Power factor correction has been used for years when high inductive loads are used and with the amount of transformers used today I would think most houses don't run at unity and a small saving could be made.
However they are designed for US at 60Hz plus 110 volt and I would be very careful buying anything which did not say exactly what it is which no real specifications as to how much it can correct.
Most items have built in power factor correction in this country the florescence light you will find already has a capacitor built into it.
Maybe in the USA they have cheap versions without this correction built in?
And I would be very surprised if you got anything like 40% saving on domestic more like 2% if lucky.
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