What is the payback on solar panels producing 3,400kWh pa with SEG?

At 5p per exported unit and assuming you use 30% of what your panels produce, your excess of 2,380kWh pa will provide £118 income. Your total savings will now be about £460 pa and your payback time reduces to 13 years. If you use more than 30% in your home the time is less.