ICA UPS CS638 Capacity 600 VA, inverter only works when the AC input voltage is low or loss. When the AC input voltage is in normal condition, ICA UPS will stabilize and filter the AC input voltage and supply a clean and stable AC voltage to the ICA UPS output and charge the battery. When the AC input voltage is low or loss, inverter will replace it and supply a stable AC voltage within a very short transfer time.