Solar penetration is steadily increasing to provide power generation as the world turns to clean and sustainable solutions to meet the rising energy demand. Consequently, grid codes and standards are constantly evolving with new requirements such as constant power injection into the grid and power reserve control (PRC) to provide grid support. As a result, existing control techniques in photovoltaic (PV) systems, such maximum power point tracking (MPPT) and flexible power point tracking (FPPT) algorithms, need to be modified to provide such functionalities. Constant power injection, achieved by FPPT, mitigates the problem of solar intermittency, whereas PRC can provide frequency response capability to the grid-connected PV power plants. PRC methods are able to determine the maximum available power which indicates the dispatchable power reserve magnitude while performing FPPT. PRC methods can be broadly classified as estimation and measurement-based. In this paper, a review of PRC methods followed by a comparative discussion on the basis of complexity, ease of implementation, accuracy, and additional hardware requirement of these methods is done.