Is PNG better quality than JPG?

PNG is lossless, so it preserves exactly what the decoder produced. That does not add detail the HEIC never had, but it means no further quality is lost, however many times you edit and re-save.

Why is the PNG so much larger?

PNG compresses without discarding anything, and photographs contain a lot of fine variation. Five to ten times the size of an equivalent JPEG is normal.

Should I use PNG or JPG for my iPhone photos?

JPG for sharing, uploading and storing. PNG when the image will be edited repeatedly, or when it contains text or graphics that must stay sharp.

Does transparency survive the conversion?

HEIC photos from a camera have no transparency, so there is none to preserve. PNG supports it, which matters if you later edit the file.

HEIC → PNG · runs on your device

HEIC to PNG, without a second round of compression

PNG stores exactly what it is given. If a HEIC photo is going to be edited, annotated or cropped repeatedly, converting to PNG stops the quality dropping a little further each time it is saved.

Most people converting from HEIC want JPG, and for sharing or uploading that is the right answer. PNG is the better choice when the image is going to be worked on rather than sent: it is lossless, so an image saved, edited and saved again is identical to the one you started with, while a JPEG loses a little more on every pass. Design tools, documentation and anything with text or sharp edges benefit from this.

The cost is size. A photograph as PNG is commonly five to ten times larger than the same photograph as JPEG, because PNG was designed for graphics with flat colour rather than for the fine noise of a camera sensor. For a screenshot or a diagram that penalty is small. For a folder of holiday photos it is considerable, and JPG is usually the more sensible target.

Conversion happens here, on your device. The HEIC is decoded by your browser and the PNG is written locally, so nothing is uploaded and no copy of your photo exists anywhere but on your machine.

Questions

Is PNG better quality than JPG?: PNG is lossless, so it preserves exactly what the decoder produced. That does not add detail the HEIC never had, but it means no further quality is lost, however many times you edit and re-save.
Why is the PNG so much larger?: PNG compresses without discarding anything, and photographs contain a lot of fine variation. Five to ten times the size of an equivalent JPEG is normal.
Should I use PNG or JPG for my iPhone photos?: JPG for sharing, uploading and storing. PNG when the image will be edited repeatedly, or when it contains text or graphics that must stay sharp.
Does transparency survive the conversion?: HEIC photos from a camera have no transparency, so there is none to preserve. PNG supports it, which matters if you later edit the file.