You can use 18w. It's backwards compatible.
Battery capacity 4520mAh, fast charging 33W fast charge, support QC3 \/PD3.0 fast charge protocol
Charge to 100% in 50 minutes and 12 seconds, and can be replaced with 66W of friends
Rear camera 48 megapixel main camera 8 megapixel ultra-wide Angle camera 5 megapixel long focus macro camera, front camera 20 megapixel
Redmi k40 is 33 watts standard charger, can use 18 watts but charge slowly