Explain round-trip efficiency in energy storage.

• A. The ratio of energy retrieved from storage to energy put in, accounting for charging and discharging losses.
• B. The ratio of solar input to wind output.
• C. The ratio of generation capacity to installed capacity.
• D. The ratio of energy lost during transmission.

Answer: (A)

Round-trip efficiency measures how much energy you actually get back from a storage system after a full cycle of charging and discharging. It is the ratio of energy retrieved during discharge to the energy put in during charging, capturing all losses that occur in both steps. For example, if you charge with 100 kWh and you can discharge 70 kWh, the round-trip efficiency is 70%. In real systems, losses come from converting electricity to the storage form and back (inverters, converters, and the storage chemistry itself), as well as any self-discharge or thermal losses during storage. This metric matters because it tells you how much usable energy remains after storing it, which directly affects how effective the storage is for smoothing renewables or meeting demand. The other options describe solar-to-wind ratios, capacity ratios, or transmission losses, which are not about the energy lost specifically in the storage cycle.