Seasonal iron as I described wouldn't be for electrical round-tripping, but for moving iron ore reduction to the summer where PV energy is abundant (assuming a PV buildout sufficient to cover winter heating heat pump demand without residual fossil fueled peaker plants) and with tricks like co-localizing these to not require the grid to be sized for the summer surplus production, thus overall enabling internationally-competitive production costs for this green-refined iron, vs. the traditional coal/coke blast furnace production.

"Grid batteries" are also a wide concept, from retired electric car lithium packs to 100-hour-scale batteries like what Form is building. You need 3 types of battery, though the two fast ones may want to be unified: - grid stabilizing/synthetic inertia (1~20 C charge/discharge rate) - day/night PV energy shifting (0.15~0.3 C charge/discharge rate) - multi-day weather pattern smoothing (around the 0.01 C Form Energy is targeting)

Other than that the grid stabilizing kind needing much higher security/criticality due to bugs/errors in it's behavior threatening e.g. the European super grid, they would all benefit from co-locating with large solar farms and being connected on the DC side to share AC-side current capacity of the as-built infrastructure (from the inverter/converter through the lines and nearby transformers).