It's a better question than it appears at first. Consider the amount of energy consumed by data centres, the proportion of data held and transferred in graphic formats and the increasing use of png. The effect on power usage in data centres and communication systems will be an important consideration for the decision makers. The end user's energy use, probably less so. The energy requirement at both ends will be affected by the level of compression. There's a good discussion about this here.
Modern computer devices are quite capable of dealing with the rendering without too much power usage. I would say that, having gained wide usage, this is a good format. I do most of my PC work on a "green" machine with a 10 Watt motherboard which is low in comparisons with most desktops. Large .jpgs are noticeable slow. I haven't noticed any problems with .png files and I have come to use the format routinely. Very large graphics in any format can slow things down as can uncompressed formats like bitmaps (.bmp).