Diary Of An Eco-Builder

Photovoltaic technology is some way from being a commercial success. but it's made us self-sufficient

Up in the crown of the tree, each fragile leaf spreads wide its five fingers under the October sunshine, creating sugars for new growth. This process is quietly miraculous, for the entire towering structure of the tree is self-built using these flimsy solar collectors.

To my delight, the canopy of Tree House has finally caught up with the tree that it seeks to emulate.

All last week a team from Solar Century (www.solarcentury.co.uk) was busy installing 30sq m of photovoltaic (PV) modules on our roof. On Friday, Trevor and Guy connected our inverters to turn the modules' DC output to 240V AC and the final switch was thrown.

It's taken a long time to get here but the moment is sweet: from this point on, we plan to consume no more power within our narrow Clapham boundaries than our glittering solar roof can supply.

At the beginning of the project, the goal of energy self-sufficiency seemed beyond our reach.

The site was small and constrained and many PV suppliers suggested that a rooftop array could typically cover no more than half of the annual power demand. But after some serious research, reflection and arithmetic, the possibilities of an atypical home became clear: an ultra-efficient house with a big southern pitched solar roof could go all the way, supplying not only our lights and appliances but also the power needed to drive our solar thermal panel and heat pump (for hot water and central heating). The goal is tough, but we are one big step nearer to achieving it.

PV is an extraordinarily under-developed resource in the UK. This is odd, given that PV modules are silent, emission free, unobtrusive, low maintenance, reliable and easy to integrate into buildings of all sizes. The main problem has always been cost: no one currently installs a PV array on their roof on the basis of a payback calculation.

The cost of PV has fallen but we could not have fulfilled our ambition without a 50% government grant (contact the Energy Saving Trust for details, www.est.org.uk).

We are also fortunate to have financial support from Halifax and Bank of Scotland, who are keen to promote good environmental design in the development of homes old and new.

Arguably, PV is a technology still lingering in the "Valley of Death", the rocky terrain between technology development and large-scale commercial success that is notoriously hard to cross. Elsewhere - Germany, Japan, the Netherlands - governments have been quicker to propel PV towards the verdant uplands, often by paying domestic PV generators artificially high rates for the power they produce.

In Britain we won't be making a killing from our rooftop power station, although having zero energy bills at the start of a long-term rise in energy prices is no bad thing.

There's little point in investing in PV if you haven't done all the other energy efficiency jobs first. But if, like us, you want to travel somewhere beyond the typical, PV is the renewable power source with the most universal application.

Unlike the philosopher David Hume, I accept that the sun will not only rise but set every day, and hence that PV can only be part of the answer. We will be grid-connected, exporting power in the day and importing at night.

But as the answer to our planet's fossil fuel dependency is needed with some urgency, every little fragment of renewable energy capacity is invaluable.

Today, Tree House is exceptional. One day, it will be an old oak amid a city-wide forest of environmentally benign solar buildings. I hope that we are around to see the day: a couple of hoary old woodsmen savouring the moment with long-awaited satisfaction.

www.treehouseclapham.org.uk