The key insight is that the "cost of inefficiency" is proportional to the number of charge/discharge cycles in the storage system. That's because, for a given

The key insight is that the "cost of inefficiency" is proportional to the number of charge/discharge cycles in the storage system. That's because, for a given storage capacity, there is a cost from the extra energy thrown away at each cycle. However, the cost incurred in building the storage system is independent of the number of charge/discharge cycles. So: in comparing diurnal (daily) storage systems, with seasonal (up to yearly) storage systems, the relative importance of efficiency and capex are radically different. In the latter, capex can be 365 times more important than in the former. For seasonal storage, one is strongly driven to minimize capex, even at the cost of making round trip efficiency worse.