A simplified thermal-storage duty: energy stored scales with heat capacity, and the heat-exchanger area needed to move that energy scales inversely with thermal conductivity. Dial the uncertainties that reference correlations honestly carry — a few percent on heat capacity, up to 15–20% on conductivity for some salts — and watch the design bands widen into the region where margin, not physics, sets the capital cost.
Deliberately simplified sizing: stored energy taken proportional to heat capacity (first-order band) and required exchanger area inversely proportional to thermal conductivity (exact worst-case 1/(1−u) form). Real designs add flow regime, fouling, transients and safety factors.