[telchar]

I think you’re on to something with that. With so few round trip conversions compared to something that is charged/discharged frequently, the conversion efficiency doesn’t matter as much compared to the cost of storage.

For a given kwh of storage that is round-tripped daily, a 1% loss of efficiency loses 3.65kwh per year. For a kWh of storage that is round-tripped once per year, you only lose 0.01kwh. At 70% conversion efficiency the yearly storage still loses only 1/10 the energy as a daily system at 99%.