Reconfigurable intelligent surfaces (RIS) can enhance wireless power transfer efficiency in communication networks by steering electromagnetic waves from a transmitter (TX) toward zero-energy devices (ZEDs) for charging their batteries. In this letter, we use the Lyapunov optimization framework to develop an algorithm that dynamically adjusts the TX power and RIS phase configuration based on the data queue lengths at the ZEDs. This approach aims to maintain queue stability while minimizing the average TX power. Our simulation results demonstrate that the proposed method provides stability and reduces the average TX power, whereas the queue-agnostic benchmark fails to achieve that even with much higher TX power.