A key uncertainty in the design and long-time behavior of EGS reservoirs, and their management, is the fracture surface area that controls the rate of heat transfer between the host rock and the circulating injected water. A workflow is developed that uses analytic solutions to estimate this surface area. We first analyze a tracer test to estimate the fracture pore volume swept, the flow geometry, and the presence of multiple fractures (or damage zones around the primary fracture). Using this information about fracture properties, an analytic solution describing produced temperature vs. time can be solving iteratively to estimate surface area. In the case of multiple fractures, we show that the equation governing the produced fluids' temperature is superposed to estimate the surface area. The workflow is shown to be robust, even in the presence of heterogeneity. The workflow is then used to make design decisions and predictions about the sustainably of a reservoir. Also shown is the use of the workflow to estimate power generation as a function of fracture properties (measured) and flow rates (controlled), and show how to scale up to multiple production wells and fracture packs. Assumptions used, and limitations of the method are discussed. (C) 2016 Elsevier Ltd. All rights reserved.