Stories from the field: Bus depots as distributed energy hubs

We quickly ran into a challenge that will be familiar to anyone working on depot electrification: power. Electrifying a bus depot like Caledonia requires roughly the same grid connection as a housing estate of around 5,000 homes. Given local grid constraints, the initial message from the distribution network operator, SP Energy Networks (SPEN), was clear - the capacity simply wasn’t available any time soon, and reinforcing the network could take many years.

For many projects, that would have been the end of the story. Instead, we started asking a different set of questions. It turned out the biggest obstacle wasn’t cables or generation capacity, but something far simpler: land. SPEN needed a site for a new primary substation to reinforce the local network, and finding a suitable location was proving difficult.

The eventual solution was straightforward. We agreed to host the new substation within the depot itself as part of the electrification project. Not only did this futureproof electrification of the depot, but also then provided much needed network reinforcement for the entire south Glasgow area. Only a third of the substation capacity is reserved for the depot itself, with the remainder strengthening the electricity network across south Glasgow.

While a simple land access and sharing agreement may seem trivial, it was also a significant realisation for us about our role in the decarbonisation journey - our electrified depots aren’t just simply large energy consumers. They can be something bigger and serve as a critical node in the energy system.

Aerial views of our Caledonia depot in Glasgow

Once you start looking at depots through this lens, new possibilities begin to emerge. Electrified depots, whether for buses, trucks or vans, concentrate several important ingredients in one place: large grid connections, high-capacity charging infrastructure and large batteries typically charging overnight. In other words, they begin to resemble energy hubs as much as transport facilities.

Let’s look at this in more detail.

Supply and infrastructure

Because electrified depots consume large volumes of power, they can become credible off-takers for local renewable generation, either through private-wire connections or via offsite power purchase agreements. This is already changing the nature of discussions we have with local authorities. A few years ago, our conversations with councils were mostly about routes, timetables and fares. Today they also cover this: “We’re looking at building a solar farm on council land half a kilometre from your depot. Could we explore supplying that power directly to you?” Transport electrification is becoming closely linked with local energy planning.

And of course, providing access to our depot chargers to businesses and the local community when the buses are out in service during the day is another great example of how electrified depots are doubling up as energy hubs. For perspective, given First’s rapid electrification trajectory over the last few years, we are now effectively one of the top 10 rapid charging network in the country with 1200+ rapid charging outlets. When access to charging infrastructure is commonly held out as a major barrier for widespread electrification in the country, particularly for other large vehicles such as coaches and eHGVs, shared infrastructure at private depots absolutely must be part of the story and solution.

For us, this is not theory. This is what we’ve put in practice since 2022 with our third party charging offering First Charge. What started with a simple agreement with DPD at our Caledonia depot has now grown to a mature offering across the country covering fleets across blue lights, utilities, logistics, coaches, trucking and in some instances, the general public.

Charging Operations and Batteries

Deferring EV charging to cheaper overnight electricity periods is hardly a trade secret, but the implications go much further than simply reducing costs.

To understand why, it helps to briefly step back and consider how electricity markets work.

Unlike petrol or diesel, electricity is difficult to store at scale. This means supply and demand must stay in constant balance at all times. If they do not, the system risks instability. That balancing act shapes the structure of electricity markets across the day and across seasons.

On a sunny, windy Sunday afternoon in August, demand is relatively low while renewable generation is plentiful. Prices tend to fall. On a cold, dark weekday evening in January the opposite is true. Demand is high, driven by heating, lighting and millions of kettles boiling for a cup of tea, while the system relies more heavily on expensive gas generation. Prices rise.

Who keeps the electricity system in balance? In the UK, that responsibility falls to the National Energy System Operator (NESO), which fine-tunes supply and demand through the ‘balancing mechanism’ on a half-hourly basis and even down to sub-second adjustments through ‘frequency response’ markets.

