Dear Mr. dik (Structural)(OP)6 Nov 21 17:43
"...Does this device advertised on Eng-Tips really work... or just more advertising stuff?... I thought it might be snake oil, but wasn't certain..."
1. It is certainly a "snake oil".
2. There is NO logic at all, that you can cut your electricity bill by 90%. You can't cut a single cent from the bill. You pay what you consume.
3. I had seen it in Singapore with my own eyes that a "snake oil" vendor demonstration; showing a significant decrease in current, with the device plugged in. Note: it is a amperemeter NOT a wattmeter . The current decrease is due to improvement of the power-factor. The wattage remained the same, but was NOT shown. The billing is based on the wattage x time (kWh).
4. Note: In most cases, for (low voltage small domestic consumers) billing, there is [no penalty on low power-factor]. There is no incentive to improve it with any devices on your own expenses.
Che Kuan Yau (Singapore)