Thermal Energy Storage Calculator
Thermal energy storage holds energy as heat in a material -- most commonly molten salt at concentrated solar power (CSP) plants -- so generation can continue long after the energy source is gone. This calculator takes the storage medium (which auto-fills a representative specific heat), the storage mass in metric tons, and the temperature differential between the hot and cold tanks, then reports the thermal energy stored in both gigajoules (GJ) and megawatt-hours (MWh). It pairs naturally with our planned LDES Technology Comparison Calculator for sizing thermal storage against flow batteries, pumped hydro, and CAES, and our planned Levelized Cost of Storage vs. Duration Calculator for the economics side of the same decision.
Selecting a medium auto-fills the specific heat below (still editable). Molten salt (solar salt) is the standard for high-temperature CSP storage; water is common for lower-temperature district heating and process-heat applications.
Molten salt (solar salt) is about 1.5 kJ/kg-K; water is about 4.186 kJ/kg-K. Use a temperature-dependent value where available.
Large concentrated solar power (CSP) plants with molten salt storage commonly hold tens of thousands of metric tons of salt inventory.
The temperature swing between the "hot" and "cold" storage tanks. Solar salt CSP systems commonly operate with a differential in this range, e.g., roughly 565°C hot / 290°C cold.
(storage mass (metric tons) × 1,000 × specific heat (kJ/kg-K) × temperature differential (°C)) ÷ 1,000,000
thermal energy stored (GJ) × 0.2778
This calculates thermal energy stored, not electrical energy. Converting stored heat back to electricity (for CSP applications) involves additional conversion losses through a steam turbine or other power block, typically reducing the usable electrical output below the thermal energy figure shown here.
Results update live as you type. For planning and field-check estimates — always verify against applicable standards and equipment ratings.
How we calculate this →Thermal energy storage scales with a simple formula: mass, times specific heat, times temperature swing. A 30,000 metric ton molten salt inventory cycling through a 200°C temperature differential -- realistic for a large concentrated solar power (CSP) plant -- stores about 2,500 MWh of thermal energy, enough to keep a CSP plant's turbines running for many hours after the sun sets. This is exactly why molten salt storage has become the standard approach for extending CSP plants beyond daylight hours, offering a simpler and often cheaper storage mechanism than converting to and from electricity with batteries.
How thermal energy storage is calculated
This calculator estimates the thermal energy stored in a sensible-heat storage medium, given the storage mass, the medium's specific heat capacity, and the temperature differential between the hot and cold tanks. Two quantities tie the calculation together.
Thermal Energy Stored (GJ) = (Storage Mass (metric tons) × 1,000 × Specific Heat (kJ/kg-K) × Temperature Differential (°C)) ÷ 1,000,000. The core physics is that the heat held by a material equals its mass times its specific heat capacity times the temperature change it undergoes. Storage mass is given in metric tons, so multiplying by 1,000 converts it to kilograms; multiplying by specific heat (in kilojoules per kilogram per kelvin) and by the temperature differential (in °C, equivalent to kelvin for a difference) gives the energy in kilojoules; dividing by 1,000,000 converts kilojoules to gigajoules. At the defaults (30,000 metric tons, 1.5 kJ/kg-K, 200°C), that is (30,000 × 1,000 × 1.5 × 200) ÷ 1,000,000 = 9,000.0 GJ.
Thermal Energy Stored (MWh) = Thermal Energy Stored (GJ) × 0.2778. One gigajoule equals 1,000 megajoules, and one megawatt-hour equals 3,600 megajoules, so one gigajoule equals 1,000 ÷ 3,600 = 0.2778 MWh; multiplying the GJ figure by 0.2778 converts it to megawatt-hours for easier comparison with electrical energy figures. At the defaults, that is 9,000 × 0.2778 = 2,500.2 MWh.
Two notes on the model. First, the specific heat figure is the single most consequential input after mass and temperature swing, and it depends on the storage medium: molten salt (solar salt, a 60/40 sodium/potassium nitrate mixture) is about 1.5 kJ/kg-K, while water is about 4.186 kJ/kg-K -- so the storage-medium dropdown auto-fills a representative value that you can still override with a project-specific or temperature-dependent figure. Second, this calculator measures thermal energy (heat) stored, not electrical energy: converting that heat back to electricity through a steam turbine or other power block introduces additional conversion losses (typically 35-42% for a Rankine-cycle CSP power block), so usable electrical output is always lower than the thermal energy figure shown here. Data sources: Sensible-heat storage physics and specific heat values from NREL (National Renewable Energy Laboratory), DOE (Department of Energy), and IRENA (International Renewable Energy Agency) energy storage technology assessments; molten salt (solar salt) properties and CSP operating temperatures from NREL and Sandia National Laboratories thermal storage research; GJ-to-MWh conversion using the standard 0.2778 MWh/GJ factor (1 GJ = 1,000 MJ; 1 MWh = 3,600 MJ). Verification: with defaults (Molten Salt, 30,000 metric tons, 1.5 kJ/kg-K, 200°C), Thermal Energy Stored = 9,000.0 GJ, Thermal Energy Stored = 2,500.2 MWh.