Supplier Cost Index by fuel type (GB)


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Source: Ofgem analysis.

Information correct as of: May 2018

The Supplier Cost Index tracks ongoing trends in wholesale costs, network costs and the charges to suppliers associated with government programmes.

This chart is no longer being updated.

At-a-glance summary

  • In the three months (between 1 February 2018 and 1 May 2018) the index increased by 5.3%, driven primarily by increases in wholesale gas and electricity costs. There was also a small increase in the costs of government obligations related to supplying electricity.
  • Overall, the Supplier Cost Index (dual fuel) increased by 14.1% between 1 May 2017 and 1 May 2018.
  • The 12 month increase was primarily driven by increases in wholesale gas and electricity costs.
  • The index starts in January 2015. Costs fell during 2015 and early 2016, with much larger reductions for gas than electricity. By the 1 May 2018, the dual fuel index is 9.7% above its level at the start of 2015. The electricity index is 19.7% higher, while the gas index is 0.1% higher.

Click the ‘more information’ tab above for a description of how the index is calculated and which costs are included.

Policy Areas:

  • Electricity - retail markets
  • Gas - retail markets

Data Table

Supplier Cost Index by fuel type (GB)
Expected supply costs for the next 12 months (Jan-15 = 100)Dual FuelElectricityGas

More information


  • We calculate the Supplier Cost Index by estimating trends in network charges, wholesale prices and the charges to suppliers associated with government programmes (note that in some cases, these government charges only apply to large and medium-sized suppliers).
  • These estimates are then combined with information on the relative scale of each of these categories of cost to calculate the trend in the overall Supplier Cost Index. The weights given to each category of costs are based on financial statements from the six large suppliers, and are as follows:

- wholesale electricity: 26.7%

- wholesale gas: 35.9%

- networks electricity: 15.4%

- networks gas: 14.4%

- government obligations electricity: 6.7%

- government obligations gas: 1.0%.


  • The index reflects estimated expected annual costs, covering the 12 months from the time of each update, based on the best information available at the time. So, for example, the value of the index for May 2018 will reflect estimated costs for the period 1 May 2018 to 30 April 2019, expressed relative to estimated expected annual costs as of the base period (1 January 2015 to 31 December 2015).
  • The estimates in the index are forward-looking, they therefore rely on forecasts and assumptions, and so will be subject to uncertainty. Information on suppliers’ realised costs is available in the financial statements published by the six large energy suppliers. See Understanding the profits of the large energy suppliers.
  • The index does not include estimates of suppliers' ‘back-office’ operating costs (such as the costs of billing or metering – including the costs of the smart meter rollout) or their profit margins, which suppliers will seek to cover when setting their prices.
  • The index is based on trends in the average prices of wholesale gas and electricity forward contracts in the month prior to the update. Suppliers will take different approaches to purchasing their wholesale energy, and many will buy their energy over an extended period. The index does not seek to estimate any impact this may have on a supplier’s costs.
  • Other elements of costs are also likely to vary across individual suppliers. For example, suppliers may have some flexibility in how they meet their obligations under government programmes. This could mean, for example, that suppliers see different year-on-year changes in costs than indicated by the index where they have chosen to meet their obligations under the ECO scheme at the start of the delivery period (the forecasts used in the index are based on a flat delivery profile). Network charges will also vary between suppliers depending on things like the regional profile of their customer base. 
  • The index is calculated for a customer with typical consumption. We have held consumption fixed over time to increase comparability with trends in suppliers’ prices (which are also typically expressed for a given level of consumption). In practice, energy use will vary from one year to the next, depending on temperatures. Energy use is also subject to long-run trends, for example as a result of increasing energy efficiency. Trends in consumption will also have a significant impact on the size of customers’ bills.
  • Capacity market payments were included in the index from winter 2017, we have categorised these as wholesale electricity costs. We consider that this allocation best reflects the nature of these costs. We intend to keep under review what further detail might be provided on the costs associated with different government programmes. 
  • Since the August 2017 update, we have included the additional costs associated with the expected exemption of Energy Intensive Industries from the costs of the Renewable Obligation scheme, following the Government’s announcement in December 2017. The Government’s decision on implementing the same exemption for the Feed-in Tariff scheme has yet to be published, and so the possible impact of a similar change to the way this programme is funded is not currently included.

Further details of how we calculate the index are provided in our methodology document.


Date correct
May 2018
Policy areas