Deep decarbonization of the power grid is only possible with mass-scale energy storage to overcome the spatiotemporal mismatch between supply from renewables and demand. Aqueous flow batteries fully decouple power and energy elements and can thus easily be scaled, a prerequisite for cheap long-duration energy storage, but low energy density is generally considered a key limitation of the technology. To date, the role of this metric for grid-scale installations has not been quantified, a crucial step for guiding further development of this potential trillion-dollar market. Here, we analyze the footprint of forty-four MWh-scale battery energy storage systems via satellite imagery and calculate their energy capacity per land area in kWh m−2, demonstrating that energy density is not critical for such installations and that the importance of this metric for grid-scale batteries is heavily overstated in academia. We suggest that a unique advantage of aqueous flow batteries, due to their intrinsic safety and vertical scalability, is their ability to provide reliable power in space-restricted sites, and we show that even with current chemistries and modest assumptions about storage tank sizes and footprints, areal energy densities five times as high as with the average lithium-ion based system can be achieved.