The Government's failure to set out its expectations for a flagship home energy efficiency programme is unacceptable, MPs warn.
The Green Deal aims to cover the upfront costs of installing measures of energy-saving technology such as insulation, with the money paid back by savings on fuel bills.
It has been designed to drive energy efficiency as part of efforts to tackle climate change, ensure energy security and help make bills affordable and the Government has also introduced an energy company obligation (ECO) which will pay for energy-saving measures for low-income families.
But the parliamentary Energy and Climate Change Committee has raised concerns that ministers do not have a well-defined set of expected outcomes for the Green Deal, such as the number of homes the scheme would help, in a report released today.
Without the information, it will be impossible to tell if the key policy is a success, the MPs said.
The committee's chairman Tim Yeo says: "Our role is to hold government to account. But it's impossible to do this if the Government itself cannot explain what it is hoping to achieve through its policies.
"It's unacceptable that, three years into the life of this Parliament, ministers are unable to explain what success would look like of one of the coalition's flagship policies. At a time when gas and electricity bills are on the rise, improving the energy efficiency of our homes could not be more important.
"My committee therefore hopes that the Green Deal will be a success. It is only right that such a high-profile policy is subject to proper scrutiny so that corrective measures can be put in place quickly if it is failing to deliver."
Scrutiny is needed on public awareness of the scheme, take-up levels, energy and carbon savings delivered, financial savings and value for money, access to the Green Deal and ECO, customer satisfaction and the supply chain and job creation, the report said.
A number of issues could prevent high take-up of the Green Deal, including the "hassle" of getting work, difficulties for those in rented accommodation and cheaper alternative sources of finance, according to the report.
The Government needs to publish detailed information on progress to ensure that problems can be addressed and the scheme is a success, the MPs said.
Consumer body Which? executive director Richard Lloyd says: "The Government must be clear about what the Green Deal is expected to achieve, and we welcome the select committee's scrutiny to help hold ministers and officials to account on behalf of consumers.
"Rising energy prices are consistently one of consumers' top financial concerns so it is right to help people save money by making their homes more energy efficient.
"The Government must act now to make sure that the Green Deal delivers real savings on energy bills. Any bad practice in selling Green Deal products must be stamped out as quickly as possible."
Energy and climate change minister Greg Barker says: "The Green Deal is an ambitious 20-year programme designed to deliver home improvement in Great Britain on an unprecedented scale.
"Our plans estimate that an initial one million households will have one or more measures installed by March 2015.
“It's far too early in this new market to go further than these projections but the potential in the Green Deal is huge; it is a world away from the old system of monopoly providers and limited choice for the consumer.
"My ambition is for the market to continue to grow and go on to deliver the retrofit of millions more homes by the 2020s."
Although it was early days, he says he hopes the programme will exceed his expectations.
"However, in the early years of the Green Deal we will not be judged by numbers of packages delivered alone, but also by the number and diversity of new market entrants, product innovation and the creation of a far more dynamic, transparent and sustainable market in energy efficiency improvements, with the consumer at its heart," he adds.