£/MWh, while UK spark spreads were in the range of 4–9 £/MWh. It is well-known that these values substantially understate the actual efficiency of modern plants. Best-in-class efficiencies (as of 2019) are near 64%, and commercial development is rapid. Clean spread In countries that are covered by the European Union Emissions Trading Scheme, generators have to consider also the cost of carbon dioxide emission allowances that will be under a cap and trade regime. Emission trading has started in the EU in January 2005. The Clean Spark Spread is calculated using a gas emissions intensity factor of 0.411 tCO2/MWh. Therefore, the clean spark spread is calculated by subtracting the carbon price per tonne (multiplied by 0.411) from the ‘dirty’ spark spread, i.e. Clean Spark Spread = Spark Spread – (Carbon Price*0.411). Clean spark spread or "spark green spread" represents the net revenue a generator makes from selling power, having bought gas and the required number of carbon allowances. This spread is calculated by adjusting the cost of natural gas for the efficiency of the generation and subsequently applying the market cost of procuring or opportunity cost of setting aside an emissions allowance such as a European Union Allowance (EUA) in the European Union Emissions Trading Scheme (EU ETS). Let S: spark spread, E: electricity price, G: gas cost, Ng: number of carbon credits necessary to cover gas operation, Pcc: price of a carbon credit. Then the Clean spark spread is defined as Clean dark spread or "dark green spread" refers to an analogous indicator for coal-fired generation of electricity. The spark green spread and the dark green spread are especially important in areas where coal-fired electricity generation is prevalent as the convergence of the spreads will lead to an important decision point. Let D: dark spread, E: electricity price, C: coal cost, Nc: number of carbon credits necessary to cover coal operation (2–2.5x that of gas), Pcc: price of a carbon credit. Then, Clean dark spread = E - C - Nc*Pcc = D - Nc*Pcc Climate spread: The difference between the dark green spread and the spark green spread is known as the "Climate Spread". Climate spread = Clean dark spread - Clean spark spread = (D - Nc*Pcc) - (S - Ng*Pcc) = (D - S) - (Nc - Ng)*Pcc. Note: (D - S) and (Nc - Ng) are positive numbers. In a carbon constrained economy a power