Good-o! British Gas owner Centrica is considering offering free power on Saturdays to nearly one million of its customers. How foolish we were to even think that it had been profiteering and this was just another fat cat power company. How responsive they have been to our complaints. Let's all turn up the heating, and spend our weekends cooking humongous roasts and doing all the washing.

But who will really benefit from this offer? The company, which has made £1.58bn profit in the past six months, is not planning to give back anything. The finer details of next year's Saturday offer have not been made public, but we do know that anyone taking it up will have to agree to pay more for their electricity during the week, meaning there will almost certainly be no advantage for anyone who is house-bound, or who needs electricity to stay warm in the week. Moreover, the deal will only apply to those customers who have had a new smart meter installed. Here lies the real agenda behind Centrica's proposal of "free" power.

Smart meters, which are able to collect accurate readings of energy consumption and feed them back to the supplier or consumer on a minute-by-minute basis, are to be installed at a cost of over £15bn in every home in Britain by 2020. Centrica has been quickest of the energy companies to roll them out. The first generation of new meters will allow companies to dispense with expensive human meter readers and ranks of people in call centres. The next generation, already being trialled, are far more sophisticated, and will allow them to modify household behaviour to save energy, or at least persuade people with financial carrots and sticks to use it on different days of the week and times of day or night.

We watched Big Brother: now we soon may all experience it. Within a few years, energy companies will offer to switch on our washing machines when the cheapest tariff kicks in, or turn off our televisions and fridges when they are not needed. They will allow us to minutely track our energy consumption on our PCs and respond to price changes. Just as the rail companies charge exorbitantly to travel at peak times, so it may cost us much more to boil a kettle at 8am than at 10am, or turn the heating on in the evening, rather than in the day. The carrot of the smart meter is potentially reduced bills and energy efficiency. The stick will be the infinite series of tariffs and myriad new offers the companies will be able to devise to persuade us to use energy when it is most profitable for themselves.

We should beware of their offers. The reality is that some consumers – those who have the time and know-how to be able to switch deals and calculate the real benefits of offers – will indeed be able to save money. But the vast majority of households will probably end up paying more, and the vulnerable, elderly and infirm are likely to be fleeced unless safeguards are built in. Is it really better for an elderly couple to pay more on weekdays and have free power on a Saturday? Few of us will have the time or knowledge to really find out.

Nor should we necessarily trust the government or industry when they insist that smart meters reduce the amount of electricity that is used, thus reducing personal or national climate emissions. People do use less when they see how much they use, but most evidence suggests this is a purely temporary behaviour change and that the savings made in the medium term are minuscule.

We are already paying heavily for these meters, the companies having persuaded a gullible government that they are essential to meet carbon targets and keep bill increases manageable. In fact the technology allows them to take advantage of us all.