Drawing No. EH–TH–041 // Thermal Engineering & HVAC
Thermal Storage Sizing Calculator
Reviewed August 2026
Find the ice mass or chilled-water volume needed to store a target cooling capacity for a given duration, and compare the two storage media side by side.
What problem does this solve?
Thermal storage lets a cooling plant run at a different time than when cooling is needed — building ice or chilling water overnight (often at cheaper off-peak electricity rates) to offset daytime peak cooling load. This calculator sizes either an ice or chilled-water storage system for a target discharge capacity and duration.
Inputs
Results
Background
Energy [kWh] = cooling load [kW] × discharge duration [h]. This is the same regardless of which storage medium is chosen — it's simply the total cooling capacity that must be available in storage.
Ice mass [kg] = energy [kJ] / latent heat of fusion (≈334 kJ/kg). Ice storage is compact because latent heat of fusion is large compared to the sensible heat of an equivalent temperature swing in water — melting 1 kg of ice absorbs roughly the same heat as cooling about 20 kg of water through an 8°C swing.
Water mass [kg] = energy [kJ] / (cp,water × temperature swing), with cp,water ≈ 4.186 kJ/kg·K. Chilled-water storage needs a much larger tank volume than ice for the same energy capacity, but is mechanically simpler (no ice-building coils) and can use standard chiller equipment without special low-temperature operation.
Frequently asked questions
Practical questions about inputs, assumptions and interpretation.
Latent heat of fusion (334 kJ/kg) is far larger than the sensible heat available from a typical chilled-water temperature swing (4.186 kJ/kg·K × 8 K ≈ 33 kJ/kg) — so ice storage packs roughly 10× more cooling capacity into the same mass, which is why ice storage tanks are much smaller than chilled-water tanks for the same capacity, an important factor where space is limited.
Chilled-water storage uses standard chiller equipment operating at normal temperatures, avoiding the more complex and less efficient low-temperature operation ice-building requires (chillers making ice must run colder, which lowers their COP). It's also simpler mechanically, with no ice-building coils or glycol/brine secondary loops needed.
No — this is an idealized capacity calculation. Real systems have some standby heat gain into the storage tank, incomplete ice melt-out or water stratification loss at the end of a discharge cycle, and distribution losses, all of which mean a real system needs some margin above this calculated minimum size.