Diary Of An Eco-Builder

Even with a green electricity tariff you have to worry about the power you use

How many builders does it take to fit a light bulb? If you don't have a power connection, there's one to dig the trench, one to dig the hole in the pavement, one to connect the cables, one to fill the hole, one to fit the meter, one to connect the meter to the consumer unit, one to certify the connection, one to install the lighting circuit and one to throw a complete wobbly when the bulb fails to light.

How many builders does it take to fit a light bulb? If you don't have a power connection, there's one to dig the trench, one to dig the hole in the pavement, one to connect the cables, one to fill the hole, one to fit the meter, one to connect the meter to the consumer unit, one to certify the connection, one to install the lighting circuit and one to throw a complete wobbly when the bulb fails to light.

Each of these events has to be carefully scheduled: if you miss one you're in trouble. Although our power supply should have been operational this week, an unexpected school run meant that foreman Steve missed the appointment with the connecter and everything else unravelled.

Without this connection, Steve is stuck with our noisy on-site generator and regular trips to the petrol station with his jerry-can. Not that power from the Grid is likely to be any more eco-friendly than diesel, so the next stage of the construction of Tree House may well be nuclear-powered - low carbon, perhaps, but as tricksy for future generations as any fossil fuel.

Happily, within a few weeks the photovoltaic panels on our south-facing roof will be in place and our little urban power station can be switched on, providing 5kW of solar power for the last stage of the build (see www.solarcentury.co.uk). In practice, however, a lot of the power generated on our roof will be surplus to Steve's requirements and duly exported (as it will be on summer afternoons even when the house is occupied). The power from little urban generators like us never gets anywhere near a high voltage transmission line and is simply soaked up locally, so whatever deals our neighbours may have with their electricity suppliers, their washing machines and day-time telly will actually be driven by our very first eco-friendly electrons.

The business of electricity generation, purchasing and consumption is a bit mind-boggling, not least because electricity is so inscrutable. The key point to bear in mind is that you don't pay your electricity supplier for the actual electrons you use; you pay them to put power into the Grid to match your consumption. They cut deals with the electricity generators within a complicated trading system that is designed to meet national demand through forward purchasing. So if you have a 100 per cent green electricity supplier, such as Ecotricity or Good Energy, you are paying them to buy power from renewable generators; what you actually turn into heat, light and Big Brother remains cloaked in subatomic secrecy.

Whatever you do, if you switch to a green electricity tariff don't assume that you no longer have to worry about how much power you consume. If you start leaving the lights on, this extra load will almost certainly be met by burning extra coal, gas or uranium, especially at times of peak demand. Currently the renewable energy generation capacity in Britain is very small, so although it's good to buy into it and support its expansion, squandering green energy is just as bad as wasting old-school dirty power.

A future-friendly power infrastructure is only achievable if we complement renewable energy with radical energy efficiency. Tree House is an expression of this principle writ small. I knew we would never be able to generate enough heat and power on our little London plot to meet the typical energy demand of a 2.8 person family (two men, four sedentary cats). Our goal of generating more energy over the year than we consume is only achievable because our roof-top power station will sit above an exceptionally energy-efficient house.

I hope that makes some sense. I'm aware that electricity can seem about as intelligible as reality TV: a mysterious energy source for a mysterious world.