Power estimation of software processes provides critical indicators to drive scheduling or power capping heuristics. State-of-the-art power estimation solutions only provide coarse-grained support for power estimation. In this paper, we therefore propose a middleware for assembling and configuring software-defined power meters. Software-defined power meters provide real-time and accurate power estimation of software processes. In particular, our solution automatically learns an application-agnostic power model, which can be used to estimate the power consumption of applications. Our approach, named BitWatts, builds on a distributed actor middleware to collect process usage and infer fine-grained power consumption without imposing any hardware investment (e.g., power meters).