It is best to use a 20 watt power adapter. More is allowed. For example, you can use the MacBook's power adapter even though it provides 96 watts of power. This will not damage your device, as Apple automatically adjusts for this. Adapters of 18 watts or less are also allowed, but charging will not take place at maximum speed. For many, this will not be a problem. If you're charging your iPhone while you sleep, speed is not a prerequisite. If you suspect that the Power adapter is broken, you can try another USB-C power adapter, with or without Power Delivery. Or you plug the cable of the MagSafe charger into the USB-C port on your MacBook.
If you think your iPhone is broken, you can easily check this by placing another device on the MagSafe charger. The magnets will always work because they are not dependent on a power source or other technical features. But the iPhone's own power circuit probably does
The typical Apple 12 wireless charge will be fully charged for about an hour 45 to two hours. If your charging time is much more than this value, then get the after-sales battery testing.