"Seconds" of capacity might be useful for "primary frequency response" but not for any of the other functions listed in table 1 of https://www.nrel.gov/docs/fy1

"Seconds" of capacity might be useful for "primary frequency response" but not for any of the other functions listed in table 1 of https://www.nrel.gov/docs/fy19osti/74426.pdf https://www.nrel.gov/docs/fy19osti/74426.pdf. Primary frequency response is only seen as important in a limited number of markets in the US (see that table).