About the DC/AC Architecture
DC/AC architecture selection for a BESS determines whether the battery connects to the AC bus (AC-coupled) or directly to the solar inverter's DC bus (DC-coupled) — a fundamental design choice that affects efficiency, cost, flexibility, and the ability to charge from solar vs grid.
How to calculate it
- 1AC-coupled: BESS has its own bidirectional inverter (PCS). Battery connects at the AC bus alongside the solar inverter. Pros: flexible, modular, works with any solar inverter. Cons: two power conversion stages (DC→AC→DC for charging), lower round-trip efficiency.
- 2DC-coupled: BESS connects to the solar inverter's DC bus via a DC/DC converter. Pros: direct solar-to-battery charging (one conversion stage), higher efficiency. Cons: tighter integration, sizing constraints, limited flexibility to add storage to existing systems.
- 3For new C&I rooftop (< 500 kW): hybrid inverter with DC-coupled battery is the default — simpler wiring, lower cost.
- 4For large utility-scale (> 1 MW): AC-coupled with dedicated PCS is the standard — allows independent sizing of solar and storage, easier grid services.
- 5For retrofit (adding BESS to existing solar): AC-coupled is usually the only practical option.
Worked example
A new 300 kWp C&I rooftop with 200 kWh storage: DC-coupled hybrid inverter (e.g., Sungrow SH250HX or SolarEdge StorEdge). Solar and battery share the inverter's MPPT and DC bus — one inverter handles everything. Total system cost 8–12% lower than AC-coupled equivalent. RTE: ~94% (DC-coupled) vs ~88% (AC-coupled with separate PCS).
What you can calculate
- DC vs AC coupled
- Efficiency delta
- CAPEX compare
- Application fit
Frequently asked questions
Can DC-coupled storage charge from the grid?
DC-coupled batteries charge primarily from solar, but many hybrid inverters also support AC-charging (grid-to-battery) by running the inverter in reverse. This is less efficient (lower RTE) than charging from solar. Some DISCOMs and net-metering regulations prohibit grid-to-battery charging (to prevent consumers from buying cheap off-peak grid power and selling at peak rates as 'solar') — check local regulations before enabling this mode.
Which is more efficient: AC-coupled or DC-coupled storage?
DC-coupled is more efficient for solar charging: 94–97% RTE vs 85–91% for AC-coupled (two conversion stages). However, AC-coupled allows the battery to be charged from both solar and grid independently, and can be added to any existing solar plant without changing the solar inverter. For new builds in India, DC-coupled hybrid inverters are the default choice for systems up to 250–500 kW; AC-coupled PCS is standard above that.
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