Convert a PNG to JPEG when it contains photographic detail, the destination accepts JPEG and transparency is not needed. Keep a suitable lossless format when crisp text, diagrams or exact pixel reproduction matter. There is no fixed saving: the result depends on the source, dimensions, chosen quality and encoder.
Check what the PNG contains
PNG uses lossless compression and can carry transparency. JPEG is normally lossy and cannot preserve an alpha channel. A photograph stored as PNG may be a good conversion candidate, while a screenshot containing tiny text can become less readable or even larger as JPEG. The extension alone does not tell you which choice is best.
Choose the background before converting
Transparent pixels need an explicit background for JPEG. Use the color intended for the final placement, then inspect semi-transparent shadows and antialiased edges. A logo that looks fine against white can show a visible rectangle on a dark page. If the background must remain flexible, choose a format that retains transparency instead.
Compare quality from the same source
Create separate candidates from the original PNG, keeping their dimensions fixed. As an illustrative test, try two JPEG quality levels in the same encoder and inspect skin texture, gradients, fine labels and edge halos. The numeric setting is not a percentage of retained information and cannot be compared reliably across different encoders.
Validate the delivered copy
Open the exported file in the receiving application, check its actual type and compare its bytes with the source. Do not repeatedly save the already-compressed JPEG to make further candidates. Preserve the PNG when it is your editing source, and keep the original delivery file if the conversion does not improve the result. A smaller photograph and a readable diagram are different optimization goals.