Power and energy balances are described for simplified gravity-driven multiphase flows. It is emphasized that the power (-> Watts) or energy input by gravity impulse must be found back as the sum of all power or energy consumptions from the flow processes (-> Application). Examples are given for bubble columns and for batch liquid-liquid separation processes, based on simplified assumptions regarding the flow behavior. For large-scale apparatus, the flow characteristic of the involved phases is more complex and needs to be modeled more precisely. That is still a key challenge for scaling-up multiphase systems. Computational fluid dynamics is an adequate instrument but needs to be further developed and enhanced for multiphase flows in large-scale processes. And of course, any rigorous model must then be verifiable by a power balance.