Solar stops at sunset — the battery is what fixes that
Rooftop solar does one thing brilliantly: it cuts your electricity import while the sun is up. But a commercial or industrial (C&I) site rarely stops working at sunset. The evening shift, the cold store running overnight, the office HVAC into the evening — all of that is bought from the grid at full tariff, often at the very hours when Time-of-Day rates and demand charges peak.
A battery is what closes that gap. It captures the solar you would otherwise export cheaply at midday and releases it in the evening, so more of your own clean generation actually offsets your own load. This guide explains how the battery physically connects to your solar, and what it does once it is there.
How the battery connects: DC-coupled vs AC-coupled
There are two ways to wire storage onto a solar plant, and the difference matters most when you are retrofitting an existing system.
DC-coupled storage puts the battery on the same DC circuit as the panels, managed by a single hybrid inverter. Because solar charges the battery without an extra DC-to-AC conversion, it is slightly more efficient — but it is more involved to add later, so it suits sites installing solar and storage together from the start.
AC-coupled storage gives the battery its own inverter and connects it on the AC side of the plant. Your existing solar inverter and panels keep running exactly as they are; the battery simply bolts on. That is why, for a C&I plant that is already generating, AC-coupling is almost always the practical retrofit route. It trades a little conversion efficiency for the ability to add storage without disturbing a working system.
What the battery actually does for the site
Once connected, a C&I battery earns its keep in four distinct ways — and they stack:
- Evening self-supply. Surplus midday solar is stored and used after sunset, so your own generation offsets your own evening load instead of being exported cheaply. When your DISCOM pays a low export rate, this alone can justify the battery — a trade-off we cover in net metering vs storage for C&I sites.
- Time-of-Day arbitrage. The battery charges when power is cheap and discharges when the Time-of-Day tariff is dear, capturing the price spread.
- Demand-charge reduction. It shaves your peak kVA so the demand line on your bill drops — often a big slice of a C&I bill, as explained in our demand-charge guide.
- Outage backup. It rides through grid failures, keeping critical loads live.
Sizing: it is about surplus and evening load, not panel wattage
A common mistake is sizing the battery to the solar array. What actually matters is how much solar you export at midday (the energy worth capturing) and how long your evening load runs after the sun sets (how much you need to release, and for how long). A site that consumes most of its solar as it is made needs little storage; one dumping half its generation onto the grid at a discount has a lot to recover. The mechanics of matching battery power and energy to a load are covered in how to size a BESS for your site.
What this means for you
If your factory or building already has rooftop solar and you are exporting a chunk of it cheaply at midday while buying expensive power in the evening, a battery is the piece that closes the loop — and on an existing plant it is almost always an AC-coupled add-on that leaves your solar untouched. If you are still at the planning stage, weigh a DC-coupled hybrid design for its efficiency edge. Either way, the right size comes from your own generation, export and evening-load data. A behind-the-meter C&I cabinet as part of a solar-plus-storage system is the typical shape this takes. To estimate what storage adds to your specific solar plant, try our savings calculator or send us your solar generation and load data for a modelled answer.
Snapshot as of July 2026. System costs, tariffs and metering rules vary by state, DISCOM and site — model your own plant before financial decisions.