This paper addresses the use of inductor current estimators in the multiloop control of uninterruptible power supplies (UPS), assuming that only the output voltage is available for feedback and taking into account the robustness against load variations. Resonant and proportional controllers are assumed to regulate the voltage and inductor current loops, respectively. Conditions to independently design estimator and controller gains are provided, allowing the use of the proposed estimator in preexisting control schemes. Closed-loop stability under estimated current feedback is assessed a posteriori. Simulation results for a single-phase 3.5 kVA UPS show compliance with the IEC 62040 standard.