Effortful behavior sustains family life, makes careers, and improves mental health. Effort is a motivational construct that is typically distinguished from physical and mental energy in diagnostic systems. However, overlapping neural substrates pertaining to the dopamine system tie these constructs together. To shed light on this conflict, here we assessed the individual contributions of mental energy, performance feedback, arithmetic skill, and caffeine to effortful decision-making in a novel, low-cost, high-throughput behavioral task that we designed to measure known dopaminergic functions. In this task, participants chose to expend high or low cognitive effort, solving arithmetic questions in working memory, while taking into account the size of reward associated with each choice, and also the likelihood of winning the reward. We found that individuals high in mental energy chose to expend high cognitive effort when the reward probability was high, but that low energy individuals made fewer high effort choices even when the rewards were likely. High energy individuals prioritized feedback from high effort tasks, whereas low energy individuals responded to feedback from low effort tasks. Arithmetic accuracy, a reward contingency, was predicted by an index of arithmetic skill and caffeine consumption. Shared variances between mental energy, arithmetic accuracy, and cognitive effort point to overlapping substrates. Insights gained have important implications for job and academic environments in which mental effort is key to success.