Define required output
Enter the steady service requirement in MWth.
Set the support window
Enter the number of hours the system must sustain that output.
Apply a usable fraction
Reserve inaccessible capacity and operating headroom by entering the share of nominal energy available.
Enter recovery efficiency
Use the expected conversion from stored energy to useful delivered heat or cooling.
Review both ratings
Use the calculated MWhth as a preliminary energy size and the entered MWth as the minimum output rating.
Thermal Storage System Sizing Calculator
The Thermal Storage System Sizing Calculator converts a required output and service duration into a preliminary nominal energy capacity for a thermal storage system. It is built for facility engineers estimating the nominal storage capacity required for a heat or cooling duty. The calculation accounts for the share of nameplate capacity that can be used and the efficiency between stored energy and useful delivery.
The result separates the minimum output rating from the energy capacity: power determines how quickly energy must be delivered, while MWhth determines how long that output can continue. This is an early-stage sizing estimate, not an equipment design. Final selection should consider peak load shape, charging constraints, reserve policy, degradation or thermal losses, ambient conditions, redundancy, equipment increments, codes, and vendor performance guarantees.
Calculator inputs
MWth
Steady delivered output the system must support.
hours
Length of the target service window.
%
Share of nominal capacity allowed for the event.
%
Stored energy converted to useful delivered output.
Result
—
Required nominal energy capacity
Required delivered energy—
Capacity allowance for limits and losses—
Minimum output rating—
Required delivered energy = Required output × Duration
Nominal energy capacity = Required delivered energy ÷ Usable fraction ÷ Recovery efficiency
Percentages are converted to decimals. The result is in MWhth; the output requirement is in MWth. The model assumes a constant load and one average efficiency over the event.
What the result means
The nominal capacity is larger than delivered energy when operating limits and conversion losses are included. The system also needs an output rating at least equal to the entered load.
Add project-specific margins only once. If usable fraction already includes a reserve or degradation allowance, do not apply the same allowance again.
Given
A facility needs 9 MWth for 6 hours. The store can use 88% of nominal capacity and returns 90% of that energy to the load.
Calculation
Required delivered energy = 9 MWth × 6 hours = 54 MWhth. Nominal capacity = 54 ÷ 88% ÷ 90% = 68.18 MWhth.
Result
The preliminary system rating is at least 68.18 MWhth of nominal storage and 9 MWth of output.
Why is nominal capacity larger than required delivered energy?
Only part of nameplate capacity is usable, and some stored energy is lost before it reaches the load. Dividing by both percentages adds the required allowance.
Does this result also size charging equipment?
No. Charger size depends on available charging time, source limits, efficiency, and operating schedule.
Should I add a separate design margin?
Add one only when required by the project and not already embedded in the usable fraction. Document the reason to avoid double-counting.
Can I use an average load instead of peak load?
Only if the system truly serves a steady average. For a varying profile, size energy from interval demand and verify that the power rating covers the highest required output.
How do I convert a result to kWhth?
Multiply MWhth by 1,000 to obtain kWhth. Keep power units consistent when checking the required output rating.