Material Insights · Understanding the Material World

Capacitors and supercapacitors: Engineering and performance

Superb for seconds, hopeless for hours

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

Supercapacitors are very efficient and almost instant, but energy and power come from the same cells, so they only make sense for discharges of seconds to a minute or two.

Round-trip efficiency

Favourable

DOE assumes 92% round-trip efficiency for an EDLC grid system, comparable to or better than lithium-ion, because there is no chemical conversion loss. The losses that matter are resistive heating at very high currents and self-discharge if energy sits idle.3

Round-trip efficiency (EDLC system) 92%3

Power and energy decoupling

Unfavourable

There is no decoupling: every extra kWh means more cells. DOE's reference plant is 1 MW for 45 seconds (0.0125 h), and the storage block alone costs about USD 19,200 per kWh, derived from USD 240 per kW. The economics therefore depend on buying power, not energy. Skeleton's hybrid SuperBattery stretches useful duration to roughly 1 to 45 minutes.23

DOE reference duration 45 seconds (1 MW)3 Storage block cost USD 19,200/kWh (USD 240/kW)3 SuperBattery duration range about 1 to 45 minutes2

Footprint and siting flexibility

Favourable

Power density is extremely high, around 28 to 36 kW per kg for a single Skeleton cell, so a large power rating fits in a small space and cells work from -40 C to +65 C. Energy per litre is low (about 20 Wh/L for Musashi prismatic lithium-ion capacitors, a company figure). At grid scale they are usually added alongside batteries: the 500 MW / 1,000 MWh Jiayuguan NingSheng project in Gansu, China, backed by China National Nuclear Corporation and grid-connected on 30 December 2025, includes a 25 MW supercapacitor section with a 60-second discharge.145

Cell specific power (Skeleton) 28.4 to 36.5 kW/kg1 Operating temperature -40 C to +65 C1 Jiayuguan supercapacitor section 25 MW, 60 s, within 500 MW / 1,000 MWh5

Safety and thermal runaway risk

Favourable

EDLCs have no stored chemical reaction to run away, so fire risk is much lower than lithium-ion. The acetonitrile electrolyte used in many cells still burns, and in a fire it can release hydrogen cyanide, so ventilation and emergency planning matter. Musashi's lithium-ion capacitors are certified to UL 810A and marketed as free of thermal runaway (company claim). In Skeleton's own abuse tests a fully charged cell heated to 250 C, overcharged to 6 V or crushed did not catch fire or explode; its vent opened at 135 C. Grid installations still fall under energy storage fire codes such as NFPA 855.249

Cell abuse test (Skeleton, company data) no fire at 250 C or 200% overvoltage2 Certification (Musashi HSC) UL 810A4 Acetonitrile combustion product hydrogen cyanide9

Sources

  1. SCF3400 and SCX5000 D60 supercapacitor cell datasheet Skeleton Technologies, 2026
  2. Skeleton Technologies company introduction 2026 (company presentation, hosted by Energy Saxony) Skeleton Technologies, 2026
  3. Technology Strategy Assessment: Supercapacitors (Energy Storage Grand Challenge, #10) US Department of Energy, Office of Electricity, 2023
  4. Hybrid supercapacitors: solution to the power-energy trade-off (trade press; manufacturer figures as reported) Best Magazine, 2026
  5. China switches on world's first GWh-scale supercapacitor-energy storage project (trade press, based on ESS News) pv magazine, 2026
  6. CAMEO Chemicals datasheet: Acetonitrile US National Oceanic and Atmospheric Administration (NOAA), 2024