High-temperature batteries: Engineering and performance

Why heat-keeping eats efficiency when the battery sits idle

What this tab covers Engineering and operating performance: round-trip efficiency, whether power and energy can be scaled separately, footprint and siting, and safety.

NaS is a compact, slow-discharging (6 to 8 hour) grid battery with efficiency in the 75 to 90% range, but heating losses penalise systems that sit idle. Fire risk is real but different in kind from lithium-ion.

Round-trip efficiency

Mixed

Published NaS efficiency figures range from 75 to 85% (EASE, 2016) to 85 to 90% (a Hungarian site report, 2024); the spread reflects whether heater energy is counted and how hard the battery is cycled. Heat matters more for ZEBRA: PNNL measured about 90% for the cells alone, 80.9% once heating and controls were included, and only 70.7 to 71.8% under peak-shaving cycles with long idle periods. Sandia's overview gives about 75% for NaS.⁴⁵⁶¹⁶

NaS efficiency 75-85% (2016) to 85-90% (2024)⁴⁵ ZEBRA overall, full cycle 80.9%⁶ ZEBRA overall, peak shaving 70.7-71.8%⁶ NaS efficiency (Sandia overview) About 75%¹⁶

Power and energy decoupling

Mixed

Power and energy are fixed by the cell, so they are not independently sizeable. NaS cells discharge slowly (C-rate about 1/6), which sets the duration at 6 to 8 hours. A container gives 250 kW and 1,450 kWh, about 5.8 hours at full power (calculated). Extra hours require more containers, so the marginal cost per kWh does not fall with duration as it does for flow batteries.²³

Container rating 250 kW / 1,450 kWh² Discharge duration 6-8 h² Cell C-rate About 1/6³

Footprint and siting flexibility

Favourable

A 20-foot container holding 1.45 MWh weighs 21 t and occupies 15 m2, about 97 kWh per m2 (calculated), which is dense for grid storage. The thermal enclosure lets units run in ambient temperatures from -45 to +55 C without air conditioning, which suits deserts and cold climates. Japan's 50 MW / 300 MWh Buzen plant uses 252 containers on a site of about 100 m by 140 m.¹²³

Container mass and footprint 21 t, 15 m2² Ambient operating range -45 C to +55 C¹ Buzen plant 50 MW / 300 MWh, 252 containers³

Safety and thermal runaway risk

Mixed

Molten sodium burns and reacts with water, so firefighting with water is not straightforward. A 2 MW NaS installation at a Mitsubishi Materials plant in Tsukuba caught fire on 21 Sep 2011. BASF says NaS operations were suspended until June 2012, when an investigation identified the cause, and EASE says the whole installed base was then taken offline in 2012 and 2013 for upgrades such as fuses, insulation boards and fire prevention and firefighting equipment. The current product's cells and modules are recognised components under UL 1973 and have been evaluated under UL 9540A, the test method that US fire codes such as NFPA 855 rely on.²³⁴¹¹

Tsukuba fire 21 Sep 2011, 2 MW system¹¹ NaS operations suspended until June 2012³

Sources

Japan's NGK discontinues manufacturing of sodium-sulfur batteries (5 Nov 2025) pv magazine / ESS News, 2025
NAS Battery: product sheet (container specifications and references), dated 02/2024 BASF Stationary Energy Storage, 2024
Sodium Sulfur Batteries NAS for Large Stationary Storage Applications: The time is right! (F. Dötz, NAATBatt Sodium Zinc Battery Workshop, Houston, 30 Nov 2023) BASF Stationary Energy Storage / NAATBatt, 2023
Energy storage technology description: Sodium-Sulphur (NaS) battery European Association for Storage of Energy (EASE), 2016
StoreMore: Best practice site visit Csillebérc, Hungary (NaS battery, site visit 9 Dec 2024) Interreg Danube StoreMore project, 2024
Evaluating ZEBRA battery module under the peak-shaving duty cycles (Shamim et al., Materials 14(9) 2280) Pacific Northwest National Laboratory, 2021
Failure Event: Japan, Ibaraki Prefecture, 21 Sep 2011 EPRI BESS Failure Incident Database (Storage Wiki), 2024
Energy Storage Overview (H. Passell, NECPUC Energy Storage Webinar Series, 19 Mar 2021) Sandia National Laboratories, 2021