A former NASA scientist who experimented with solar panels and cells cannot axe a mature oak tree in his Mawson garden which he says blocks sunlight from his $10,000 solar panels.

Dharma Sharma's expertise in energy efficiency in Australia and India and while working on satellite experiments did not sway the ACT Conservator of Flora and Fauna, nor the Civil and Administrative Appeals Tribunal, who reconsidered and confirmed the conservator's decision.

Dr Sharma said the oak, in the north-west section of his yard, threw a partial shadow over his solar panels for most of the day during winter and blocked sunlight during summer afternoons, which interfered with solar generation.

In summer he lost about six hours a day of production. In winter he lost about three hours a day.

Dr Sharma, 69, and his wife suffer from arthritis so he had installed a 5kW solar system on his north-facing roof to offset an annual heating bill of $1600 a year.

About the same time his neighbour, Sam Leone, installed two 5kW systems on his west-facing roof.

Dr Sharma, a nuclear technology, physics, science, chemistry and mathematics scholar, calculated he should have been generating 1.4 times more power than Mr Leone's units and presented data to support his case.

Not long after he moved into his house it rained, flooding it, and he had discovered tree roots in pipes which he replaced for $3000.

In 2008 he had to pay ACTEW $1500 to prune the tree and would have to pay ACTEW $450 to cut the power before a contractor could safely prune the tree, which also dropped leaves into his gutters.

An injured right shoulder prevented him from climbing on to the roof to remove branches and clean gutters and his sons, who previously maintained the roof, had left town.

The tribunal found nothing within the Tree Protection Act, which aims to protect exceptional trees in the urban landscape, would support damaging the big oak. A tree of this nature can only be removed if it is causing an unacceptable risk to public and private safety, or is damaging a building or service or blocking sunlight to someone's land during winter between 9am and 3pm.

Allan Moss, a horticulturist for 40 years, inspected the tree and believed at least two-thirds of sunlight would get through its canopy in winter.

Mr Moss said the oak's 23 metre-wide canopy provided an economic benefit in summer, shielding the home from the west.

Given its healthy state, he reckoned it would stand for 100 years.

According to the tribunal, pruning trees is a normal part of garden and property maintenance, as is removing fallen branches from roofs and leaves and other debris from gutters and downpipes.