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Drop one HEIC or two hundred. Batch conversion is not a paid feature here.
An iPhone photo is going into a design tool, a document or a print job where a second round of JPEG artefacts would show.
The HEVC still is decoded once and stored as PNG, which is lossless. No new compression artefacts are introduced, and nothing is thrown away after the decode. Expect the file to grow several times over.
PNG is lossless but the HEIC it came from was not, so the artefacts already baked into the original are preserved perfectly rather than removed. PNG also drops the depth map an iPhone portrait shot carries.
JPG for anything you send or upload, because a photo in PNG can be ten times the size. PNG only when the image will be edited and re-saved repeatedly.
PNG has no EXIF block in the way JPEG does, so camera and GPS metadata is not carried across in the same form. Convert to JPG if you need that data intact.
Drop one HEIC or two hundred. Batch conversion is not a paid feature here.
Portable Network Graphics is lossless, so nothing is thrown away in the conversion.
Converted and deleted the moment it finishes. Never written to permanent storage.