Underground, submarine and superconducting cables: Efficiency and losses
Low losses, but weeks of downtime when something breaks
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.A long DC subsea cable loses roughly 2.6 to 3.1% of its power per 1,000 km at full load (calculated), plus about 1% at each converter station. Cables hold no stock of energy, so the main 'holding' cost is downtime: a damaged cable can be out for weeks to months.
Chart: Efficiency ranges
Lost in transitLosses by measure
| Measure | Range | Type | What is measured |
|---|---|---|---|
| DC cable conductor loss, 2,000 MW at ±525 kV, 2,500 mm² copper | 2.6 to 3.1%2 | Lost in transit | Share of power lost per 1,000 km at full load (calculated from the IET cable design and copper resistance at 20 to 70 C) |
| HVDC converter station loss | 1%7 | Lost in transit | Per converter station; down from about 3% before 2000 |
| Superconducting cable heat leak, Shenzhen 400 m, 10 kV | 2 W per metre9 | Used to drive transport | Heat entering the cryostat that refrigeration must remove; total cooling duty about 7 kW at 70 K (minus 203 C) |
| AC cable length limit | 50 km7 | Theoretical limit | Beyond about this length most AC current is charging current, not useful power |
Holding periods and inventory
| Measure | Value |
|---|---|
| Energy held in the cable | None in practice: power passes through in milliseconds, so losses occur only while current flows7 |
| Underground 400 kV cable fault | Two to six weeks to find and repair; on average out of service 25 times longer than a 400 kV overhead line3 |
| Offshore wind cable fault | About 40 days (array cables) to 60 days (export cables); cable failures make up about 80% of offshore wind insurance claims6 |
| Estlink 2 after anchor damage, 25 December 2024 | 650 MW unavailable for about six months; about 1 km of cable was replaced and the link returned to the market on 20 June 20251314 |
The conductor loss figure is a simple calculation, not a measurement: two poles each carrying about 1,900 A through a 2,500 mm² copper core give about 52 to 62 kW of heat per km, or about 2.6 to 3.1% of 2,000 MW per 1,000 km (calculated). Real links run below full load much of the time, and loss falls with the square of current, so annual average losses are lower. AC cables add losses in the insulation and metal sheath on top of the charging current.
A conventional cable needs almost no energy to run. A superconducting cable has no resistive loss in the conductor but must be refrigerated continuously, and that cooling power is its main own use of energy.
Sources
- Electricity transmission costs and characteristics: final full report IET / Mott MacDonald, 2025
- Undergrounding high voltage electricity transmission lines: the technical issues (Issue 4) National Grid, 2015
- 80% of insurance claims in offshore wind are related to subsea cable failures DNV, 2024
- HVDC transmission (presentation by L. Tang, HVDC 2013) US Department of Energy, 2013
- Development and demonstration of concentric-type HTS power cable for distribution grid in Shenzhen urban area Frontiers in Energy Efficiency, 2023
- Smooth repair work brings EstLink 2 connection back to market already on Friday Elering, 2025
- Estlink 2 electricity transmission link returns to commercial use Fingrid, 2025