But maintaining stability isn’t just about the next few minutes or hours. Longer-term balance is self-regulated through demand and supply in wholesale energy markets, from years ahead to day-ahead or even within-day, where suppliers like EDF, Octopus Energy, and British Gas buy electricity from generators on behalf of their customers. On top of all this, the UK also runs a capacity market designed to ensure there’s enough generation investment to meet the country’s future needs.

A simple analogy is managing food in your kitchen. The capacity market is like making sure you have a fridge and cooker large enough for the year ahead. Wholesale energy markets are like stocking the fridge based on what you expect to cook tomorrow. The balancing mechanism come into play when an unexpected guest arrives for dinner and you need to adjust your plans. Frequency response is the equivalent of catching a plate before it hits the floor.

Each layer helps keep the system running smoothly, and increasingly flexible demand such as EV fleets at bus depots can have a role to play across several of them.

At First Bus, we already participate in the capacity market and actively optimise our bus charging profiles across the wholesale energy markets as well. 

First Bus has been active in the capacity market since 2023. From the grid’s perspective, reducing demand at critical moments can be just as valuable as increasing generation. And under the capacity market we’ve committed to temporarily reducing our charging load, given four hours’ notice, during periods of system stress. In effect, our flexibility helps reduce the need for investment in additional power stations.

Now let’s consider the wholesale energy markets. While this varies across the country, our electric buses typically return to depot between 6pm and 8pm, where they are plugged in for overnight charging. But rather than charging immediately, we often delay charging until later at night (10-11 PM) when the typical national peak in electricity demand during the evening has passed.

Energy market optimisation in practice at our Bramley depot on 15/3/2026 with minimal charging loads between the hours of 7-10 PM

Along with lower costs our actions also help the electricity system by flattening demand peaks and making better use of overnight renewable generation, particularly wind.

Making this work requires more than simply setting a timer. Charging schedules must be optimised across multiple factors, including energy prices, site power limits, vehicle charging curves, and the need to ensure every bus is ready for service the next morning.

And as with many systems, the human element matters more or just as much as the technology. We’ve worked hard to ensure our depot managers are confident that the process works and that buses will always be ready for service. Also, we’ve engaged with our frontline staff plugging in vehicles so that they understand why a bus may not start charging immediately and know that the system can be overridden if necessary.

The future

Moving up the spectrum, the balancing mechanism has historically been the purview of generators, who submit offers and bids to turn up or down their generation to the NESO every half hour to help manage demand and supply. But thanks to electricity market rule changes across the last couple of years (BSC P344, P376 among others for the curious and energy nerds), assets such as EV fleets and bus depot loads can now participate more easily in these markets. There is also a similar shift in the frequency markets as well.

While the space is still nascent, these market evolutions have also coincided thousands of electric buses being in-fleeted across the UK. And given that EV fleets and bus depots are some of the largest flexible loads in the system, this makes them a very good match for these markets, providing the grid with a critical source of flexibility. The helpful thing for fleet operators looking to participate is that it’s a natural evolution from wholesale energy market optimisation and all the skills, internal know-how and knowledge will translate across very well. Watch this space!

Lastly, no discussion of depots as energy hubs would be complete without mentioning vehicle-to-grid or V2G. In all the above discussions, our buses don’t send energy back to the grid, but simply adjust their consumption up or down, and as such this is not V2G. Whereas the north-star for complete EV-grid integration is when EV batteries actually discharge into the grid. This is presently not feasible at scale for a variety of technical, regulatory and commercial reasons which is beyond the scope of this article.

But in our view, the near-term future is broader than V2G. Rather than exporting electricity only back to the grid, bus batteries could also provide energy to other vehicles, buildings, batteries or local systems.   At First Bus we are actively exploring this space and are already working with vehicle manufacturers to promote capabilities such as vehicle-to-vehicle charging, which could add valuable operational flexibility.

In conclusion, the electrification of transport is often framed as a challenge for the electricity system. But our experience over the last few years suggests something slightly different. Across the country, hundreds of buses quietly charging in depots each night represent something remarkable: one of the largest emerging sources of flexible demand in the UK electricity system. Seen through that lens, the humble bus depot starts to look less like a transport facility and more like a cornerstone of the future energy system.