This will see "a select group" of customers on a water meter charged a lower rate than at present during the 'winter', but a higher rate than now over the 'summer'.

‘Summer’ will be classed as the start of April to the end of September, with the ‘winter’ tariff between October and March.

However, the move has left some customers selected for the trial angry, and accusing the company of “trying to impose it out of the blue”.

One told the Packet that the first they heard about the new tariff was when they received a letter through the door, telling them they had been selected – and their request to opt out was refused.

The letter reads: “We want it to feel rewarding for you to use less water. The summer months bring higher temperatures and higher demand, so to help protect our environment and make bills fairer we’re trialling a new seasonal tariff. Your water will be cheaper in winter and more in the summer.”

It goes on to add: “The charges in summer will be around three times the charges in winter.”

The selected customers will be automatically transferred over to the new tariff on October 1 and the trial is expected to last two years.

What will be the new charges?

The letter says the exact charge per litre for each season will be published in February 2025 on the company’s website.

The website currently states that at the moment metered customers are charged a fixed rate, together with a variable rate of £2.07 for every 1,000 litres of water used. From October 1, those on the new seasonal tariff will be charged a winter variable rate of £1.02 for every 1,000 litres.

Further small print in the ‘Household Charges Scheme 2024/25’ document further states that using the differential of three – as identified in the letter – the summer rate would be £3.06 for every 1,000 litres.

It gives tips on “making the most of the new tariff” such as saving rain in water butts, as well as to “save a flush a day”, and “shave 30 seconds off a shower in the summer”.

The letter concludes: “The more people who remain on this trial, the more accurate our findings will be, meaning we’ll be able to bring a fairer way of charging to more people in the south west.”

What customers have said

However, one customer who received the letter told the Packet: "South West Water, being a monopoly supplier of a vital public service, have 'selected me' in differentiation from their other customers to be placed on a potentially more onerous tariff for a period, they say, of two years.

"As I object to the imposition of being used as a sort of lab rat for their experiment I called the telephone number in the documentation to ask to be removed from the experiment - South West Water refused to remove me from the scheme, quoting their interaction with Ofwat.”

This is despite the company stating on its website: “We’ve been careful to exclude customers on social tariffs who may be struggling to pay their bills. But, if you feel like you cannot be part of the trial due to financial concerns, you’re moving house or you have a health condition that would be exacerbated by being on the trial, please let us know by filling out the form below.

“We’ll be in touch to review your situation and how you’ll be impacted by the tariff, and agree with you the best way forward.”

The customer continued: "How can a regulated supplier of a vital public commodity - water - arbitrarily or otherwise force some of its customers but not others in the same circumstances to pay a charge for their water different from the standard tariff, without getting the customer’s consent?

"If the company, as a public service monopoly supplier, wishes to introduce seasonal tariffs for the residents of Cornwall it should publicise the intent properly and treat everybody equally, and not treat some customers differently or experiment on them without getting their consent."

They went on to add: “As a permanent resident otherwise, the only way to save on such a tariff over what we are already doing would be to not live at the house so much in the summer every year, which is impossible - or do they expect us to stop washing, or start digging holes in the garden to avoid using the toilet so much in the summer?”

They accused the company of “trying to impose it out of the blue, without any publicity or public consultation.”

A spokesperson for South West Water said: “Our customers have told us they want us to find new ways of charging and we want to make sure that the bills our customers pay are fair and reflect their usage.

“As part of this work we are trialling two new innovative tariffs from October 1 to understand what works well and what our customers consider to be fair ways of charging.”