Background - Grid Voltage and Harmonised Standards with Europe
Since electricity supply standards were harmonised with Europe in 1988, it is law that the electricity supply to your house should provide 230 volts give or take, +10% -6%. That's a huge range, from 216.2 and 253 volts. Before the harmonization of standards, the nominal voltage was 220V in Europe and 240V in the UK. Nothing on existing networks has changed though. The Uk grid is still 240V, parts are even operating at 250v due to legacy equipment and this is deemed to be within the acceptable range. This way of creating a common standard allows new equipment and networks to be designed and installed to the new standard, whilst not requiring any expensive changes to power networks across Europe.
Matching supply to equipment
The upshot of this is that although all CE marked electrical equipment is designed to work at UK voltages, it is not necessarily designed to work efficiently or provide an optimum service life. Electronic equipment tends to be manufactured for the European market as a whole and as such is designed for 220V. This is where fitting a voltage optimiser can help. By stepping down the voltage in your building by around 20 volts, your supply and equipment are better matched. No electricity is wasted by equipment that's poorly suited to the supply and the service life of that equipment may be extended.
This all depends on what equipment is in use in a given installation. The savings that can be made typically come from electronic equipment such as computers, fluorescent lighting electronic ballasts, LED lighting, and motors with variable speed drives. These are known as voltage independent loads. For other load types the picture is more complicated. For restive loads such as an electric heater or a kettle that use electricity to heat an element, when we turn down the supply voltage we will also turn down the power output. This will not save electricity, the kettle will just take a little long to boil but the kettle itself may last longer as it's not working quite as hard but it may have a longer life as a result. An office is the perfect place for a VO unit to realise sizable savings. Any industrial setting would need very careful consideration due the impact of lowering voltages as it may effect motorised processess
For the average household?
Research conducted into the VPhase VX1 voltage optimisation unit registered with the energy regulator OFGEM, is large and detailed enough for me to confidently shed some light on the matter. The report found average electricity savings in domestic houses of 5%. HOWEVER, The savings made across the study were found to be very variable, as high as 19% but as low as -4%, (yes, MINUS!) The takehome message here is that voltage optimisation units in domestic installations can save electricity, but it's very hard to give a specific figures because each installation is so different. I could not find anything in the report to explain the instances of high savings rates so can't say confidently who would be likely to see a worthwhile benefit. (click here to view the full report)