These power saver plugs are a scam that has been going on longer than the Internet. They used to be sold through newspapers and magazines, but are now pushed via modern media platforms with carefully fabricated (but fake) stories about how some young kid invented it, and then defied "big oil" to make it available to everyone so they could save money.

The only active thing in these plugs is the indicator light, and some of the plugs may pose a fire risk or damage your sockets due to bad circuitry and rogue plug sizes.

Some of the websites selling these items may charge you for a multiple of them when you only wanted one, or set up a subscription link to your card as well as taking the card details for other uses.

If you really MUST have one then search on AliExpress for "power saving plug" and get a cheap one for the feelgood factor.
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https://www.youtube.com/watch?v=aQEegthk5bs