- Enter stored energy. Use the thermal energy available in the tank or storage medium, expressed in kWh.
- Set the discharge load. Enter the expected building or process thermal demand while storage supports the heat pump system.
- Account for delivery efficiency. Include expected heat-exchanger, piping, and conversion losses.
- Choose a reserve. Hold back part of nominal storage when operational limits or temperature requirements prevent full discharge.
- Review duration. The result shows how many hours the usable delivered energy can meet a constant load.
Heat Pump Storage Duration Estimator
This estimator calculates how long a thermal storage volume can support a heat pump load before the stored energy is depleted. It divides usable stored thermal energy by the expected heat demand and adjusts for delivery efficiency and any reserve that should remain unused.
The result is useful when evaluating buffer tanks, hot-water storage, chilled-water storage, or load-shifting designs. It can help determine whether storage can bridge a peak-price window, reduce compressor cycling, or maintain service during a planned operating interval.
The estimate assumes a steady discharge load. Real duration may differ when demand changes, tank temperature stratification is poor, or heat losses increase over time.
Storage and load inputs
kWh
kW
%
%
Result
—
Estimated storage support duration
Usable delivered energy—
Energy held in reserve—
Discharge load—
Usable delivered energy = Stored energy × (1 − Reserve %) × Delivery efficiency
Storage duration (hours) = Usable delivered energy (kWh) ÷ Discharge load (kW)
Percentages are converted to decimals. The calculation assumes constant load and average efficiency throughout discharge.
What the result means
The result is the estimated number of hours storage can supply the selected constant thermal load.
Standby losses and changing temperatures may reduce actual duration.
Given: Stored thermal energy = 180 kWh; discharge load = 30 kW; delivery efficiency = 88%; reserve = 10%.
Calculation: Energy after reserve = 180 × (1 − 0.10) = 162 kWh. Usable delivered energy = 162 × 0.88 = 142.56 kWh. Duration = 142.56 ÷ 30 = 4.752 hours.
Result: The storage can support the selected load for approximately 4.75 hours.
Does the duration stay the same if the load changes?
No. Duration is inversely proportional to load. If the thermal load doubles while usable energy stays constant, the estimated duration is cut in half.
What should be included in delivery efficiency?
Include expected losses between stored energy and useful energy delivered to the load. Avoid counting the same loss again if nominal stored energy is already stated on a usable basis.
Why keep a storage reserve?
A reserve can protect minimum tank temperature, preserve operating flexibility, or prevent operation outside equipment limits. Use the project control strategy to choose it.
Can this model represent a changing building load?
Not directly. Use an average load for a rough estimate, or perform an hourly simulation when peaks and load variation are important.
Is this the same as sizing the heat pump?
No. Storage duration describes how long stored energy can support a load. Equipment sizing determines the capacity needed to meet or recharge that load.