Photoplethysmography (PPG) is used to determine blood oxygen saturation and heart rate from light reflected on patient\'s skin. Both health metrics are useful in detecting COVID-19 in asymptomatic patients but remains impacted by individual physiological differences. For wearable self-powered devices, PPG sensing is relatively high-power compared to available on-body energy. This paper presents a PPG sensing model to demonstrate circuit design tradeoffs, the impact of a patient\'s skin phototype on sensing accuracy and measured results from on-body testing.