The hidden costs of forcing Britain to switch to digital television were revealed yesterday with experts warning that the cost of household electricity bills will rise because of inefficient set-top boxes.

Chris Goodall, an analyst with the strategic consultants Enders Analysis, put the price of the broadcasting revolution at almost £7bn over the course of the next 20 years.

Some £3.5bn of the cost is attributable to consumers' electricity bills because of the increased energy demands of set-top boxes. People living in flats would also be hit hard by the need to upgrade television aerials when the government turns off the analogue broadcasting network.

He described the forced switch over to digital - now expected to take place in 2012 - as "an illiberal policy" that would cost far more than the government had admitted, hitting the poor and vulnerable in particular.

Mr Goodall revealed that many of the set-top boxes are not compliant with the European Union energy codes and consume far more electricity than viewers might expect.

He said that the cost of installing or upgrading aerials for households that presently have inadequate devices for digital reception would be £865m. About 1% of homes may not be able to receive digital television unless satellite aerials were supplied, and around 12% would require new aerials, he said.

Many people living in flats, which account for about a fifth of all households in Britain, would need to have their aerial system upgraded; 30% of households use only portable aerials and some of them will need a rooftop antenna.

Mr Goodall also said that helping the old and vulnerable to install new digital equipment could cost between £250m and £400m, on the assumption that much of the assistance would be provided by voluntary organisations, without charge.

Another £2.1bn would be spent over 20 years on a public-service publisher, which Ofcom, the media watchdog, sees as a way of topping up public-service broadcasting.

He was speaking as part of a panel with Professor Patrick Barwise and Jeremy Kline, a senior consultant at the research group Generics, at the Oxford Media Convention yesterday.

Mr Kline said that research he had carried out for the Department of Trade and Industry showed that only 38% of people agreed with the idea of analogue switch-off. Of those who already had digital television, just 50% agreed with the process which the government is hoping to complete by 2012.