The advanced metering infrastructure (AMI) is revolutionizing electrical grids. Intelligent AMI smart meters report real time usage data that enables efficient energy generation and use. However, aggressive deployments often outpace security efforts: new devices from a dizzying array of vendors are being introduced into grids with limited understanding of the security problems they represent. In this paper we develop an archetypal attack tree approach to guide penetration testing across multiple-vendor implementations of a technology class. In this, we graft archetypal attack trees modeling broad adversary goals and attack vectors to vendor-specific concrete attack trees. Evaluators then use the grafted trees as a roadmap to penetration testing. Our experiments with multiple vendors generate real attack scenarios using vulnerabilities identified during directed penetration testing, e.g., manipulation of energy usage data, spoofing meters, and extracting sensitive data from internal registers. We provide a detailed example of one such attack as tested using our developed methodology.