Roundtrip efficiency

Roundtrip efficiency is a key performance metric for an energy storage system (ESS) that characterizes the energy lost during a full charge–discharge cycle. It is defined as the ratio of the energy output from the system during discharge to the energy input supplied during charging. A higher round-trip efficiency indicates lower energy losses and operational costs.

The efficiency can be expressed as a percentage using the formula:

{{\text{Roundtrip Efficiency}}(\%)}={\frac {\text{Energy Out}}{\text{Energy In}}}\times 100

Round-trip efficiency greatly affects the economics of energy storage systems, particularly for applications in grid stability, renewable energy integration, and peak demand management.

Factors affecting efficiency

The round-trip efficiency of a storage system accounts for losses from multiple sources. These can include:

Conversion inefficiencies
Heat dissipation

For the green hydrogen and green ammonia the main factors are:

water electrolysis voltage required for production of hydrogen (the energy required for ammonia synthesis is relatively small). The hydrogen production energy linearly depends on the required voltage (that in turn depends on the catalyst used in anode and cathode);
efficiency of the power plant that burns the fuel (combined cycle gas turbine provides the highest efficiency of 64% assumed for high-end estimates).

Comparison of storage methods

Different energy storage technologies exhibit a wide range of round-trip efficiencies. The technology is often selected based on its intended application, such as providing power quality and distributed power or serving as bulk energy storage.

Comparison of Round-trip Efficiency for Energy Storage Methods
Storage Technology	Median Efficiency (%)	Efficiency Range (%)
Lead-acid battery	~75%	~60% – 90%
Li-ion battery	~90%	~85% – 95%
Sodium–sulfur battery		~60% – 90%
Flywheel	~92%	~85% – 95%
Supercapacitor		85% – 95%
Superconductive	~90%	~85% – 97%
Compressed air	~52%	~41% – 90%
Thermal	~40%	~30% – 50%
Pumped hydro	~75%	~65% – 85%
Redox flow		60% – 75%
Green hydrogen	~40%	28 – 52%
Green ammonia		23 – 42%

References

1 2 3 4 Penthia 2025, p. 290.
1 2 Kojima 2025, p. 2.
↑ Ma, Glatzmaier & Kutscher 2011.
↑ Ma, Glatzmaier & Kutscher 2011, p. 9, Figure 9.
↑ Kojima 2025, p. 5, Fig. 5.
↑ Nadeem et al. 2019, p. 4575, Table 9.
↑ Ma et al. (2011) claims 70%–95% without citation, but newer sources that are better cited, e.g. Nadeem et al. (2019), report 85%–95%.
↑ Sources vary significantly: Ma et al. (2011) indicates ~60%–70%, while newer data from Nadeem et al. (2019) reports 75%–90%.
↑ Nadeem et al. (2019) note a range of 41–75% for underground systems and 70–90% for overground systems.
↑ Includes conversion to electricity
↑ Nadeem et al. (2019) note 30–50% for low-temperature systems. The ~80% upper bound for high-temperature systems reported there likely represents thermal-to-thermal efficiency rather than full round-trip electricity conversion.
↑ Headley & Schoenung 2015, p. 3.
1 2 Kojima 2025, Abstract.

Sources

Headley, Alexander J.; Schoenung, Susan (2015). "Hydrogen Storage". U.S. DOE Energy Storage Handbook (PDF). Albuquerque, NM & Livermore, CA: Sandia National Laboratories. SAND2015-1002.
Kojima, Yoshitsugu (2025). "Round-trip efficiencies of green ammonia and green hydrogen". Next Energy. 8 100340. doi:10.1016/j.nxener.2025.100340.
Ma, Zhiwen; Glatzmaier, Greg C.; Kutscher, Charles F. (August 7–10, 2011). "Thermal Energy Storage and Its Potential Applications in Solar Thermal Power Plants and Electricity Storage". Proceedings of the ASME 2011 5th International Conference on Energy Sustainability & 9th Fuel Cell Science, Engineering and Technology Conference. Washington, DC: ASME. ESFuelCell2011-54077.
Nadeem, F.; Hussain, S. M. S.; Tiwari, P. K.; Goswami, A. K.; Ustun, T. S. (2019). "Comparative Review of Energy Storage Systems, Their Roles, and Impacts on Future Power Systems". IEEE Access. 7: 4555–4585. doi:10.1109/access.2018.2888497.
Penthia, Trilochan (2025). "Energy Storage Systems for Electrical Vehicle Chargers". In Kumar, A.; Bansal, R.C.; Kumar, P.; He, X. (eds.). Handbook on New Paradigms in Smart Charging for E-Mobility: Global Trends, Policies, and Practices. Elsevier. ISBN 978-0-323-95202-6. Retrieved 2025-10-17.

[FOOTNOTEPenthia2025290-1] 1 2 3 4 Penthia 2025, p. 290.

[FOOTNOTEKojima20252-2] 1 2 Kojima 2025, p. 2.

[FOOTNOTEMaGlatzmaierKutscher2011-3] Ma, Glatzmaier & Kutscher 2011.

[FOOTNOTEMaGlatzmaierKutscher20119Figure_9-4] Ma, Glatzmaier & Kutscher 2011, p. 9, Figure 9.

[FOOTNOTEKojima20255Fig._5-5] Kojima 2025, p. 5, Fig. 5.

[FOOTNOTENadeemHussainTiwariGoswami20194575Table_9-6] Nadeem et al. 2019, p. 4575, Table 9.

[7] Ma et al. (2011) claims 70%–95% without citation, but newer sources that are better cited, e.g. Nadeem et al. (2019), report 85%–95%.

[8] Sources vary significantly: Ma et al. (2011) indicates ~60%–70%, while newer data from Nadeem et al. (2019) reports 75%–90%.

[9] Nadeem et al. (2019) note a range of 41–75% for underground systems and 70–90% for overground systems.

[10] Includes conversion to electricity

[11] Nadeem et al. (2019) note 30–50% for low-temperature systems. The ~80% upper bound for high-temperature systems reported there likely represents thermal-to-thermal efficiency rather than full round-trip electricity conversion.

[FOOTNOTEHeadleySchoenung20153-12] Headley & Schoenung 2015, p. 3.

[FOOTNOTEKojima2025Abstract-13] 1 2 Kojima 2025, Abstract.

Roundtrip efficiency

Factors affecting efficiency

Comparison of storage methods

See also

References

Sources