The distinguishing feature of so-called no break or non-interrupting power supplies over ordinary standby or emergency power systems is that no supply interruption (even momentary) occurs when the commercial power source fails or when commercial power again becomes available. This invention comprises a non-interrupting modular power supply system which can be configured for loads of from 15 to 100, watts or for 75 to 500 watts, respectively. The system combination supplies single or multiple regulated DC outputs meeting the requirements of the equipment being operated. A source of AC power is converted to DC for utilization by a load and for charging a back-up battery. When the source of AC power fails, the battery supplies power to the load. Means are provided to monitor the battery voltage to control its charging.