An apparatus, system, and method are disclosed for determining power source failure. A sampling module samples an alternating current power waveform as input to a power supply, at a sampling frequency which is a multiple of a predetermined frequency, to obtain a sampled amplitude at a known point within the predetermined period. A comparison module compares the sampled amplitude to a stored threshold amplitude to obtain a comparison result, corresponding to the known point. An accumulation module accumulates most recent comparison results. A warning module asserts an early power off warning signal if a predetermined number of the most recent comparison results each indicate that the sampled amplitude is smaller in absolute value than the stored threshold amplitude.