The Untapped Potential of Embedded DC Solar in the London Underground
- Vikram Kumar
- May 22
- 3 min read
Dear Reader,
This is a work in progress. Please validate before use and feel free to challenge or improve it if you spot inaccuracies. This was written in coordination with Google Gemini and ChatGPT AI, built on my own research and technical analysis.
Summary:
London Underground consumes over 1.2 TWh of electricity annually most of it for traction power. Yet the opportunity to generate and deliver energy more efficiently through embedded DC solar remains largely unexplored. New FOI data confirms the scale. Now is the time to act.
Solar Where It's Needed Most
Much of the public debate around decarbonising transport focuses on vehicles themselves. But what powers the vehicles particularly large rail systems is equally important. In cities like London, the opportunity is especially clear.
Transport for London (TfL) recently confirmed through an FOI response that the London Underground alone used 1,215,978,706 kilowatt-hours (kWh) of electricity in 2022–2023 over 1.2 TWh. Conservative engineering estimates suggest that 80–85% of this is used for DC traction power, the electricity that drives the trains.
This electricity is delivered via approximately 300 traction substations, each converting 11–22kV AC grid input into a nominal 630V DC four-rail system. It’s a network purpose-built for DC. So why aren’t we feeding it with DC power directly?
Efficiency Gains Waiting to Be Captured
Solar PV and batteries generate direct current (DC). Yet nearly all UK solar today is grid-tied via inverters, feeding alternating current (AC) into the wider network. For a system like London Underground’s which ultimately requires DC this means energy is converted from DC to AC, then back again to DC, incurring 5–10% loss in the process.
If we integrate generation and storage on the DC side, we can bypass those losses. That’s 50 to 100 GWh/year of energy saved, worth £10–20 million annually at today’s electricity prices.
TfL Is Already Making Moves
TfL isn’t standing still. Plans are underway to install up to 64 MW of solar PV across depots, rooftops, and brownfield sites expected to deliver around 80 GWh per year, or 5% of the Tube’s electricity use. With additional investment and land access, this could rise to 10% (160 GWh/year).
At current market conditions:
10 MW of solar in London can save over £2 million per year
Capex per MW is falling below £1 million
3–5 year paybacks are now realistic without subsidy
Why This Is the Ideal Use Case
Unlike national rail or unpredictable commuter loads, the Underground operates with:
Fixed, centralised traction infrastructure
Predictable daily and seasonal energy demand
A legacy DC system with known parameters
It’s a near-perfect match for co-located solar, battery storage, and private-wire power delivery. The DC architecture is already in place the system simply hasn’t been asked to do this yet.
Integration Is Technical, Not Impossible
Yes, this is not a plug-and-play scenario. DC-side integration requires:
Proper voltage matching (630V DC nominal)
Safe DC-DC converter design
Coordination with regenerative braking systems
Local control and load balancing hardware
But these are solved problems in other sectors. Network Rail has already tested DC-side solar for third-rail systems. Industrial campuses and ports are integrating local storage and DC microgrids. The barrier here is institutional inertia, not technology.
The Broader Impact
Beyond cost savings, this strategy reduces peak grid load, stabilises local power supply, and lowers operational emissions all critical as TfL moves toward a net-zero target. It’s also a template for other metro systems across Europe, many of which share similar infrastructure characteristics.
Final Word
The FOI data confirms it: the electricity use is real, the infrastructure is ready, and the economics now work. The opportunity is not theoretical, it’s measurable, fundable, and scalable. For once, we’re not talking about future energy scenarios. We’re talking about an investment-grade energy upgrade hidden in plain sight.
London Underground could be the first major metro system in the world to fully integrate embedded DC renewables at scale. The only question now is who will move first and who will follow.
By Vikram Kumar, with technical contributions from Gemini AI and ChatGPT
Sources:
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