DERSec Sentry is a threat detection system designed to protect distributed energy resource (DER) assets including EV chargers and solar+storage systems from network and power-system attacks. The solution works in coordination with DERSec Detective to provide fleet-wide protection for operators. The system implements energy-protocol-specific deep packet inspection to parse network traffic and extract monitoring and control signals, validating them against power system rules. It detects power-based attacks, malicious firmware updates, nefarious control operations, false data injection attacks, and insider threats that target DER assets. DERSec Sentry can be deployed on hardened industrial compute devices attached to a switch SPAN port for detection mode or as a bump-in-the-wire for prevention mode. Security checks on packets and local human interfaces are completed in less than 500 milliseconds. When falsified power data or malicious control signals are detected, the system suppresses incoming network traffic and notifies network operators. The detection algorithm uses multiple techniques including customer-tunable heuristic rules, stateful detection, digital twins with power simulation, and machine learning to classify DER measurements and control data. The solution is designed to complement existing network and security infrastructure including firewalls and SIEM systems.