An Uninterruptible Power Supply, UPS, system, the UPS system comprising: an input configured to receive input power; an output configured to provide output power to a load; power conversion circuitry (512, 516) coupled to the input and the output; a capacitor (518, 520) coupled to the power conversion circuitry; and a controller coupled to the power conversion circuitry, the controller configured to: determine a first value indicative of a first equivalent series resistance of the capacitor during a calibration period; and identify a baseline equivalent series resistance based on the first value; determine, subsequent to the calibration period, a second value indicative of a second equivalent series resistance of the capacitor; characterised in that the controller is further configured to determine if the second value exceeds the baseline equivalent series resistance by at least a first threshold amount, the first threshold amount being a first multiple of the baseline equivalent series resistance; execute, responsive to determining that the second value exceeds the baseline equivalent series resistance by at least the first threshold amount, a first set of one or more actions to address degradation of the capacitor; determine, subsequent to determining the second value, a third value indicative of a third equivalent series resistance of the capacitor; determine if the third value exceeds the baseline equivalent series by at least a second threshold amount, the second threshold amount being a second multiple of the baseline equivalent series resistance, and the second multiple of the baseline equivalent series resistance being greater than the first multiple of the baseline equivalent series resistance; and execute, responsive to determining that the third value exceeds the baseline equivalent series resistance by at least the second threshold amount, a second set of one or more actions to address degradation of the capacitor, the second set of one or more actions being different than the first set of one or more actions. an input configured to receive input power; an output configured to provide output power to a load; power conversion circuitry (512, 516) coupled to the input and the output; a capacitor (518, 520) coupled to the power conversion circuitry; and a controller coupled to the power conversion circuitry, the controller configured to: determine a first value indicative of a first equivalent series resistance of the capacitor during a calibration period; and identify a baseline equivalent series resistance based on the first value; determine, subsequent to the calibration period, a second value indicative of a determine a first value indicative of a first equivalent series resistance of the capacitor during a calibration period; and identify a baseline equivalent series resistance based on the first value; determine, subsequent to the calibration period, a second value indicative of a second equivalent series resistance of the capacitor; characterised in that the controller is further configured to determine if the second value exceeds the baseline equivalent series resistance by at least a first threshold amount, the first threshold amount being a first multiple of the baseline equivalent series resistance; execute, responsive to determining that the second value exceeds the baseline equivalent series resistance by at least the first threshold amount, a first set of one or more actions to address degradation of the capacitor; determine, subsequent to determining the second value, a third value indicative of a third equivalent series resistance of the capacitor; determine if the third value exceeds the baseline equivalent series by at least a second threshold amount, the second threshold amount being a second multiple of the baseline equivalent series resistance, and the second multiple of the baseline equivalent series resistance being greater than the first multiple of the baseline equivalent series resistance; and execute, responsive to determining that the third value exceeds the baseline equivalent series resistance by at least the second threshold amount, a second set of one or more actions to address degradation of the capacitor, the second set of one or more actions being different than the first set of one or more actions. determine if the second value exceeds the baseline equivalent series resistance by at least a first threshold amount, the first threshold amount being a first multiple of the baseline equivalent series resistance; execute, responsive to determining that the second value exceeds the baseline equivalent series resistance by at least the first threshold amount, a first set of one or more actions to address degradation of the capacitor; determine, subsequent to determining the second value, a third value indicative of a third equivalent series resistance of the capacitor; determine if the third value exceeds the baseline equivalent series by at least a second threshold amount, the second threshold amount being a second multiple of the baseline equivalent series resistance, and the second multiple of the baseline equivalent series resistance being greater than the first multiple of the baseline equivalent series resistance; and execute, responsive to determining that the third value exceeds the baseline equivalent series resistance by at least the second threshold amount, a second set of one or more actions to address degradation of the capacitor, the second set of one or more actions being different than the first set of one or more actions.