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Things start to get more interesting once you've made your way down a long, gloomy corridor and reached the first floor. This open-plan showroom is an Aladdin's cave of solar-powered treasures: racks of glinting panels over here, rolls of purple photovoltaic film over there.

Jeremy Leggett bounds over, extending a huge hand and launching into a whistle-stop tour of the premises. "These are lovely, no question about it," he purrs, gesturing at a shelf full of glass tiles coated with shimmering polychromatic crystals. "Want to see a wholly solar-powered house?"

He dashes across to a photograph of a Victorian terraced home roofed with solar slates that generate more wattage than the owners require. "It's a net exporter of power," he says.

Leggett is the founder and chief executive of Solar Century, a firm that designs and installs building-mounted photovoltaics. In business for just three years, it has captured one-third of the UK market, and is snapping at the heels of sector leader BP Solar.

Open-shirted and scuffy-shoed, he looks like an outback farmer on a trip to town, and talks like a cross between an evangelist and a scientist. In fact, you feel you could connect wires to his sandy head and run a few lightbulbs on his fizzing energy.

The tour continues into a storeroom, where Leggett lovingly opens a slim package just arrived from the Kanaka corporation in Japan. Inside is a stack of wafer-thin transparent sheets. "Thin-film amorphous silicon," he declares. "The first time they've been used in the UK – a 100 kW array for the national indoor athletics training centre in Birmingham".

He moves across the room and whips the cover off a large, sculptural metal object.

"I shouldn't really be showing you this," he says. It's a prototype bus stop for London Transport, topped with a photovoltaic panel that will light up the timetable and dot-matrix display board at night. "Ken Livingstone wants London's street furniture to be solar powered."

Solar will be the single most important contributor to climate change reduction. Investors are queuing up: the market is growing 40% a year globally

Leggett, 48, is an entrepreneur, visionary and eco-campaigner in equal parts. He started out as a petroleum geologist, helping firms such as BP and Shell prospect for oil. After a crisis of conscience, he joined Greenpeace to head its climate campaign, and later advised Margaret Thatcher on environmental issues. In 1999, he changed career for a third time. With £8m in venture capital behind him, he set up Solar Century with the aim of kickstarting the UK's laggardly renewables industry.

His timing was impressive. Earlier this year, the government announced a £20m package of grants for solar buildings and his phones have not stopped ringing. "We're just entering a whole new era," he says. "The level of interest has been huge."

Now we are standing on the roof, admiring the open-air showroom of working solar arrays. It's raining steadily, but Leggett assures me that the panels are still soaking up invisible sunbeams. Do they power the whole building? "Not entirely," he concedes. "It's a 30-year-old, poorly lit office block. But it provides enough to power the computers."

He points across the chimneys to Waterloo Station's vast canopy, which is being refurbished. "It's maddening. They're just putting functionless glass in there. That could be the biggest solar power plant in the UK."

We head back down the stairs to the third floor suite of cushion-strewn rooms he calls his "solar hotel". This is where his 35 staff can relax, pull a book from the stack – Naomi Klein's No Logo, George Soros' The Crisis of Global Capitalism, Leggett's own, definitive work The Carbon War – or catch up on the World Cup. This is also where Leggett, a self-confessed workaholic, beds down during the week. "I'm completely dysfunctional," he says. "I work way too much – most weekends, very early starts, late finishes."

Settling into an armchair, he switches to eco-preacher mode. "Solar will be the single most important contributor to climate change reduction. Investors are queuing up: the market is growing 40% a year globally, so it provides unbeatable returns.

"But this government hasn't yet done enough to encourage high levels of investment in this country. We're close to the bottom [of the international league] at the moment."

The problem with solar, I venture, is that the technology is prohibitively expensive – it can take decades to recoup the initial investment. "We encourage people not to think about payback time," he says. "You don't ask about the payback time of a decorative facade. There are other ways of calculating value."