solar Battery Go in an ADU?

By Buz Artiano, BuildX | September 2026

Watch: How Solar Really Works for Builders, Homeowners, and ADUs | BuildX Podcast #35

You have a floor plan for a 700 square foot ADU and a layout that works. Then someone mentions a battery and you're thinking of it as a box on the wall like the water heater is a cylinder in the closet. But when the contractor asks you which wall, the answer changes your plans.

My name is Buz Artiano, and I am the founder of BuildX. We've built dozens of ADU projects across Massachusetts, mostly around the South Shore and Plymouth County areas. We are the only team you need: we do the design, permitting, and construction. So we see every phase of a project from the first site visit up until you move in. We evaluate septic constraints, zoning issues, utility connections and the requirements for a permit from the city. Massachusetts allows one accessory dwelling unit by right in single family zoning areas, although municipal building code, health code, and dimensional requirements still apply to every project. You can review the state requirements at mass.gov. Energy storage placement isn't just a preference in this state. It gets checked against the manufacturer instructions and the electrical code on inspection day.

Our opinion on this isn't complicated. In your ADU a battery isn't an accessory, it's a requirement of the floor plan, and it takes up floor space on your plan before you even pay for it. We do our design around every one of these requirements before we are finished.

Where Does the Battery Actually Go in an ADU?

On the outside wall of a garage, attached or detached. That is the normal place for it in almost every ADU we build. Energy storage system is the industry term for the home battery plus the electronics that you need to charge it and switch it over. The garage wall works because then the equipment is inside, out of the weather, and on the far side of a wall from anywhere people sleep or sit.

When the ADU doesn't have a garage, where to place it is still a problem. The floor plan has to have another wall that would work. And that usually means a mudroom or utility space big enough to hold the battery on an outside wall with an empty floor in front of it. Depending on how many batteries the system needs, how much space you have, and what ventilation the units require, the battery may have to have a space of its own instead of a shared wall in a room people walk through.

Each of these requirements affects your floor plan, and that's the part that gets skipped when a battery is thought of as just a decision about equipment instead of a design issue. Here is what the battery needs and what each thing does to the plan.

Why Can't My Battery Share a Wall with Living Space?

Two separate rules apply here. The first is that the battery doesn't go inside your living space. The second is that moving it to the outside of a wall doesn't fix anything if the other side is the bedroom. Both of these are requirements and the same wall has to meet both.

The reason is because a failure can cause serious issues. We have been to battery fires. They don't happen often, and the people who put in these systems all the time aren't nervous about them. The solar contractor we talked to for this episode above has two batteries in his own detached garage. Just because it's rare doesn't mean you don't have to design for it and there has to be separation. Clearance and separation are what keep it a rare event.

That's why we treat the battery wall the same way as an electrical panel wall. We plan three feet of clear floor in front of it, we keep rooms that are lived in off the other side of it, and we tell homeowners not to fill up that area with storage after they move in.

Cold Weather Is the Reason Your Battery Lives Indoors

The pictures you see for home battery advertisements almost always shows on one mounted outdoors on a stucco wall in full sun. Those photos are taken in California, Hawaii, and Florida. New England isn't like that. A battery mounted outside here will have months in the cold that will affect the output and shortens its usable life. Homeowners can insist on mounting it outside and the installers will do it, but there's a downside.

The problem is the timing. A battery is a backup plan, so the nights you most want it heating the house will be the coldest nights of the year. Those are the exact conditions that affect an outdoor battery.

Nobody in our conversation gave an exact number on how much output or lifespan cold exposure will cost you, and we're not going to make one up. The number that matters is the temperature derating range in your battery specifications, and that's the published curve from the manufacturer showing just how much capacity your battery loses when the temperature goes down. Your solar contractor can show you that before you choose a location.

What Does a Battery Cost You in Floor Plan Square Footage?

In a 600 to 900 square foot ADU the scarce thing isn't floor area. It is uninterrupted outside wall. Windows will be put on it, and so will a sliding glass door for the bedroom. The battery needs part of it with three feet of empty floor in front and nothing habitable behind it.

That's why we plan out the battery wall before we lay out interior walls on any ADU. The reason is the order of things. If the kitchen and the bedrooms are laid out first, the only wall left over is almost always one that backs a bedroom, and then the options come down to a bump-out, a dedicated enclosure, or no battery at all. If you place the battery wall first it costs you nothing on paper but placing it last can cost square footage, and in an ADU this size that has to come out of a room that somebody needs.

That's the real cost of this decision and so we won't treat a battery as a line item without looking at the plan first. The additional space isn't the area the battery takes up, it's whatever room had to be sized to make room for it.

Do I Actually Need a Battery in My ADU?

A battery isn't a necessity. It's the homeowner's decision. There's one question that just applies to ADUs that needs to be answered first: does the main house already have a generator, and if it does, will that generator have extra capacity to carry the ADU too? If so then the idea of a second backup in the ADU loses importance. If it doesn't, a battery is one way to fix that.

That question gets asked far too late in most projects. By the time a family is talking about batteries, the ADU plan is drawn and the main house electrical capacity is treated as a fixed given. Ask it at the design table instead, where the answer can still change the floor plan and the budget.

Whether an ADU battery and a main house generator can back each other up is an engineering question and not one just anyone can answer. Discuss that with both your solar contractor and your electrician in the same room and get it settled for your property.

Who Sets the Clearance and Enclosure Rules?

Two authorities, and your builder isn't one of them. The first is the company that makes the batteries and you can check by looking through the installation instructions in the manual. The second is the electrical code, which is enforced by your town's municipal inspector of wires.

In Massachusetts that code is 527 CMR 12.00, the Massachusetts Electrical Code, administered by the Department of Fire Services and built on the national electrical code with state amendments layered on top. The current edition and the amendments are published at mass.gov.

The three feet we plan for is a planning number, based on where we put the electrical panel and confirmed drawn from how we space an electrical panel and the solar contractors we work with will check to make sure it's correct. Just consider it a number to plan around, not something to argue about with an inspector. Check the exact clearance, separation, and enclosure requirements against the listed instructions for the specific battery on your project, and with your local inspector, before the plan is finalized.

There is a red flag that we want to mention here. A solar contractor who won't add the battery on your floor plan before you sign tells you something. Ask for it to be drawn on the plan with dimensions.

When a Battery Is the Wrong Call for Your ADU

We build ADUs and we use solar contractors for many of them, so you might think we'd tell every family to add storage. We don't. There are three situations where you'll get a no from us.

• The main house generator already has enough capacity for the ADU and some to spare
• The floor plan can't give up a wall without making the bedroom too small for what the family actually needs
• The family priority is to have a second bedroom or a larger living space, and the battery would use that space

None of those are actually solar problems. They're floor plan issues and those get fixed on the plan. Talk to more than one solar contractor, and ask each of them to draw the battery on your plan before you make a decision.

Draw the Battery Before You Approve the Plan

The decision you have to make isn't if a battery is good technology or not. It's if your ADU floor plan has an exterior wall where you can put one, with clearance in front of it and no bedroom behind it, and whether you'd rather find that out now or later. It's always better to find out at the design table.