ampere-hour

The ampere-hour survives as a battery unit for a historical reason that is still practical: for a given cell chemistry the voltage is roughly fixed, so charge alone ranks capacity within a product family. It also matches how charge controllers and shunts actually work, since they integrate current over time. Coulomb counting is exactly the ampere-hour done continuously, $Q = \int I \, dt$, and every battery management system in a solar installation or an electric vehicle is doing that sum.

To compare across chemistries or voltages, convert to energy: $E \text{ (Wh)} = Q \text{ (Ah)} \times V$. Then remember the two derating factors that separate nameplate from usable. Depth of discharge limits how much you may take out, conventionally 50 percent for flooded lead-acid and 80 to 90 percent for lithium iron phosphate. Round-trip efficiency, roughly 80 to 85 percent for lead-acid and 92 to 96 percent for lithium, tells you what fraction of the energy you put in comes back out.