Vehicles As Storage

Vehicles as storage: Why the industry needs to prepare

Following on from the draft for public comment stage for the forthcoming Code of Practice for Electric Vehicle Charging Equipment Installation, this article looks at the role of vehicles in the electricity grid and why future-proofing charging infrastructure according to the new guidance could be important.


The decarbonisation of British electricity has been remarkable. Coal power plants used to provide over 40 per cent of our electricity, today they supply to less than 7 per cent. Electricity demand has fallen with consequent declines in fossil fuel use and there has been a staggering growth in wind and solar technologies which, in 2018, provided more electrical energy than Britain’s nuclear power stations[1]. As a result, the annual average carbon intensity of British electricity has almost halved in just a few years.

Decarbonisation of transport and heat is now rightly entering the British consciousness. At present, transportation represents the largest sector for final energy consumption in the UK with almost all of this energy coming from fossil fuels (Figure 1). The future of decarbonised transport is projected to be a combination of battery and hydrogen technologies, the former placing new demands and presenting new opportunities to the grid as this article explores. But what could a fleet of millions of vehicle batteries mean for the grid?

Figure 1: Final energy consumption of different energy vectors

Vehicles as storage: EVs present challenges and opportunities for the power system

Today’s electricity grid is designed to run our homes, services and businesses. In this traditional power system, everything from washing machines to computer servers have a demand for electricity which is very difficult to change – appliances are used when people need them to be used. It’s intuitive that battery vehicles will increase demand for electricity and electricity generation

For operators of the electricity system, vehicle batteries are intuitively attractive. Storage facilities were shown to add value in the summer of 2019 in reducing the scale of a major power outage, but they are expensive and only serve the power system. What if vehicle batteries could also contribute to our power system?

Vehicle batteries are a potentially massive storage resource which, service vehicle owners and could also serve the grid. A conservative estimate of the flexibility in five million electric vehicle batteries (Figure 2) shows that these would almost quadruple British electricity storage capacity. This is flexibility that opens up opportunities for vehicles to support the decarbonisation of electricity such as encouraging vehicles to be charged when low carbon generators are operating or to stop charging when there are high conventional electricity demands, such as in the evening peak.

The scale of battery storage in vehicles shows unprecedented scale of this flexibility and the case impact they could have on power. One could imagine a scenario whereby vehicles are predominantly plugged in in the early to late evening. A worst-case might be all these vehicles charging at the same time and potentially overloading the grid. If just 2.5 m electric vehicles were all charged using a slow charger at the same time, it would add 7.5 GW of power demand to the grid. For comparison, the midday system demand for electricity in April 2019 was below 40 GW. That’s a large ramp-up in power and makes vehicle services an essential consideration for the electricity system.

Figure 2: Comparison of power and capacity in electric vehicles relative to pumped storage plants operational in 2016. (Pumped storage data from the British Hydro Association. Vehicle data assumes 5 million vehicles 50 per cent of battery available to grid and 50 per cent of vehicles plugged in at a time)

It’s right to start thinking about EV charging infrastructure

With approximately 202,000 registered EV vehicles (Dec 2018) EVs still represent only 0.5 per cent of vehicles on the UK’s roads. However, influences related to political, economic, social, technological, legal and environmental changes will all play a role in shaping how fast EVs are adopted and the rate of adoption is increasing every day. The decarbonisation of transport is certainly expected to, excuse the pun, accelerate in the coming years. Vehicle manufacturers are chasing aggressive emission compliance targets against a backdrop of an increasingly competitive marketplace and a changing mobility landscape (with autonomous cars and fleet growth). As shown in Table 1, the batteries in cars are only going to get larger as manufacturers move from batteries in hybrid cars, to plug-in hybrids to full electric vehicles – and emissions targets from the EU being driven by the number of vehicles sold.

Figure 3 shows the advancing scenarios by National Grid showing that EV’s use will see exponential growth from the mid-2020s, by which time legislative, infrastructure and commercial influences will make EV’s the default choice for the masses. It’s important for us all to get ready for this challenge with appropriate charging infrastructure.

Figure 3: Number of electric vehicles as predicted by National Grid Future Energy Scenarios (Data is for two degrees or gone green scenarios which are amongst the greenest scenarios)


Table 1: Types of vehicles with some form of electric traction

EV Type

Fully electric

Charged from grid

Vehicle as storage opportunity

Typical battery capacity

Mild Hybrid Electric Vehicles (MHEV)




48V battery system

Hybrid Electric Vehicles (HEV)



Very Low


Plug-in Hybrid Electric Vehicle (PHEV)





Battery Electric Vehicle (BEV)





How and why you might use a car battery to support the power system

In this article, we are not arguing that all of a vehicle battery is used to support the grid. We are suggesting the strategic use of some of the battery capability and smart charging to provide services to support the power system – and accordingly reduce the costs of managing electricity networks, renewables, generators. For vehicle owners, that might mean strategically changing the rate of charge, different tariff structures or allowing say up to 10 miles range to be sold back to the grid when needed. The interaction of vehicles and the grid work best when they have an unnoticed impact on the vehicle but a very positive impact in maintaining the electricity system.

