Short answer: They don't really work to well.
Long answer.....
Which? magazine reviewed them and concluded that in some cases they increase consumption, but i have my doubts over the quality of the review tbh.
First thing to do is measure what voltage you are getting into your house, you may, if your lucky already be getting a low voltage, approx 220v, then it would be even more pointless having one installed.
Problems with these units include:
Only rated for 8A continuous draw, over this they can't cool themselves and begin to overheat and shutdown, 16A max peak i believe and it instantly shuts down.
As Paddy above said, they do not work for resistive loads or anything controlled by thermostats, its just takes longer to heat up for example, same total energy used (assuming ideal closed system), so this rules out any savings from immersion heaters etc.
Also to have one installed you would need the rest of your house brought upto current standards, e.g. bonding to water and gas, rcd protection, these v phase units are normally sold as an add on to having the fuse box upgraded, so the rest of the work is already being done.
Also they are a pain to fit, need to reorganise the fuse box to take a 50A mcb to supply the v phase unit, then split circuits in the board into mains supply and v phase supply, anything high powered, resistive or thermostat controlled goes on the mains so no saving there.
Main savings are running lights AFAIK, sockets are ok until you plug in something high powered then the unit switches off.
If the savings are as bad as i imagine, say £300 to install (will be more), it could take say 5-10 years until you are in profit, you could maybe save more just changing to energy saving bulbs etc.