The system can be seen as a type of transformer, with the power coupler (inductor) forming the primary coil and the gas discharge arc in the bulb forming the one-turn secondary coil and the load of the transformer. The ballast is connected to mains electricity, and is generally designed to operate on voltages between 100 and 277 VAC at a frequency of 50 or 60 Hz, or on a voltage between 100 and 400 VDC for battery-fed emergency light systems. Many ballasts are available in low voltage models so can also be connected to DC voltage sources like batteries for emergency lighting purposes or for use with renewable energy (solar and wind) powered systems.