- 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 geothermal 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.
Geothermal Heat Pump Storage Duration Estimator
This estimator calculates the discharge duration of thermal storage connected to a geothermal heat pump system. It converts usable stored thermal energy into hours of support at a selected load, after accounting for delivery efficiency and a required reserve.
The result can help size buffer tanks, evaluate peak-load support, and determine whether stored heating or cooling can cover a demand-response interval. It also makes it easier to compare storage capacity with the actual thermal load rather than the heat pump's nameplate rating alone.
The model assumes a constant discharge rate and does not simulate changing water temperature, stratification, loop interaction, or standby losses. Detailed designs may require an hourly thermal model.
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 geothermal 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.