That’s because households liable to pay extra tax must file a self-assessment tax return by midnight on January 31. Those who fail to turn in and pay their return on time could be hit by a fine.

And many solar panel owners now make profit from their rooftop installations by selling energy they haven’t used back to the grid.

Mr Penny explained that many of the UK’s 1.6M solar households make over £300 a year by selling excess solar energy through the Smart Export Guarantee (SEG).

This means more than 600,000 solar panel owners could be exceeding HMRC’s tax free earnings thresholds and will need to file and pay their owed tax or face a £100 late fine.

He said: “After a record-breaking year for solar installations in 2025, around 1.6 million UK households now have solar panels, with more homeowners than ever earning money by selling unused electricity back to the grid.

“While households can earn £300 or more a year through the Smart Export Guarantee (SEG), those that earn additional income on top of this through side hustles can quickly exceed HMRC’s £1,000 tax-free trading allowance, creating a legal requirement to declare the income.

“Based on HMRC self-assessment figures and data that shows around 39% of Brits have a side hustle, an estimated 605,966 solar panel owners are likely to submit an online Self Assessment return by 31 January. However, HMRC figures show that around 9% of taxpayers miss the deadline annually, meaning more than 54,500 solar households could face an automatic £100 late-filing fine, which could effectively wipe out a third of annual earnings from SEG payments.

“Ahead of the deadline, it’s vital that solar panel owners check exactly how much they’ve earned,  including SEG payments. If total income exceeded £1,000 between April 2024 and April 2025, it must be declared to avoid penalties.”