iv installed one of these that a customer supplied about a year ago.
it reduced voltage to 220v, after speaking to him he belives he saves around 9/10% on his electric bill.
based on this you customer should save around £170 a month if his bill is £1700.
i think it would be worth fitting one, they cost around £300ish so will it would have payed for its self in 2 months.
not sure if it could work if he had a 3phase supply though. its also worth noting that his heaters took a little longer to heat up.
i would fit one to my house but my bill is only about £20-£25 a month so it would take a long time to pay for itself.