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A UPS normally contains an inverter, a 12VDC battery, and a charging circuit. When plugged in to the mains, the UPS allows power to pass through to the load while also charging it's own built-in battery. Then the input power is cut, the UPS instantly switches on the inverter and replaces the pass-through power, from the mains, with power from the battery supplied (via the inverter) to the load. When a UPS is used in a solar circuit, the built-in charging circuit of the UPS is removed and replaced by the charging circuit of the solar panel.