The place to look, of course, is the legislation framework.
When it comes to income tax declaration (to HMRC) the answer appears to be that the GENERATION tariff for a normal domestic user, it is not income. But when it comes to the EXPORTING electricity tariff, this is income and should be declared. But I'll wager that few people realise this or do it.
The evidence for the above is buried deep in this document.
http://www.hmrc.gov.uk/manuals/bimmanual/bim40510.htm
This question appears to be about one's income in respect of some Council tax benefit. I would expect the 'rules' to be the same as HMRC's definition of solar power income.