Each set of HeatTrak Residential Mats and HeatTrak 120V Industrial Stair Mats requires one HR-P Power Unit to connect the system to an electrical outlet. If you don't buy a HR-P Power Unit for these systems, your mats will not work. This item is not required for HeatTrak Industrial Walkway Mats. The HR-P Power Unit connects your system to a 120V electrical outlet and allows power to flow to the mats. You can connect up to 13 amps on a single Power Unit. If your connected mats (either Residential Mats or 120V Industrial Stair Mats) exceed 13 amps, you will have to split up the mats and add a second Power Unit to plug into a separate outlet. The Power Unit contains a watertight connector that will allow it to connect to the first or last mat in your system. The unit is rainproof and is designed to be left outside for the entire winter season. Power Unit contains a 6 foot cord and inline Ground Fault Circuit Interrupter. If your electrical outlet is further than 6 ft away, consider using a watertight cable extender (see Model No. HR-EXT25).