A flow battery, because a four-to-ten-hour discharge window makes the technology the most adaptable option across any grid service.

A lithium-ion battery sized for twelve hours of discharge, because longer discharge duration always takes priority over faster response.

A flywheel system, because very fast response within seconds is the exact requirement and flywheels are built for this short-duration, high-cycle duty.

A pumped-hydro reservoir, because the large energy capacity ensures that any frequency event is covered regardless of duration.