[en] For fusion to become available for commercial use in the 21st century, R and D must be undertaken now. But it is hard to justify these expenditures with a cost/benefit oriented assessment methodology, because of both the time-frame and the uncertainty of the future benefits. Focusing on the factors most relevant for current consideration of fusion's commercial prospects, i.e., consumption levels and the outcomes for fission, solar, and coal, many possible futures of the US energy system are posited and analyzed under various assumptions about costs. The Reference Energy System approach was modified to establish both an appropriate degree of detail and explicit time dependence, and a computer code used to organize the relevant data and to perform calculations of system cost (annual and discounted present value), resource use, and residuals that are implied by the consumptions levels and technology mix in each scenario. Not unreasonable scenarios indicate benefits in the form of direct cost savings, which may well exceed R and D costs, which could be attributed to the implementation of fusion