Adding a battery to a solar system raises one question that drives the whole design: how does the battery talk to the rest of the system? The answer comes down to two things, the type of inverter you use and whether your storage is AC coupled or DC coupled. This article explains both, and shows how they shape a quote and its battery analysis.
Inverters: the heart of the system
Every solar system needs an inverter to convert DC electricity (from panels and batteries) into the AC electricity your home uses. There are three types:
- String inverters. A central inverter wired to one or more strings of panels. The traditional, cost-effective choice.
- Microinverters. One small inverter per panel, converting DC to AC right at the roof.
- Hybrid inverters. A single inverter that manages both solar input and battery charging and discharging. This is the key piece for storage.
A hybrid inverter is special because it has a DC input dedicated to a battery. It can route solar power to the panels' loads, send excess to charge the battery, and pull stored energy back out when needed, all from one box.
The big distinction: AC coupled versus DC coupled
This is the concept that determines whether a battery needs a hybrid inverter at all.
AC-coupled storage
An AC-coupled battery has its own built-in inverter. It connects directly to the home's AC bus and operates independently, converting its own DC to AC on board.
- No hybrid inverter required. The battery is self-contained.
- Easy to add to an existing solar system (including one with microinverters or a standard string inverter).
- Energy makes an extra conversion trip: solar DC to AC to battery DC to AC again, which costs a little efficiency.
DC-coupled storage
A DC-coupled battery is a "bare" battery pack with no inverter of its own. It connects to the DC input of a hybrid inverter, which handles all the conversion.
- Requires a hybrid inverter to function.
- More efficient, because solar can charge the battery directly as DC, with fewer conversions.
- The hybrid inverter is shared: several DC batteries can sit behind one hybrid.
Integrated (all-in-one) storage
Some products combine a hybrid inverter and battery into a single enclosure, for example the Tesla Powerwall 3, EP Cube, or Sigenergy SigenStor. These count as both an inverter and storage at once, with no separate battery to pair. They are the simplest option to specify because the manufacturer has already matched the inverter and battery for you.
How this shows up in a quote
When you add storage to a quote, the system automatically works out how each battery is powered and shows it in the BESS (Battery Energy Storage System) validation card. Each battery falls into one of these states:
- Built-in. An AC-coupled battery using its own onboard inverter. Ready to go.
- Paired. A DC-coupled battery successfully linked to a hybrid inverter (the card shows which hybrid and its discharge rate).
- All-in-one. An integrated-storage hybrid.
- Pairing needed. A DC-coupled battery with no hybrid inverter available. This is flagged in red, and the battery analysis cannot run until it is resolved.
Tip: If you see "battery pairings needed," it means a DC-coupled battery is on the quote without a hybrid inverter. Either add a hybrid inverter, or swap in an AC-coupled battery instead.
The numbers that matter
Once the system is correctly configured, the battery analysis uses these specs to model performance:
- Total capacity. The full energy the battery can hold, in kWh.
- Usable energy. What you can actually use, total capacity times depth of discharge.
- Depth of discharge (DoD). How much of the battery can be drained without shortening its life.
- Round-trip efficiency. The percentage of energy you get back after a full charge-and-discharge cycle. Energy is lost in conversion, so this is always under 100%.
- Charge and discharge rate. How fast the battery can take in or push out power, in kW.
For DC-coupled batteries, the hybrid inverter often sets the ceiling on charge and discharge rates, and its efficiency stacks with the battery's. For AC-coupled batteries, these figures come straight from the battery's own specs.
What the battery analysis tells you
With the system configured, the quote calculates two key benefits for your customer.
1. Backup power duration
The analysis simulates an outage across four seasons (winter, spring, summer, fall) to show how long the battery can keep the home running, both overnight on battery alone and through the day with solar topping it up. In favorable conditions, where daytime solar exceeds overnight demand, the system can sustain the home indefinitely.
This is where the Backup Load % slider comes in. It sets how much of the home is on backup:
- 100%. Whole-home backup (every circuit).
- ~50%. Essential loads (HVAC, fridge, lights).
- ~10%. Minimal loads (a few lights and outlets).
The lower the percentage, the longer the battery lasts during an outage. Adjust the slider to match what your customer wants to protect.
2. Energy savings
Depending on the utility's rate structure, the battery saves money in one of two ways:
- Time-of-use arbitrage. Charge the battery when electricity is cheap (off-peak) and use that stored energy during expensive peak hours. The savings come from the spread between peak and off-peak rates.
- Self-consumption. Store excess solar instead of exporting it back to the grid at a low rate, then use it later instead of buying power at a high rate. The savings come from the gap between import and export rates.
Quick reference: which setup is right?
- Adding storage to an existing solar system? AC-coupled storage is usually simplest, with no need to replace the inverter.
- Designing a new system for maximum efficiency? A hybrid inverter with DC-coupled storage minimizes conversion losses.
- Want the easiest spec with the fewest moving parts? An all-in-one integrated unit handles inverter and battery together.
Whichever path you choose, the quote's BESS validation card will confirm the system is wired correctly before the battery analysis runs, so you always know the numbers in your proposal are sound.
Related articles
- Adding Inverters to Your Catalogue. Setting up hybrid, string, and microinverters and their specs.
- Adding Batteries to Your Catalogue. The AC Coupled switch, battery specs, and what the analysis and SLD need.
- Utilities, Rates, and Consumption. Where the time-of-use and self-consumption rates behind battery savings come from.
