A changing landscape

To this point, energy policy in the built environment industry has been largely focussed on reducing demand for energy as a means of reducing CO2 emissions. This makes sense, as historically emissions have been directly proportional to the amount of energy used: you burn more gas, you emit more carbon dioxide. However, as we move to an electrified nation, the notion of less demand = less carbon dioxide is only part of the story.

Unlike natural gas, the carbon intensity of electricity is constantly changing, reflecting the mix of fossil fuel and zero carbon generation feeding the grid at a given time. Looking at the half-hourly carbon factors reported by National Grid ESO[1] for last year, over the course of a day, the carbon intensity of electricity varies by an average of 32% (see Figure 1). In winter months, where gas demand for domestic heating means more coal is required to meet the UK’s demand for electricity, the carbon intensity can vary by over 50%!

This means moving demand for electricity away from peak times can be a critical component in reducing carbon emissions, alongside reducing the demand itself and increasing renewable generation.

Figure 1: Half hourly carbon intensity of and demand for grid-supplied electricity in 2019, averaged over the year. Sources: carbonintensity.org.uk[2], National Grid ESO[3].

Balance is key

Demand can be actively managed through utilising energy storage or by timing non-essential uses to be during off-peak periods. In either case, the objective is to move the consumption of grid electricity away from times of peak demand on the system. This is likely to necessitate a transition to a different charging mechanism for domestic electricity consumers.

Domestic Time Of Use Tariffs (TOUTs) will see a variable rate for electricity, charged based on the current available capacity: electricity on a windy night or a sunny afternoon will be very cheap, whereas electricity on a still, overcast evening at 5:30pm will be more expensive.

Peak electricity is not only the most expensive to generate and distribute, but also the highest carbon (see Figure 1). This means the cost of energy and the carbon intensity of its consumption are linked.

The smart meter rollout has been well publicised. However, all the reports seem to have missed a key driver for the rollout: they enable the ability to charge a variable rate for electricity. Opening up the domestic market to TOUTs is the primary motivation for the smart meter rollout, and the government is targeting completion by 2024; Western Power is currently installing around 2,500 smart meters per day[4] to meet this target.

National Grid estimates that we’ll need a 60% uptake of TOUTs by 2050 to achieve our national net zero ambitions, resulting in up to 11.4% (1.5GW) of peak demand being shifted[5].

Providing sufficient electrical capacity to fuel our transition to electric vehicles is an oft touted challenge. However, the Electric Nation project, piloted in the South West by Western Power, has shown that Electric Vehicles (EVs) are plugged in for an average of 11 hours whilst only charging for 2. This offers lots of potential to diversify their demand.

Market and technological evolution

Whilst TOUTs and devices to exploit them are in their infancy, once the potential savings are realised by consumers, this is likely to catalyse progress in the industry. TOUTs will also increase the business case for energy storage, as power can be stored when the price is lowest allowing the consumer to use energy whenever they wish whilst paying the cheapest rate.

EVs offer domestic storage without the need for a supplementary battery. One step further is so-called Vehicle-to-Grid (V2G) where EVs are used as distributed storage to buffer the grid at peak times. National Grid projects that up to 45% of EVs could be used for V2G by 2050[6]. This means that electric vehicles could provide a net contribution of 13GW to bolster the grid at times of peak demand – that’s enough to power over 4 million homes!

The Electric Nation project is also the first pilot of V2G technology in the UK, offering Nissan Leaf owners the opportunity to automatically sell their car’s stored electricity back to the grid to assist Western Power in maintaining grid stability.

Why policy and reporting needs to change

The ultimate message is that, with the current grid mix, a building may be able to reduce its carbon emissions by upwards of 30% simply by changing when it consumes electricity, but there is currently no mechanism, in policy or otherwise, that enables this ‘time-of-use’ benefit to be claimed in annual carbon reporting.

If companies are to acquire an accurate picture of the carbon footprint of their assets and homeowners are to understand when and when not to charge their car and put on their dishwasher – both of which will be essential in pursuit of decarbonising the built environment to net zero – energy demand and carbon emissions must be considered at a half-hourly resolution.

Reducing demand should rightly remain a focus, but in this journey to electrification, we must shed the entrenched belief that it is only ‘how much’ electricity we consume that matters, not when we use it.


[1] National Grid ESO, “Carbon Intensity API,” [Online]. Available: https://carbonintensity.org.uk/.

[2] National Grid ESO, “Carbon Intensity API,” [Online]. Available: https://carbonintensity.org.uk/.

[3] National Grid ESO, “Historic Demand Data 2019,” 2019. [Online]. Available: https://data.nationalgrideso.com/demand/historic-demand-data/r/historic_demand_data_2019.

[4] Western Power Distribution, “Smart Meter Strategy,” June 2020.

[5] National Grid ESO, “Future Energy Scenarios 2020,” 2020.

[6] National Grid ESO, “Future Energy Scenarios 2020,” 2020.


UK Government, “Energy consumption in new domestic buildings 2015 – 2017 (England and Wales),” 2019.
National Energy Foundation, “Energy Efficiency Benchmarks For SuperHomes: A Normalised Performance Index Approach,” 2016.
UK Government, “Electric vehicle smart charging consultation,” [Online]. Available: https://www.gov.uk/government/consultations/electric-vehicle-smart-charging.
Tesla, “Tesla Powerwall 2 technical specifications,” [Online]. Available: https://www.tesla.com/sites/default/files/pdfs/powerwall/Powerwall2ACDatasheet_EN_UK_feb2020.pdf.