You aren't kidding when you say that "many products use these transformers"! Walk around any American home and you see them everywhere. In my house I found them attached to my printer, scanner, speakers, answering machine, cordless phone, electric screwdriver, electric drill, baby monitor, clock radio, camcorder… you get the idea. A typical home probably has 5 to 10 of these little transformers plugged into the wall at any one time.
It turns out that these transformers consume power whenever they are plugged into the wall, whether they are connected to a device or not. They also waste power when powering a device. If you have ever felt one and it was warm, that is wasted energy turned to heat. The power consumption is not large -- on the order of 1 to 5 watts per transformer. But it does add up. Let's say that you have 10 of them and they all consume 5 watts each. That means that 50 watts are being wasted constantly. If a kilowatt-hour costs a dime in your area, that means you are spending a dime every 20 hours. That's about $44 every year down the drain. Or think of it this way -- there are roughly 100 million households in America. If each household wastes 50 watts on these transformers, that 5 billion watts. As a nation that's half a million dollars every hour or $4,380,000,000 wasted every year! Think what you could do with 4 billion dollars…
Where these small loads really take a toll is in remote locations powered by things like solar cells and wind generators. In these systems you are paying something on the order of $10 to $20 per watt (once you add up the cost of the solar cells, the batteries to store the power, the power regulators and inverter, etc.). 50 watts at $20 per watt means that you have to spend $1,000 more just to power the transformers. In these kinds of systems, small loads are something you avoid by unplugging the transformers when not in use or by eliminating the transformer and powering the device straight from the battery bank to improve efficiency.
However, the added electricity expense is offset by the
manufacturing cost savings passed along to the customer hopefully
as a lower product selling price. For example, it costs a manufacturer
considerably less to manufacture and stock one universal "flavor"
of printer that runs on 12 volts DC. The manufacturer then packages
the printer with a the country-dependent voltage ac wall transformer
for each country. New printers can come out, and the manufacturer
needs no power supply retooling.
Here are several interesting links: