A system includes a converter and a controller to control a compressor and operates without receiving power supply from a thermostat. The converter receives a demand signal from the thermostat that is used to power the controller and charge a capacitor. When the thermostat de-asserts the demand signal, the charged capacitor powers the controller, which saves system parameters in a nonvolatile memory and enters a power save mode. The life of the nonvolatile memory is extended by alternately storing the system parameters in different memory locations. The system normalizes outdoor ambient temperature (OAT) during a demand cycle. The system determines OAT slope, which is used to select durations to operate the compressor at different capacities, by performing time based calculations during a demand cycle, demand cycle based calculations at the start of a demand cycle, or time and demand cycle based calculations during a demand cycle.