How do you size thermal storage in kWh?

Multiply the average heat load in kW by the required hold duration in hours, then add a loss and reserve allowance. At 750 kW for 4 hours with a 15% allowance, the base 3,000 kWh becomes a sized requirement once the allowance is applied.

Why add a loss and reserve allowance?

Stores leak heat through insulation and you should never plan to fully discharge them. The allowance covers standing losses over the hold period plus a usable reserve so the store still delivers at its low end.

What duration should I use?

Match it to the gap you need to bridge — the longest off-peak-to-peak window, the defrost recovery time, or a maintenance interval. Sizing for the longest realistic no-run window gives the safe capacity.

Should I size on average or peak load?

Average load gives the energy capacity, which is what this tool returns. But check the peak draw separately — if the process can pull well above average for a stretch, verify the store and its discharge rate can keep up at that instant.

Does this account for the storage medium?

No — it gives the energy requirement in kWh. Converting to a physical size depends on the medium: water storage uses the usable temperature swing, while phase-change material uses its latent heat. Size the vessel from this kWh figure and the medium's properties.

How does storage help with electricity cost?

By storing heat made during cheap or low-carbon hours, the heat pump can avoid running during expensive peaks. The store must hold enough kWh to cover the full peak window, which is exactly what the duration input captures.

Industrial Heat Pumps & Electrified Thermal Systems calculator

Process Heat Thermal Storage Sizing Calculator

Thermal storage sizing determines how many kWh of stored heat a buffer tank, hot-water store, or phase-change vessel must hold to carry a process through a defined window without the heat pump running. Engineers electrifying process heat use it to decouple the heat pump from instantaneous demand — letting it run during cheap off-peak power, ride through defrost cycles, or shave peaks. Because real stores lose heat and you never want to drain them to empty, a loss and reserve allowance is added on top of the bare energy requirement. Getting this number right is the difference between a buffer that smooths operation and one that runs dry mid-process.

What this calculator does

Estimate usable thermal storage capacity for process heat buffering from average load, required coverage time, and loss or reserve allowance.
Use it when a process engineer or energy manager is sizing hot water tanks, buffer vessels, or thermal batteries for peak shaving, batch heating, or heat recovery smoothing.
It computes required thermal storage capacity in kWh by multiplying average heat load by storage duration, then scaling up by the loss and reserve allowance.

Formula used

Base thermal storage energy = average process heat load × required storage duration
Required thermal storage capacity = base thermal storage energy × storage loss and reserve allowance

Inputs explained

Average process heat load:
Required storage duration:
Storage loss and reserve allowance:

How to use the result

Use it when sizing a buffer or thermal store to shift heat pump operation to off-peak hours, bridge defrost or maintenance gaps, or cap peak electrical demand.
It assumes a steady average load; a process with sharp peaks may briefly draw more than the average and can underflow a store sized purely on the mean.

Current U.S. benchmarks

Global copper trades at $13,484 per tonne (IMF via FRED, May 2026), up 41.5% in a year, and U.S. industrial electricity averages 8.66 cents per kWh. Both feed electrified-hardware unit economics.
The U.S. has 21,668 machinery manufacturing establishments employing about 1,086,146 workers (Census County Business Patterns, 2023).

Common questions

How do you size thermal storage in kWh? Multiply the average heat load in kW by the required hold duration in hours, then add a loss and reserve allowance. At 750 kW for 4 hours with a 15% allowance, the base 3,000 kWh becomes a sized requirement once the allowance is applied.
Why add a loss and reserve allowance? Stores leak heat through insulation and you should never plan to fully discharge them. The allowance covers standing losses over the hold period plus a usable reserve so the store still delivers at its low end.
What duration should I use? Match it to the gap you need to bridge — the longest off-peak-to-peak window, the defrost recovery time, or a maintenance interval. Sizing for the longest realistic no-run window gives the safe capacity.
Should I size on average or peak load? Average load gives the energy capacity, which is what this tool returns. But check the peak draw separately — if the process can pull well above average for a stretch, verify the store and its discharge rate can keep up at that instant.
Does this account for the storage medium? No — it gives the energy requirement in kWh. Converting to a physical size depends on the medium: water storage uses the usable temperature swing, while phase-change material uses its latent heat. Size the vessel from this kWh figure and the medium's properties.

Last reviewed 2026-05-12.