If your project is lacking a C wire and have at least four wires at your thermostat to work with, you’ll want to install the Power Extender Kit (PEK). The PEK gets installed at the furnace control board and is wired in with your existing thermostat wires to create a C. 
ecobee Power Extender Kit (PEK)
If your project is lacking a C wire and have at least four wires at your thermostat to work with, you’ll want to install the Power Extender Kit (PEK). The PEK gets installed at the furnace control board and is wired in with your existing thermostat wires to create a C. 
Easy Installation 
PEK gets installed at your furnace control board and is wired in with your existing thermostat wires to create a C.
ecobee Thermostat Compatibility:
ecobee SmartThermostat with voice control 
ecobee SmartThermostat Pro with voice control 
ecobee4
ecobee4 Pro
ecobee3 lite 
ecobee3 lite Pro 
ecobee3