IT'S not easy being green—nor is it cheap. With the best will (and some of the most generous handouts) in the world, solar power still makes little sense for the average homeowner, even in sunny southern California. Under pressure from his environmentally conscious ten-year-old daughter, your correspondent has spent the past week talking to experts around the state and running the numbers to see if he could reduce Mayhem Manor's carbon footprint.

Solar power ought to be the answer. The house faces south-east, lacks trees or other shadow-casting obstructions, and its flat roof offers ample space for a sizable solar array. At 720 feet up the hillside, it is well above the “marine layer” (the locals' fancy name for morning fog) and gets about 300 sunny days a year. So what's the problem?

It's not even as though the place gobbles electricity. When the house was being rebuilt five years ago, the new roof came with over a foot of thermal insulation. The floor-to-ceiling windows along two sides of the structure were replaced with double-glazed “low-E” glass (the sort that blocks infra-red radiation), and thermal linings were included in all the exterior walls. Even during the summer, the air conditioner usually stays off.

Admittedly, the architect went overboard on lighting. Fully illuminated, the house demanded seven kilowatts of raw lighting-power before fluorescent lights replaced thirsty tungsten filaments. Overall electricity consumption is now a reasonable 8,300 kilowatt-hours (kWh) a year.

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But does it come in mauve?

Given the local utility's rate of 10.8 cents per kWh, that adds up to a modest $900 for the year—or just $2.50 a day. At those prices, solar energy simply cannot compete with juice from the local power-station, even with California-level subsidies.

The problem is that solar-energy technology has been improving incrementally, but its costs have been falling slowly.

If solar cells had abided by Moore's Law, they too would have halved in price every 18 months or so—and we would all be running our homes on sunshine. But getting photons from sunlight to dislodge more and more electrons in semi-conducting materials like silicon, and so generate electricity, is harder than building a better microchip.

The first solar cells—built more than a century ago—had conversion efficiencies of around 1%. Since then, their efficiency has doubled once only every 30 years—a veritable snail's pace compared with the speed of microchip development.

It was not until the 1950s, for instance, that America's famed Bell Labs stumbled on a way of boosting a solar cell's conversion efficiency by “doping” its silicon with certain impurities. It then took another 50 years to raise the efficiency to nearly 20%.

The performance of solar cells has picked up recently. But that's only for the most exotic cells used in space. Today's satellites have solar panels based on thin films of gallium arsenide that boast efficiencies of over 35%. Meanwhile, in the laboratory, exotic “quantum wells” promise photovoltaic conversion efficiencies of 45% or more.

But the solar panels used in space cost millions to make and last for a decade at most. Back on earth, the only ones affordable enough to be used commercially are early models based on crystalline and amorphous silicon with efficiencies of around 15%.

And even these aren't exactly cheap. Sanyo's 200-watt module, one of the better panels used by the industry, offers 17% efficiency and costs $1,500 retail.

As a rule of thumb, the industry reckons that a solar panel capable of generating one kilowatt of power at peak times will average roughly 20% of that over the whole day. In other words, every kilowatt of installed capacity should be good for 4.8 kWh of daily consumption—or around 1,750 kWh per year. By that reckoning, Mayhem Manor would need 4.8 kilowatts of solar capacity to be able to generate the amount of electricity normally consumed from the grid.

Unfortunately, that ignores all the losses that occur between the sun's rays striking the solar array and that direct current being converted into alternating current to run the house. Such losses can easily mop up 25% of the solar panel's output. So, better install at least 6.4 kilowatts worth of solar panels on the roof.

Here's where going green gets tough. At today's prices, your correspondent would have to stump up $48,000 for the solar panels alone. Add the cost of the switching modules, the power controller, the fault protector, the DC-to-AC inverter and the service panel—not to mention the installation charges and the contractor's profit—and the final bill could easily come to $65,000.

What about incentives and tax credits? That depends on where precisely you live and how effective an installation you have. To get anything like a full grant in your correspondent's neck of the woods, the array would have to be facing due south and tilted at an angle of 34 degrees to the sun. The first might be possible; the second would definitely not. At best, Mayhem Manor would qualify for about $12,000 worth of local assistance plus a $2,000 federal grant.

Borrowing the balance at today's interest rates would mean repayments of roughly $600 a month for ten years, even after setting the interest charges against tax. And all that just to feel good about saving $75 of electricity a month. Better to buy a couple of tons worth of carbon offsets each year for $70 and have done with it.