Trash to Treasure is a less energy-intensive approach for upcycling plastic waste into value-added products to generate sustainable, cost-effective, eco-friendly materials. The carbonization of polyethylene terephthalate (PET) via a single-step process into hard carbon (HC) delivers a high-performing anode material for sodium-ion batteries (SIBs) and potassium-ion batteries (PIBs). Direct pyrolysis of PET waste increases the interlayer spacing with partially closed slit-shaped micro and mesopores that significantly improves the lower potential plateau-based capacity to 68% of the total capacity, validating a battery-type anode material. The HC carbonized at 1000℃ delivers a reversible capacity of 337 mAh/g with capacity retention of 88% after 100 cycles at 30 mAh/g. The HC exhibits 80% capacity retention even after 1000 cycles at a 1C rate and is promising for fast cycling-high power density applications. In PIBs, the HC carbonized at 800℃ performs the best and shows a reversible capacity of 305 mAh/g, with a battery-type low voltage plateau contribution of 32%. The charge storage mechanism in SIBS and PIBs follows a three-stage storage mechanism, i.e., adsorption, intercalation, and pore-filling.