A wall-mounted battery with a leaf logo is still a conversion machine. It buys kilowatt-hours, stores them imperfectly, and sells them back with a haircut. The haircut is heat—not a moral failure, just physics. The carbon question is: what generation mix produced the extra kilowatt-hours you lost?
Round-trip losses are indirect emissions
Efficiency (\eta) means you must import (C/\eta) kWh to deliver (C) kWh to the house (per full cycle on capacity (C)).
The gap ((1/\eta - 1) \times C) is wasted as:
- I²R in conductors
- Inverter switching
- Cell overpotential and thermal management
If that gap was charged from a grid averaging 420 gCO₂/kWh, it has a footprint even if discharge replaced clean solar hours later. You already paid the carbon on the loss slice at charge time.
Example intuition
| RTE | Loss per 10 kWh cycle |
|---|---|
| 90% | ~1.11 kWh |
| 85% | ~1.76 kWh |
| 95% | ~0.53 kWh |
250 cycles/year at 1.1 kWh loss ≈ 275 kWh wasted. At 420 g/kWh → ~116 kg CO₂/yr from losses alone.
Why renewable charging matters
Marginal renewables (curtailed solar, behind-the-meter PV surplus) often displace near-zero additional grid generation on that kWh. Charging the BESS then makes loss energy carbon-light.
Grid-midnight charging in a coal-heavy region can flip the story—financial arbitrage savings may still exist while carbon rises.
Not all “green tariffs” are equal
- Matched RECs — contractual zero if additionality is credible
- Average grid mix label — may not change hour of charging
- Community solar — good if production aligns with charge window
Operational green status is a schedule + source problem, not a sticker problem.
Motors and appliances are separate
Voltage drop and motor stress live on the delivery circuit. BESS carbon lives on the charge source + efficiency plane. Do not confuse bill savings with climate savings without running both numbers.
Action list
- Run BESS Carbon Cost with your real cycles/year
- Log when the battery actually charges (app export)
- Shift charge windows toward solar surplus or published low-carbon hours
- Re-run calculator with lower effective gCO₂ if charging improves
Related reading
- BESS Carbon Cost Guide — full methodology
- BESS Charge Scheduling — timing tactics
Green storage is renewable-fed, efficient storage—not storage alone.