windfarms. In 2023 there is an effective windfall tax. Costs The economics of wind power are driven by factors such as the capital, operating and finance costs, as well as the operational performance or capacity factor. These factors are in turn affected by issues such as location, turbine size and spacing and, for offshore windfarms, water depth and distance from shore. Operating costs and performance change over a windfarm's life, and several years of data are required before an assessment of the trajectory of these figures can be made. A review of financial accounts published by the Renewable Energy Foundation in 2020 showed that UK offshore windfarm capital costs rose steadily from 2002 to around 2013, before stabilising and perhaps falling slightly. Operating costs have risen steadily up to the time of the study, but financing costs have fallen. This picture has been confirmed by a comprehensive review of audited accounts data for UK offshore windfarms, which found that levelised costs rose from around £60–70/MWh for early projects, to around £140–160/MWh by 2010–13, before stabilising. The Renewable Energy Foundation study also examined onshore wind costs, finding that capital costs had risen to around 2011 before declining slightly thereafter, while operating costs had risen steadily. Estimates of the levelised cost of UK onshore wind are older. A 2011 study by the engineering consultancy Mott MacDonald put onshore wind costs at £83/MWh, below new nuclear at £96/MWh. Auction bids In the UK's contract for difference auctions of 2017 and 2019, offshore windfarms made bids to supply the grid at strike prices much lower than anything seen before: £57.50/MWh in the 2017 auction and £39.65/MWh in the 2019 one. These values are below the ostensible windfarm costs outlined in the previous section, and have therefore been widely taken as evidence of a fundamental change in the economics of offshore wind power; in other words that technological advances have led to much lower costs. There has been no similar reduction in bidding prices from onshore windfarms. The lowest successful bid under the CfD regime has been £79.99/MWh. Effects on electricity price The building of UK wind farms has been supported through the Renewables Obligation and, since 2016, by price guarantee support through the Contracts for Difference regime too. The 2018 levelised cost of electricity (LCOE) of offshore wind was in the range £100–150/MWh. However, in recent CfD auctions, strike prices as low as £39.65/MWh