The Office of the Renewable Energy Regulator has told a Senate estimates committee hearing that of 1841 inspections of roof-top solar panels to the end of January, 4 per cent were deemed unsafe and immediately shut down.

The information came as the Climate Change Department conceded, in the same hearing, that the budget revenues projected by the government carbon pricing package could be lower if the floating carbon price traded around the $15-a-tonne floor price in the package.

But the compensation under the scheme would also be lower if the carbon price was lower.

The questioning by South Australian Liberal senator Simon Birmingham was sparked by continued weakness in the price of emissions units in the EU scheme. After trading around $20 in the middle of last year, the market has been hit by the European economic slump and is now around $9.

But when the Australian carbon price floats it will be prevented from falling lower than a floor price of $15 with international permits subject to a top-up payment if their price is lower.

Senator Birmingham asked whether a carbon price trading around a floor price of $15 when the scheme moved to a floating price in 2015-16 would produce significantly lower revenues than the $9.2 billion projected at the fixed price of $25.40 a tonne in 2014-16.

But Climate Change Department secretary Blair Comley cautioned that “costings are never quite as simple as they appear” and that Treasury was responsible for economic forecasting. Department officials are set to fly to Europe for meetings with EU officials before a vote next month that will debate raising the EU’s emissions reductions targets in a move that would in part be designed to restore the carbon price.

Asked if international and domestic prices were likely to be lower than $15 in 2015, Mr Comley said: “I don’t think that’s true.”

In later evidence on solar-panel inspections, Andrew Livingston, from the office of the Renewable Energy Regulator, said a further 1368 inspections were planned for this year and state authorities had been notified of faults. He said some states had suspended installers over the results but most installers and electricians had been “very prompt” in rectifying faults.

Where a solar installation was found to be faulty it was immediately disconnected, he said.

The move came as the hearing was told that the government was chasing more than $21 million from installers who had wrongly claimed money in the government’s home installation scheme.

The department had collected $844,000 of this amount and written off $9200. It was in negotiations with a debt collector about the outstanding funds.