When PNG is the right destination
Three cases, all about what happens next to the file:
- Editing. Every save of a lossy format loses a little more. Converting to PNG before a round of edits stops that accumulating.
- Software that rejects WebP but accepts PNG, which is most older desktop and office software.
- Transparency matters and JPG is therefore out.
If the image is going straight onto a web page, converting to PNG makes it larger for no benefit — leave it as WebP.
Lossless does not mean restored
This is the point people get wrong. PNG stores exactly what it is given. If it is given a lossy WebP, it stores the artefacts perfectly and produces a bigger file containing the same visible damage.
Convert from the best original you have. If that is the WebP, accept that this is as good as it gets.
Expect the file to grow
A photograph converted from lossy WebP to PNG commonly triples in size. That is not a fault of the conversion; it is what lossless storage of photographic detail costs.
Questions
Does this restore quality that WebP lost?+
No. If the WebP was lossy, the detail it discarded is gone and PNG will faithfully preserve the damaged version at a larger size. Converting to a lossless format prevents further loss; it cannot undo previous loss.
Is transparency kept?+
Yes, exactly. Both formats have a full alpha channel, so a transparent WebP becomes a transparent PNG with no background painted in. This is the reason to choose PNG over JPG here.
Why is the PNG so much bigger?+
Because PNG is lossless and WebP usually was not. You are storing every pixel exactly instead of an approximation. For a photograph the file can easily triple; for a flat graphic the difference is much smaller.