Railway electrification and wireless power: Efficiency and losses
Recycling braking energy and charging cars through the road
What this tab covers How much energy is lost while it moves and while it waits in storage or transit, and how much energy the transport itself consumes, in per cent.Electric rail is among the most efficient ways to move people and goods: rail carries 8% of the world's passengers and 7% of its freight with only 2% of transport energy (IEA, 2019). The largest controllable losses are braking energy that is not reused; wireless charging reaches 96% in the best tests, while laser beaming manages only about 20%.
Chart: Efficiency ranges
Lost in transit Used to drive transportLosses by measure
| Measure | Range | Type | What is measured |
|---|---|---|---|
| Regenerative braking energy actually reused, DC metro | 80.6 to 95.5%4 | Lost in transit | Share of braking energy recovered and used; 80.6% with normal operation, 95.5% with system-optimised timetabling (case study) |
| Substation energy saved by optimised operation | 38.6%4 | Used to drive transport | Reduction in energy drawn from substations in a modelled DC metro case study |
| Driver advisory with coasting control | 16%4 | Used to drive transport | Traction energy saved against normal driving, field test on Beijing Yizhuang subway line |
| Static wireless charging, 100 kW | 96%5 | Lost in transit | Grid-to-battery transfer efficiency in an ORNL vehicle test, about 13 cm gap |
| Laser power beaming | 20%8 | Lost in transit | Over 20% from laser output to receiver electrical output at shorter distances (DARPA POWER tests; efficiency was not the test's aim) |
Holding periods and inventory
| Measure | Value |
|---|---|
| Battery swap time | About five minutes per swap9 |
| Laser record transmission | 30 seconds; more than a megajoule moved across the whole test campaign8 |
In metros the trains stop and start often, so braking energy is a large part of the energy budget. If no other train is accelerating nearby when one brakes, the regenerated power has nowhere to go and is burned off in resistors. Timetables that line up braking and accelerating trains, stationary batteries and 'reversible' substations that can send power back to the grid all raise the share recovered.
Battery swapping adds the normal charging and battery losses of any electric vehicle, but no extra transmission step; its gain is speed rather than efficiency.
Sources
- System energy optimisation strategies for DC railway traction power networks (PhD thesis) University of Birmingham, 2017
- Novel technology reaches 100-kW wireless power transfer for passenger vehicle Oak Ridge National Laboratory, 2024
- DARPA program sets distance record for power beaming DARPA, 2025
- China is betting on battery swaps to tackle EV chargers shortage Rest of World, 2024