Energy, power and carbon
Last updated on 2026-02-18 | Edit this page
Estimated time: 40 minutes
Overview
Questions
- What is energy?
- What is power?
- How power and energy relate to carbon emissions?
- What other sources of carbon involve digital research?
- How do we calculate carbon emissions?
Objectives
- Explain what energy and power are
- Explain how energy is produced
- Explain what low-carbon energy sources are and how they opperate
- Explain what embeded carbon is
- Use the greenhouse gas (GHG) protocol to estimate carbon emissions
(This episode will be heavy on pointing to the Green software practitioner course sections)
Energy and power
Energy is a physical property that can be used to do work. This can be lifting a weight, pushing a piston or even running a computation on a computer. The SI unit of energy is the Joule (J) but commonly the kilowatt-hour (kWh) is also used.
Power is a rate at which energy is consumed i.e., how much energy is used in a given amount of time. The SI unit of power is the watt (W) however kilowatts (kW) are commonly used as well.
Joules, kilowatts and kilowatt-hours
The units used for power and energy can be confusing, particularly kilowatt-hours as a unit of energy. A useful relation to bear in mind is that \(1 W = 1 J/s\). By multipling watts by another unit of time we recover units of energy with a scaling factor.
Kilowatt-hours are commonly used because they tend to work out nicely for everyday situations, e.g. a kettle may have a power rating of 1 kW so running it for an hour gives 1 kWh of energy used.
Praticing units of power and energy
Which of the below are not equal to 1 kWh.
- A - 200 W consumed for 12 minutes.
- B - 1000 J
- C - 3,600,000 J
- D - 5000 W consumed for 12 minutes.
- A - 0.2 kW x 0.2 hours = 0.04 kWh
- B - 1000 J = 0.00027 kWh
- C - 3,600,000 J = 1 kWh
- D - 5 kW x 0.2 hours = 1 kWh
Energy sources and carbon emissions
Energy famously cannot be created or destroyed but the energy used for research activities has to come from somewhere. In practice the majority of energy used for digital research comes from a national electricity grid so this will be our focus.
The electrical grid serves to transport energy from electricity generators to end users. Economies of scale tend to mean that electricity generation is a large scale activity. The electrical energy supplied to the grid comes from a variety of different sources. This can be fossil fuels like coal and gas or green energy sources like solar and wind.
A key feature of electrical grids is that supply must be balanced with demand. Demand for electricity can vary greatly throughout a year or even an individual day. The grid responds to increases in demand by purchasing additional electricity from suppliers.
Energy Mix and Carbon Intensity
Different methods of electricity generation have different properties. Some of the important include:
- Cost - The cost of generating each kWh of energy.
- Carbon Intensity - A measure of the kgCO2e emitted per kWh of energy.
- Dispatchability - How easily or quickly generation can be scaled up in response to demand.
- Predictability - How easy it is to predict the amount of generation available.
The table below provides a quick summary of how different energy sources compare on their key properties:
| Energy source | Cost | Carbon intensity | Dispatchability | Predictability |
|---|---|---|---|---|
| Gas | Medium | Medium | High | High |
| Solar | Low | Low | Low | Low |
| Wind | Low | Low | Low | Low |
| Nuclear | High | Low | Medium | High |
| Hydro | Variable | Low | Low | High |
While solar and wind are very good in terms of cost and carbon intensity, they are unable to respond effectively to changes to demand. Gas, and to some extent, nuclear, while less appealing otherwise, can respond to these quick changes and hence complement green sources.
The energy sources used by the grid will change on an hourly timescale and some sources such as wind and solar can be subject to seasonal and climate effects. The relative cost of different sources can also be impacted by global events and markets. The sources of electricity used by the grid are referred to as the energy mix. The energy mix of the grid leads to an overall carbon intensity value given as gCO2/kWh of electricity generated. This can also be broken down by geographical region or given as an average for a time period.
Carbon Intensity in the UK
The following graphs show a typical UK day in 2026.
The following dynamics are at play:
- At midnight initial energy demand and carbon intensity is low.
