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A sand battery and a water tank store heat; a lithium-ion battery stores energy for electrical output. The right comparison depends first on what you need later: electricity, hot water, steam, hot air, or another heat supply. Comparing all three by one efficiency figure obscures that fundamental difference.
What does a sand battery store?
A sand battery is a thermal energy storage system, not a battery that supplies electricity to ordinary electrical loads. It uses electricity to heat sand or a similar solid material, then delivers the stored energy as heat. Polar Night Energy lists hot water, steam, and hot air as possible outputs and specifies output temperatures up to 400°C for its system. The actual delivery temperature depends on system design and application. Polar Night Energy’s Sand Battery page describes the commercial system.
The U.S. Department of Energy defines thermal energy storage as energy held in a material as a heat source or cold sink for use at a later time. Thermal storage can deliver heat directly when heat is the required service, avoiding the extra conversions involved in turning electricity into heat and then back into electricity. The DOE overview of thermal energy storage explains this role.
How do the three storage options differ?
| Comparison | Sand thermal storage | Water-tank thermal storage | Lithium-ion battery |
|---|---|---|---|
| What is stored? | Heat in sand or similar solid material | Heat in water | Electrochemical energy for electrical output |
| Direct useful output | Heat, potentially delivered as hot water, steam, or hot air, depending on system design | Hot water or heat for a heating system, depending on tank and system design | Electricity |
| Temperature information | Polar Night Energy specifies output up to 400°C for its system; this is a vendor specification | IRENA’s 2020 overview gives a broad 10–90°C range for water-tank thermal storage, not a specification for a particular tank | Battery operating temperature is not a delivered heat service |
| Useful comparison measures | Required temperature, thermal capacity, charge and discharge power, storage duration, heat losses, footprint, and process integration | Required temperature, tank volume, storage duration, heat losses, footprint, and integration with the heating system | Electrical power and energy capacity, duration, efficiency, installed and lifecycle costs, service life, safety, and replacement needs |
| Key limitation when comparing | Reported performance depends on the project and the system boundary | Specifications vary by tank and system; a broad category range is not a product comparison | It supplies electricity rather than heat directly, so compare it with thermal storage only when the required service is explicit |
IRENA’s 2020 outlook lists water-tank storage at 10–90°C and solid-state storage across a broad −160–1,300°C category range. These are technology-category ranges, not direct measurements of the commercial installations discussed here and not evidence that one option is more efficient or less expensive. IRENA’s 2020 Innovation Outlook: Thermal Energy Storage provides the ranges.
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What do reported sand-battery projects show?
Polar Night Energy describes its Kankaanpää, Finland, installation as a commercial district-heating system with 200 kW of heating power and 8 MWh of capacity, operating since 2022. Its project page reports approximately 60–75% overall efficiency, while the company’s 2024 explainer gives 60–70% for the same plant. These are the company’s reported figures; the sources do not provide an independent operational test report that reconciles the difference. The Kankaanpää project page and the 2024 explainer give the respective values.
Polar Night Energy also reports that its Pornainen installation was commissioned in June 2025 with 1 MW of thermal power, 100 MWh of storage capacity, and around 2,000 tonnes of crushed soapstone. These are company-reported project specifications, not independent measurements. The same company explainer estimates approximately 85% round-trip efficiency for a 2 MW system and 90% for a 10 MW system. Those estimates are not results reported for the Kankaanpää or Pornainen installations and should not be treated as independently verified performance. Polar Night Energy’s explainer reports these figures.
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How should efficiency be compared?
Always name the input, output, and system boundary behind an efficiency figure. For a thermal store, a figure may mean useful heat delivered divided by electricity used to charge it. For an electrical battery, round-trip efficiency ordinarily concerns electricity delivered divided by electricity used to charge it. Those outputs are different services, so the percentages do not make the technologies equivalent.
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- Performs in extreme temperatures from -40 degrees F to 140 degrees F to ensure reliable use in all seasons
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- Lightweight household batteries last up to 25 years in storage
Which system fits a heat load, and which fits an electrical load?
Choose by the service you need
- For electrical loads: evaluate electrical storage such as lithium-ion, specifying the power, energy capacity, duration, and operating requirements the loads need.
- For lower-temperature heating or hot water: assess a water tank against the required delivery temperature, volume, duration, losses, and heating-system integration.
- For higher-temperature heat, steam, or industrial or district-heating integration: a sand-based thermal store may be relevant if its temperature, power, capacity, and process interfaces suit the load.
Compare proposals on a common basis
For a real project, ask every supplier to model the same location, duty cycle, charge source, delivery temperature, output profile, storage duration, and project horizon. Request separate figures for thermal output and electrical output rather than a single unqualified efficiency percentage. Include charging costs, duration, service life, augmentation, replacement, and power equipment in lifecycle-cost comparisons. DOE’s 2022 Grid Energy Storage Technology Cost and Performance Assessment describes these as relevant lifecycle-cost inputs.
No comparable installed-cost figure or like-for-like lifetime dataset is established for these three options here, so a universal cost or efficiency winner cannot be named. Sand storage is commercial energy infrastructure rather than a consumer battery category; the cited project specifications describe district-heating-scale installations, not household substitutes for lithium-ion batteries.
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