WebP typically produces files 25-35% smaller than an equivalent PNG while still supporting a full alpha channel, which is why it's a standard swap for site icons, logos, and UI graphics when you're chasing a better PageSpeed or Core Web Vitals score. A small number of older browsers and some CMS media pipelines don't fully support WebP, so it's worth keeping the original PNG as a fallback if you need very broad compatibility.
WebP is really two formats
WebP has a lossy mode derived from VP8 video keyframes and a lossless mode with its own separate compression engine, and both are wrapped in the same file extension. Converting a PNG typically uses the lossy path, which is what produces the substantial size reduction. The important part is that both modes support a full alpha channel, so unlike a move to JPEG this conversion keeps your transparency intact.
That combination, transparency plus lossy compression, is exactly what neither PNG nor JPEG could offer on its own, and it is why WebP took over as the default for site imagery.
Where the savings actually show up
For UI graphics, logos and illustrations the typical reduction over PNG is around a quarter to a third of the file size at visually equivalent quality. On a page with thirty such assets that is a real difference to how quickly the page finishes painting, which is why image format is one of the standard recommendations a Lighthouse or PageSpeed audit will hand you.
Browser support stopped being a concern some years ago and every current browser handles WebP. The remaining friction is elsewhere: some older CMS upload forms, email clients, and desktop applications still reject the extension, and a few image pipelines will silently convert it back. Keep the PNG as a fallback for anything that leaves your own site.