The Standards Stack Behind the Smart Grid
OCPP, OCPI, ISO 15118, OpenADR and HEMS don't compete — each covers one interface in the chain between a vehicle and a DNO's control room.
Most people working in and around EV charging and flexibility encounter OCPP, OCPI, ISO 15118, OpenADR and HEMS as a wall of acronyms — mentioned constantly, rarely explained in relation to each other. That's a genuine gap, because these standards don't compete with one another. Each one covers a specific interface, a specific link in the chain between a vehicle and a distribution network operator's control room, and none of them can substitute for another.
The useful way to understand the stack isn't as a catalogue to memorise — it's as a layered map, where each layer answers one question: who is talking to whom, and about what.
Five layers, five conversations
Figure 1. No single standard covers the whole journey — a fully interoperable system needs all five layers to work, and to work together, not any one of them working perfectly in isolation.
ISO 15118
This is the conversation that happens the moment a cable connects — authenticating the vehicle, negotiating how much power will flow and in which direction, and (in its newer 15118-20 revision) supporting genuinely bidirectional exchange for V2G. It's the layer closest to the physics of charging, and the one where compatibility problems are hardest to paper over after the fact.
OCPP
The Open Charge Point Protocol is what lets a charge-point operator remotely start and stop sessions, monitor charger health, push firmware updates, and send smart-charging profiles that tell a charger to throttle or delay based on network conditions or tariff signals. It's the most widely adopted layer in this stack, and the one most CPOs treat as table stakes.
OCPI
No driver wants to carry a different app or card for every charging network, and no regulator wants live charger status locked inside one operator's app. The Open Charge Point Interface is what lets networks share live availability, enable cross-network roaming, and publish the pricing transparency data increasingly required by UK regulation.
OpenADR
This is the layer that turns individual chargers and batteries into a coordinated flexibility resource a network operator or market platform can actually call on — sending demand-response signals out and receiving confirmation of delivery back. A controlled test reported through OpenADR Connections in June 2026 reduced close to 3 MW of demand across roughly 1,000 chargers within seconds — a useful illustration of what coordinated, standards-based dispatch can achieve operationally when the layer beneath it is solid.
Identity, certificates and lifecycle
Certificate management
Every device in the stack needs a trusted identity, issued and rotated on a manageable schedule — at fleet scale, this is its own operational discipline.
Software lifecycle
Five standards, each with their own versioning, mean five separate update cadences to track across every device in the estate.
Cybersecurity posture
A stack that can start, stop and reroute power flow at scale is a meaningful attack surface if any one layer is weakly secured.
Backward compatibility
Standards evolve — 15118-2 to 15118-20 is a good example — and a mixed estate has to keep supporting both for years.
Conformance testing isn't real-world interoperability
A charger and a vehicle can both pass independent conformance tests against the same standard and still fail to work together the first time they meet in the field — implementation choices, edge cases, and version mismatches all live in the gap between "conforms to the spec" and "interoperates in practice." SSES is moving into implementation, and OpenADR's own industry material identifies the DSO-to-customer boundary specifically as a scaling challenge — which is exactly where this stack has the most links still to prove out at volume.
None of these standards are going away, and none of them are optional if the goal is a genuinely interoperable smart grid. The practical work now is less about choosing which standard to support and more about proving, at real-world scale, that all five layers hold together — which is precisely the work SSES and its adjacent programmes are moving into.
How a read moves across the estate
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Chris Gales
Chris Gales leads CEHG. He converts complex technical and commercial choices into clear decisions and dependable outcomes for utilities running connected device estates at scale.
He writes these field notes from the practice — on bearer migration, smart metering architecture, procurement and operations — to help estates start counting down early.