Managerial accountants define residual income as the amount of operating revenues left over from a department or investment center after the cost of capital used to generate the revenues have been paid. In other words, it’s the net operating income of a department or investment center. You can also think of it as the amount that a department’s profits exceed its minimum required return.
Under throughput analysis, the only factor that matters is the impact of a proposed investment on the ability of a business to increase its total throughput (revenue minus totally variable costs). Under this concept, the main focus is on either enhancing throughput through the bottleneck operation or in reducing operating expenses. This analysis requires a consideration of bottleneck usage by the likely mix of products to be manufactured, and their margins. This is a much more detailed analysis than is contemplated under the more simplistic residual income approach.
Residual income is the amount of net income generated in excess of the minimum rate of return. Residual income concepts have been used in a number of contexts, including as a measurement of internal corporate performance whereby a company's management team evaluates the return generated relative to the company's minimum required return. Alternatively, in personal finance, residual income is the level of income that an individual has after the deduction of all personal debts and expenses have been paid.