Microbes are the most abundant living forms on earth and major contributors to the biogeochemical cycles. However, our ability to model their dynamics only relies on empirical laws, fundamentally restricting our understanding and predictive capacity in many environmental systems. Preeminent physicists such as Schrödinger and Prigogine have introduced a thermodynamic interpretation of life, paving the way for a quantitative theory. From these seminal contributions have emerged microbial thermodynamics, which allows the prediction of microbial energy allocation. Nevertheless, the link between energy balances and growth dynamics is still not understood. Here we present a microbial growth equation relying on an explicit theoretical ground sustained by Boltzmann statistics, thus establishing a relationship between microbial growth rate and available energy.