Round-trip efficiency is the percentage of energy you get back from a system after storing it and later using or exporting it. It measures losses across a full “charge → store → discharge” cycle. A higher round-trip efficiency means less energy is wasted as heat and conversion losses.
In EV charging ecosystems, round-trip efficiency is most commonly discussed for:
– Battery energy storage systems (BESS) used for peak shaving and buffering
– Bidirectional charging (V2G / V2B / V2H), where EVs can export energy
– Site-level energy strategies combining solar PV, batteries, and chargers
Why Round-trip Efficiency Matters for EV Charging Economics
Round-trip efficiency directly impacts both cost and carbon outcomes:
– Lower efficiency means more grid energy is needed to deliver the same usable kWh later
– It reduces the financial benefit of energy arbitrage (buy low, use/sell high)
– It changes the true value of peak shaving and demand charge reduction
– It affects how much solar PV energy can be “captured” and later used for charging
– It influences the ROI of V2G and stationary storage projects
When evaluating storage-based charging hubs, round-trip efficiency is a key input in ROI models.
What Determines Round-trip Efficiency
Round-trip efficiency is influenced by several loss sources:
– Converter losses (AC↔DC conversions in inverters/rectifiers)
– Battery losses (internal resistance, temperature effects, state-of-charge window)
– Auxiliary loads (cooling, control electronics, standby consumption)
– Cable and transformer losses across site distribution
– Operating conditions (power level, charge/discharge rate, temperature)
Efficiency typically varies based on whether the system runs at partial load or near rated power.
Round-trip Efficiency in Common Charging Energy Setups
Different setups have different “round-trip” paths:
– Stationary BESS buffering a charging site
– Grid AC → inverter/charger DC → battery → inverter AC → site/chargers
– Losses occur in both conversion stages plus the battery itself
– V2G / bidirectional charging
– Grid AC → EV charger conversion → EV battery → reverse conversion back to AC → grid/building
– Additional losses depend on charger design and vehicle onboard systems
– PV + storage + charging
– PV DC → inverter/DC bus → storage/chargers → later discharge to chargers or grid
– System architecture (AC-coupled vs DC-coupled) affects conversion steps and efficiency
How Round-trip Efficiency Is Measured and Reported
Round-trip efficiency is typically calculated as:
– Round-trip efficiency (%) = Energy delivered out ÷ Energy put in × 100
Important measurement details include:
– Define the measurement boundary (battery-only vs full system including inverters and auxiliaries)
– Use consistent time windows and include standby/aux loads if comparing business cases
– State the operating profile (power level, temperature, SOC range), because efficiency is not constant
Practical Implications for System Design
To improve effective round-trip efficiency and project outcomes:
– Minimize unnecessary conversion steps (optimize AC-coupled vs DC-coupled architecture)
– Size converters so typical operation stays in efficient ranges
– Use good thermal management to reduce temperature-related battery losses
– Account for efficiency in energy cost and carbon reporting (effective kWh delivered)
– Include efficiency losses in peak shaving and arbitrage calculations to avoid overstating savings
Related Glossary Terms
Battery energy storage system (BESS)
Bidirectional charging
Vehicle-to-Grid (V2G)
Vehicle-to-Building (V2B)
Peak shaving
Energy arbitrage
Load management
Power conversion stage
Inverter
Idle power consumption