Round-Trip Efficiency

What gets lost in the round trip

Storing and retrieving electricity is not lossless. When solar energy charges a battery, it passes through power electronics - rectifiers, inverters, battery management circuitry - and each conversion step wastes a small fraction as heat. The same happens on discharge. Round-trip efficiency measures both losses combined.

For a DC-coupled system (solar charges battery directly at DC, no conversion to AC in between):

• Charge losses: ~2–4% in the charge controller
• Discharge losses: ~4–6% in the hybrid inverter converting DC to AC
• Total round-trip: roughly 90–95%

For an AC-coupled system (solar → AC inverter → AC → battery inverter → DC battery → inverter → AC again):

• More conversion steps mean more losses
• Total round-trip: roughly 85–92%

Real numbers from current products

Published round-trip efficiencies for common Australian residential products:

• Tesla Powerwall 3: ~97.5% (DC-coupled, highly integrated)
• Sungrow SBR series: ~98% (DC-coupled with compatible hybrid inverter)
• BYD Battery-Box HVM (AC-coupled): ~94%
• Enphase IQ Battery 5P: ~89% (AC-coupled microinverter ecosystem)

Higher is better, but the difference between 90% and 95% on a 10 kWh daily cycle is 0.5 kWh - worth about 17 cents at a 34¢/kWh tariff. Over a year that’s ~$62. Not irrelevant, but unlikely to swing a purchase decision.

Why it matters more in some cases

Round-trip efficiency becomes more significant when:

High cycle rates - commercial or VPP applications cycling twice daily amplify the loss percentage by doubling daily throughput.

Poor feed-in tariff economics - if your excess solar is worth 4–5¢/kWh exported but 28–35¢/kWh avoided, the round-trip loss costs you the difference only on the lost fraction. At 90% efficiency, you lose 10% of stored solar to heat rather than exporting it at 5¢. The maths still usually favours storage.

Comparing quotes - a battery with 10 kWh usable capacity and 90% round-trip efficiency delivers effectively 9 kWh of value per cycle. One with 95% delivers 9.5 kWh. A fair comparison between systems should account for this, not just nominal capacity.