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Compare these systems by the service you need, not by a single “battery efficiency” figure. Lithium-ion stores electricity and supplies electrical loads; sand batteries and water tanks store heat and are most useful when the eventual demand is heat. The right comparison depends on output temperature, power, storage duration, losses, system boundaries and project costs.
What does a sand battery store?
A sand battery is a thermal energy store, not an electrical battery. In Polar Night Energy’s system, electricity heats sand or similar solid material; later, the stored heat can be delivered as hot water, steam or hot air. The company specifies output temperatures up to 400°C for its system, making it relevant to district heating and some industrial heat uses. The delivered temperature depends on system design and application. Polar Night Energy’s product description presents this as commercial energy infrastructure, not a household battery category.
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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 later. Because heat is delivered directly, thermal storage can avoid converting electricity into another form and back when heat itself is the required service. DOE’s overview of thermal energy storage describes this role.
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Polar Night Energy reports that its Kankaanpää installation has 200 kW of heating power, 8 MWh of capacity and has operated since 2022. Its project reference page gives approximately 60–75% overall efficiency, while a 2024 company explainer gives 60–70% for the same installation. These are vendor-reported figures; they should not be treated as independently verified test results or merged into a single range.
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Polar Night Energy also reports that the Pornainen installation, commissioned in June 2025, has 1 MW of thermal power and 100 MWh of storage capacity and uses around 2,000 tonnes of crushed soapstone. Those are company-reported project specifications, not a general specification for all sand storage systems.
How do sand batteries compare with lithium-ion and water tanks?
| Comparison point | Sand thermal storage | Water-tank thermal storage | Lithium-ion |
|---|---|---|---|
| What is stored? | Heat in sand or similar solid material | Heat in water | Electrochemical energy for electrical output |
| Direct useful output | Heat; Polar Night Energy lists hot water, steam or hot air | Heat in water for heating or hot-water service | Electricity |
| Temperature information | Polar Night Energy specifies output up to 400°C for its system; actual delivery depends on design and application | IRENA’s 2020 broad technology category lists 10–90°C; this is not a specific tank specification | Not a heat-delivery technology; battery temperature is not the delivered service |
| Useful comparison axes | Required temperature, thermal capacity, charge/discharge power, duration, losses, footprint and process integration | Required temperature, tank volume, duration, losses, footprint and system integration | Electrical power, energy capacity, duration, efficiency, installed and lifecycle costs, service life, safety and replacement |
| Main caveat | Reported performance varies by scale and system boundary; available examples are vendor-reported | Broad category ranges are not product specifications | Compare with thermal options only after making the required output service explicit |
IRENA’s 2020 outlook lists water-tank thermal energy storage at 10–90°C and solid-state storage across a much broader −160–1,300°C range. These are broad technology-category ranges, not measurements of the commercial installations above or evidence that one option is more efficient or less costly. IRENA’s 2020 thermal storage outlook provides that categorization.
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Which system fits the load?
Choose an electrical storage comparison for electrical demand
If the job is to supply electricity to electrical loads, compare lithium-ion with other electrical storage options. A sand or water thermal store does not directly provide the same output. Turning stored heat back into electricity requires an additional conversion step, so the resulting electricity-to-electricity performance must include that step. DOE’s Energy Storage Grand Challenge workshop text discusses the distinction between stored energy forms and delivered services.
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Compare thermal stores for heat demand
If the required output is heat, compare thermal options against the actual heat duty. A high-temperature industrial process and a building hot-water loop may need different supply temperatures and integration designs. Establish the temperature and discharge rate the load needs, how long storage must cover it, and how the store connects to the heating network or process. Sand’s potential high-temperature output can matter for some industrial uses; a water tank is a different thermal-storage configuration and should be evaluated against its own design conditions.
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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
How should efficiency be compared?
“Round-trip efficiency” is meaningful only when its input and output are specified. For a thermal store charged with electricity, heat-out over electricity-in is a thermal round-trip measure. It is not equivalent to electricity-out over electricity-in, which would include the added conversion needed to generate electricity from heat.
Polar Night Energy’s figures illustrate why boundaries and attribution matter. For Kankaanpää, the company’s reference page reports approximately 60–75% overall efficiency, while its 2024 explainer reports 60–70% for the same plant. The 2024 explainer also estimates approximately 85% round-trip efficiency for a 2 MW system and 90% for a 10 MW system. These latter figures are company estimates, not independent measurements of the cited installations. Do not compare them directly with a lithium-ion figure unless the compared systems use the same input/output definition, operating conditions and system boundary.
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How should cost and project performance be assessed?
The available figures do not establish a like-for-like cost winner among sand storage, water tanks and lithium-ion systems. A credible project comparison needs proposals for the same location, duty cycle, output service, temperature and project horizon. A thermal system sized for heat delivery is not interchangeable with an electrical battery sized for electrical output.
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- Define whether the required output is electricity or heat, and specify the temperature if it is heat.
- Match power and stored energy to the required discharge rate and duration.
- Use the same efficiency boundary for every option; include the heat-to-electricity conversion if electricity is the final output.
- Compare system losses, footprint, integration requirements and the project’s operating schedule.
- Assess lifecycle costs over the same project horizon, including charging, service life, augmentation and replacement.
- Keep vendor-reported specifications and estimates distinct from independently verified performance.
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