I can't help but be cynical when I see percentage saved quoted as typical. My retort is to say most of my electric bill comes under Economy Seven, and pay for the heat provided, this energy measured in KW hours.
If the device lowers the voltage, the rate of heating drops, and my thermostats extend the proportion of time the thermostats stay on.
By law the electricity I pay for is proprtional to the energy provided, and it makes no difference if that is measured in watt hours. or calories, or RMS volts and amps.
However, a reduction in volts could make my lighting dimmer, my dishwasher, washing machine, fridge and freezer less efficient.