I don't know about consumer (non)laws elsewhere, but in the US, ONLY if the amplifier is a unit measured and sold independently, then its watt must be measured with actual Volts, Amperes, and Ohms, the distortion must be within limits, and must be sustainable over the entire claimed bandwidth for a defined period of time.
Anything goes as a Watt when it comes to computer speaker systems. It is possible in the U.S. to sell a 50-watt speaker system that runs on 4 "D" cells. Inside you will find two bridged one-wat chips. Even the big brand names do it. The seller is not responsible for any claims. Buyer beware.