Potential uses for vehicles as storage fall into a number of categories, and in this article, they are broadly split into ancillary services, decarbonized energy matching services (those which help ensure the provision of low cost, low carbon electricity), consumer services and network services (those which help protect networks).

Ancillary services are those services which help to keep the power system stable and provide high-quality power. As of 2019, these are largely procured via the national electricity system operator, National Grid, although regional ancillary service markets also exist that are focused on networks. Two easy to understand national ancillary services are reserve and frequency response. 

Reserve is the calling upon of sustained load or generation to ensure balancing of supply and demand. For example, when a major power station trips, it is necessary to either reduce demand or increase supply in order to ensure that major loads can be met. To provide this, a fleet of electric car batteries can, in theory, be called upon to either reduce load (by stopping charging) or supply the grid (by discharging the battery into the grid) in the event of the power station tripping.

‘Frequency response’ is similar in the requirements of the battery. The frequency of the electricity grid in Great Britain, 50Hz, is a reflection of the instantaneous balance of supply and demand for power across the grid; frequency falls during undersupply and rises during oversupply. The provision of an automated frequency response by EVs plugged into the grid could, in theory, provide a frequency response mechanism. Indeed, it is felt that EVs could provide this to the grid at a much lower cost than stationary storage and so reduce the need to procure large stationary grid batteries altogether – or become a mandatory requirement for grid-connected storage.

Energy matching enables EVs to be charged more fully using lower cost and lower carbon electricity or at times which suit the electricity system. For example, during high summer solar generation EVs might be encouraged to charge to allow enough demand to consume excess low carbon generation. Similarly, EVs could be charged at night to raise baseload and provide enough demand to keep nuclear power stations online. These services are likely to manifest through time of use/differential pricing from suppliers, allowing customers to choose how fast or how much to charge or discharge vehicles contingent on the price (or forecast price) of electricity.

Consumer services are those which use the vehicle as storage to directly assist consumers. This might include charging of EVs from distributed generation (such as domestic solar PV or solar carports). It might also be scheduling the EV to charge when electricity is cheap. Automatic chargers already exist and are widely installed in UK homes to achieve both of these services.

Alternatively, the vehicle(s) might be used to provide backup to a home or business when there is a power outage.

Network services are the use of EVs to support or prevent failure of cables and substations. There are a complex variety of network services, and the simplest to explain is slowing down or restriction of EV charging at peak times where the power consumption of EV charging risks overloading network cables. As a result of the regionalised and time-dependant nature of network issues, these are likely to be localised, available at different times of the day, and procured/regulated by network companies trying to ensure the lowest cost of running their systems.

Getting ready for flexibility

It is valid to recognise that not all of the services that EVs can offer to the grid are available to vehicle owners today. Similarly, there is no agreement between vehicle manufactures on whether their vehicles can or should provide such services: some manufacturers actively promote vehicle services whilst others take a hard line against them. However, over the life of charging infrastructure a range of vehicles and services could be connected and as such electrical requirements for the charger must be considered to provide safe use throughout the charger’s life. In addition, charge point infrastructure owners might use smart charging to take advantage of lower-cost electricity or to maintain charging rates within the power constraints of their grid connections.

To provide these services requires charging infrastructure, which is designed for vehicle services and not just as dumb electrical supplies. That is because utilising vehicle batteries to support the grid means changing when the battery is charged and also discharging that electricity back into the grid when needed. The following examples show why the differences are so acute and why the updated code of practice is so important:

  • Vehicle charging infrastructure is not just a plug to take power from the grid, but also becomes a potential point of generation supplying electricity at times of need. This places particular requirements on notifications to network operators and the design of protection.
  • Control and communication access needs to be considered, particularly for infrastructure where an external organisation such as an aggregator or vehicle fleet manager wishes to manage when their vehicles charge. It is important not to underestimate how much vehicle manufacturers are looking at changing their operating models with many considering leasing a fleet of vehicles to a pool of customers and accordingly wanting to manage how their vehicle batteries are used.
  • In the most extreme example, infrastructure developers may need to consider providing backup power, with grid forming inverters keeping on essential loads in our homes. This places unique requirements on infrastructure, such as the provision of safe earthing in the event of a lost network earth, updated protection under reduced fault currents and safe isolation.
  • Operating charging in parallel with solar and battery storage requires communication and control that is coordinated. For example, the charging infrastructure needs to measure the solar generation and coordinate charging with any on-site battery storage.


Should there be wide adoption of battery electric vehicles, Britain can expect a large increase in the consumption of electricity as well as a huge increase in the amount of energy storage on the grid. As we have seen, there are a number of means by which this storage can be used to help the grid.

The likelihood of EV batteries operating in this role remains controversial, with different viewpoints from different vehicle manufacturers. The rate of adoption of different grid services is likely to be a result of a combination of the views of the vehicle owner, the economic/technical value of grid services, the economic incentives available such as time of use tariffs, the ownership models, the impact on vehicle battery life and the physical need for services (which is contingent on the national electricity supply mix).

However, as installers of vehicle charging infrastructure, it is our role to provide equipment which is ready for all eventualities. The IET Code of Practice for Electric Vehicle Charging Equipment Installation, 3rd Edition provides a simple guide to the safety, electrical and practical considerations. That and other publications are key to helping vehicles play their role in the future grid.