AC-Coupled vs DC-Coupled explained
When connecting battery storage to a solar system, there are two main approaches:
DC-coupled (hybrid inverter):
- Solar panel DC power charges the battery directly without first converting to AC
- The hybrid inverter then converts from battery DC to home AC in one step
- Round-trip efficiency: approximately 95–97%
- Best for: new installations designing solar and storage together from scratch
AC-coupled (e.g. Tesla Powerwall on existing solar):
- The existing solar inverter converts DC to AC as normal
- The AC battery (which has its own inverter) then converts AC back to DC to charge, and DC back to AC when discharging
- Round-trip efficiency: approximately 85–90% (two extra conversions)
- Best for: adding storage to an existing solar installation without replacing the inverter
The efficiency difference (roughly 5–10% per cycle) isn't enormous in absolute terms — on a 10 kWh daily cycle, it's about 0.5–1 kWh/day difference. For most homeowners retrofitting storage, the convenience and lower disruption of AC coupling outweighs the small efficiency penalty.
For a new installation, DC coupling via a quality hybrid inverter is the preferred approach: fewer components, higher efficiency, and usually lower total cost.