Surplus value is a central concept in Karl Marx's critique of political economy. Marx did not himself invent the term, he developed the concept. "Surplus value" is a translation of the German word "Mehrwert", which simply means value added (when the sales revenue is less than the cost of materials used up). Conventionally, value-added is equal to the sum of gross wage income and gross profit income. However, Marx's use of this concept is different, because for Marx, the Mehrwert refers to the yield, profit or return on production capital invested, i.e. the amount of the increase in the value of capital. Hence, Marx's use of Mehrwert has always been translated as "surplus value", distinguishing it from "value-added". According to Marx's theory, surplus value is equal to the new value created by workers in excess of their own labour-cost, which is appropriated by the capitalist as profit when products are sold.