A wearable device with an energy harvesting circuit calculates a trendline based on a plot of multiple calorie amounts, each calorie amount associated with an energy amount. Each calorie amount is based on one or more sensor measurements over a particular time period corresponding to a fitness activity from one or more sensors of the wearable device. Each energy amount is an amount of energy produced by the energy harvesting circuit during the time period corresponding to the fitness activity. The wearable device uses the trendline to determine how many calories the user should burn in order for the energy harvesting circuit to produce enough electric charge to charge the wearable device to a predetermined battery charge level and outputs a user alert based on this amount of calories.