- Around 5am, energy usage begins to increase as people wake up and businesses open. As demand increases, the proportion of gas in the energy mix increases as more gas generation is brought online to keep the grid balanced. This also drives an increase in carbon intensity.
- Carbon intensity peaks in the morning around 7am. Although energy demand continues to rise, gas usage and carbon intensity drop slightly as cheaper imported energy becomes available. Slightly later a small amount of solar power also becomes available as the sun rises.
- Demand remains steady throughout the day before increasing in the evening. This is driven by domestic usage as people come home, cook and use domestic appliances. Again additional gas generation is brought online to meet the demand and carbon intensity rises to its peak value.
- As the evening progresses and people go to bed, demand drops again and carbon intensity also falls as gas generation goes offline. Overall carbon intensity ends up lower at the end of the day than the beginning as more imported energy is available.
Takeaways
The pattern shown is typical for a day in the UK. There are however many other factors that can determine the relationship between demand and carbon intensity which can play out at a variety of timescales.
There is considerable variability in the carbon intensity of electricity throughout the day - a factor of two in the above example. A simple strategy to reduce the emissions from digital research is therefore to shift electricity usage to times when carbon intensity is low. This is known as demand shifting. A simple rule of thumb is to favour running computationally intensive work at night.
Gas is a key part of the UK’s energy mix because of it’s dispatchability i.e., it’s ability to rapidly respond to changes in demand. Some green technologies like solar and wind have low dispatchability as they depend on factors like the weather.
The above graph demonstrates how carbon intensity can vary throughout the year. Whilst there is little pattern month to month, it is interestirng to observe that the mimimum and maximum carbon intensity of the grid can vary between ~50 gCO2/kWh and ~250 gCO2/kWh, a factor of five.
Carbon Intensity Forecasts
For the UK there are publically available forecasts for the carbon intensity available at https://carbonintensity.org.uk.
Data sources
The above graphs were generated from publicly available data provided by the National Energy System Operator. Data was sourced from the UK Carbon Intensity API and the NESO Data Portal. The scripts used to generate the graphs available on GitHub in ImperialCollegeLondon/digital_research_sustainability_visualisations.
Embodied carbon and carbon awareness
So far we’ve focussed on the relationship between carbon emissions and electricity usage. This is relevant to the operation of equipment used in digital research and is usually the dominant component of the operational carbon. Another key source to consider however are embodied emissions.
Embodied carbon is the greenhouse gas emissions produced during the full lifecycle of a product or system before it starts being used: raw material extraction, manufacturing, transport, construction and eventual disposal or recycling. It represents the “upfront” carbon locked into goods and infrastructure. Accounting for embodied carbon helps teams choose lower‑carbon options by considering repair, reuse, material choices and service life in addition to operational energy use.
We’ll discuss in detail the embodied carbon contributions associated with digital research activities in the next episode.
The Greenhouse Gas (GHG) Protocol and how to use it
So far we’ve discussed several sources of emissions. A key requirement to managing and reducing emissions is to measure and account for them. The Greenhouse Gas Protocol provides a framework for identifying and categorising different emission sources. It’s holistic and covers both direct and indirect emission sources.
The GHG protocol breaks down emissions into three categories called scopes:
Scope 1 are direct emissions. These come from activities that directly emit carbon such as burning fuel. This would cover fuel used in a vehicle or an on-site heating system or electricity generation.
Scope 2 are indirect emissions. These come activities that consume energy produced elsewhere. This is primarily the emissions associated with electricity generation covered in detail above.
Scope 3 are “Value chain emissions”. These come from everything upstream i.e., requirements you need to carry out research activities and everything downstream i.e., emissions associated with the use of your research outputs, even by others. Upstream emissions includes things like the embodied emissions of hardware whilst downstream emissions might include use of software or data you’ve created.
The GHG protocol is most often applied to businesses, countries or cities but it can be applied at any scale including an individual or research group. It’s easy to get hung up on which scope to place emissions in but perhaps the key takeaway is to take a broad view of different emissions sources.
According to the GHG protocol, what are the carbon emissions of…?
- Using a laptop in the office for coding 4h a day, 5 days a week. No calculations run.
- Brewing 5 cups of coffee per day, at home, 5 days a week.
No idea. We need to do it.