Material Insights · Understanding the Material World

Electrical loss reduction: Science and thermodynamics

Why tiny percentages of loss add up to huge totals

What this tab covers The thermodynamics: the grade (temperature or pressure) of the energy being recovered, energy versus exergy efficiency, and how harsh the waste stream is on equipment.

Electrical losses are small as a percentage but huge in total, because motors and transformers handle most of the world's electricity. Most of the value lies in avoiding the loss; reusing the low-grade heat it creates usually needs a heat pump.

Grade of energy

Mixed

Losses in transformers and motors end up as low-grade heat in oil, windings and cooling air. A UK study found transformer oil leaving at only about 33 to 43 C at 40 to 60% load, so the heat needs a heat pump to feed a district heating network at 75 C. Reactive power and harmonics are different. They are not lost energy as such, but they carry extra current that heats cables and transformers. Regenerative inverters deal with the highest-grade stream, braking energy in a motor drive, which goes straight back into grid-quality electricity. Electric motor systems used about 53% of global electricity, 13,462 TWh in 2023. US transmission and distribution losses averaged about 5% in 2018 to 2022.1512

Transformer oil outlet temperature about 33-43 C at 40-60% load12 Motor systems' share of global electricity 53% (13,462 TWh, 2023)1 US T&D losses about 5% (2018-2022)5

Exergy versus energy efficiency

Mixed

Transformers are typically more than 99% efficient. ABB's 2,500 kVA example loses 2.78 kW with no load and 23.7 kW at full load, about 98.95% efficiency at full load and unity power factor (calculated). The UK study models losses at about 0.7% of rating at full load (calculated from its loss formula). Heat quantity is small and exergy is low, but heat pumps lift it efficiently: the study's transformer heat pump reached a seasonal COP (heat out per unit of electricity in) of 3.40, against 2.68 for air-source heat pumps. Motor losses split between stator copper (25 to 40% in older standard motors), rotor copper, iron core, friction and stray losses. Regenerative drives return high-exergy electricity. Otis and Danfoss quote harmonic current distortion of about 5% for their regenerative units (vendor claims).2361214

2,500 kVA transformer losses (vendor case) 2.782 kW no-load, 23.682 kW load3 Transformer heat pump seasonal COP 3.40 (air-source 2.68)12 Stator I2R share of motor losses 25-40% (older standard motors)2 Regenerative drive THD (vendor) 5% or less614

Media compatibility and corrosiveness

Favourable

The media are clean: insulating oil, air and electricity. The engineering risks are electrical. A poorly applied passive filter or capacitor bank can resonate with the network and amplify harmonics. Passive filters suit stable loads with known harmonics. Active filters, which inject cancelling current in real time, suit variable loads such as drives. Compliance is judged against IEEE 519 at the point where the plant connects to the utility. Transformer heat recovery needs an oil-to-water exchanger that cannot leak water into the transformer.11

Variants compared34679101112131421

VariantWhat it recovers or avoidsTypical gainEvidence in this pass
Power factor correctionReactive current; tariff penaltieskVAh billing for HT consumers since 2020 makes low PF costly (Maharashtra); earlier penalty below 0.90MSEDCL and MERC documents
Harmonic filtersHarmonic heating, trips, resonanceNot quantifiedEC&M (qualitative)
Low-loss transformersCore and winding lossesNo-load loss 2.78 to 1.75 kW in ABB exampleABB, US DOE
Transformer heat recovery with heat pumpWarm oil at 33-43 C56-81% of available heat reused (modelled UK case)LSBU study, UKPN Full Circle
Regenerative and bidirectional invertersBraking energy from drivesLifts up to 75% (vendor); metros 99% of recoverableOtis, Alstom, Danfoss

Sources

  1. Electric motor systems: why are they important? Energy Efficiency Policy Brief EMSA 9 IEA 4E Electric Motor Systems Annex (EMSA), 2025
  2. Premium Efficiency Motor Selection and Application Guide: A Handbook for Industry US Department of Energy, Advanced Manufacturing Office, 2014
  3. Transforming revenue: ABB's technology cuts transformer losses (ABB Review 2/2015; vendor case) ABB, 2015
  4. DOE finalizes energy efficiency standards for distribution transformers (press release, 4 April 2024) US Department of Energy, 2024
  5. How much electricity is lost in electricity transmission and distribution in the United States? US Energy Information Administration, 2023
  6. ReGen drive (product brochure, UK; vendor claims) Otis
  7. Capturing wasted energy in our cities (Hesop reversible substations; company press release) Alstom, 2025
  8. kVAh billing: frequently asked questions Maharashtra State Electricity Distribution Co. (MSEDCL), 2019
  9. Methodology for power factor penalty and incentive (MERC order in Case 195 of 2017) Maharashtra State Electricity Distribution Co. (MSEDCL), 2019
  10. Harmonics: what electrical contractors need to know, Part 2 (R. Ignall) EC&M, 2026
  11. Heat recovery opportunities from electrical substation transformers (Lagoeiro, Davies, Marques and Maidment), Energy Reports 10, 2931-2943 (open access) Elsevier, 2023
  12. Full Circle (transformer heat recovery project) UK Power Networks, Strategic Innovation Fund, 2024
  13. VLT Active Front End drive (product sheet; vendor claims) Danfoss, 2014
  14. Commercial Circular No. 323: implementation of MERC MYT order in Case 322 of 2019 (kVAh billing) Maharashtra State Electricity Distribution Co. (MSEDCL), 2020