“Carbon adders” are a means of accounting for possible future costs of mitigating GHG emissions. A carbon adder is an expected future price for CO2e that is assumed when comparing investment options. Carbon adders are used to compare the cost of fossil fuel and renewable generation, as well as demand-side management investments. The carbon adder is used for utility planning purposes only, and is not assessed to consumers. Taking the cost of carbon into account means that an investment is considered more cost-effective if it avoids a ton of CO2 emissions at an incremental cost equal to the value of the carbon adder. One significant advantage of this approach is that the carbon adder anticipates the future compliance costs of a carbon constraint, rather than attempting to estimate the cost of damages from future climate change. Such damage calculations are highly uncertain and controversial. There is a smaller range of uncertainty in assessing future compliance costs than in predicting the economic impacts of future climate change.