A trip to London in February gave me a great slogan for the future: Mind the Gap! I must have heard that dozens of times in one week riding the famous London Underground, the “Tube”. Designed to warn riders getting on and off trains to pay attention to the couple of inches’ space between the train and the platform, Mind the Gap! is a phrase both economical and effective. Posted signs said that only one person a week was injured last year getting on or off trains. That’s amazing, given the hundreds of thousands of riders each week and the crush of people getting on and off in just a handful of seconds. So, “Mind the Gap!” announcements must be working.

I attended the EcoBuild conference and trade show in London, held over three days in February. There were 35,000 announced attendees at the event, this in a country one-fifth the population of the U.S. Imagine if our Greenbuild show had 165,000 attendees!

Learning about the UK government’s commitment to zero carbon new homes by 2016, I was reminded of this slogan, because of the gap between the political class (some of whom may even believe that gravity is a law enacted at some distant time long ago that can be repealed at will) and the design world that must deal daily with the reality of energy use and carbon dioxide emissions in buildings, products and transportation. So I thought, we really must pay more attention to the gap between expectations and reality.

Consider that nearly 50 percent of electricity use in the home comes from appliances and electronics and that electricity use is about half of home energy use. Homes account for some 20 percent of all US carbon emissions (we can debate the numbers, but that’s not a bad “first approximation,” as a scientist or engineer might say). So if we cut out all heating, cooling and hot water use in the home, we’re still left with an almost intractable 5 percent of carbon emissions in the home that must be cut with renewable energy or other sustainable sources.

Now, what would it take to provide that from renewables such as solar power? An average home in southern Arizona uses about 12,000 kilowatt-hours (kWh) per year of electricity, largely owing to the heavy air-conditioning (A/C) demands. Assume that all non-A/C loads are about 6,000 kWh. A photovoltaic (PV) array will produce about 1500 kWh per year per KW of rated capacity, so to get 6,000 kWh of solar energy production I’ll need a 4 kW array, about 24, 170-watt modules. That will cost me about $30,000, at today’s prices, and I’ll get federal and state tax credits, plus (in my area) utility payments, of about $12,000, for a net cost of about $18,000. Still pretty expensive, especially compared with other things I can do to reduce power use, such as putting in superior windows, superinsulating the attic, etc.

Let’s go to the macro scale now. Assume we tried to retrofit 100 million existing American homes over the next 25 years, at the rate of about 4 million homes per year (about the rate of sale of existing homes right now), just to take care of the appliances and electronics. We could do this by requiring that each home sold had to be retrofitted with a 4-KW solar array, taking up about 200 sq.ft. of roof area (that’s the easy part). We’d need 16 million kW of solar panels installed each and every year. That’s 16,000 megawatts, about 40 times the current rate of production and sales in the U.S. and the rest of the world.

Since silicon production for solar panels right now about equals silicon production for microprocessors, this means effectively increasing silicon chip production by 10 times its current level. Pretty daunting task! That’s why most of the “smart” money is investing in PV technologies not based on silicon wafers. We just can’t justify the costs and capital of investing in silicon in such a massive way.

Why do I find this example interesting? First, it suggests a massive difficulty in providing rooftop renewables for completely carbon-neutral homes, even if we succeeded in retrofitting all homes to eliminate energy use for things we think about: heating, cooling and hot water. Second, it implies a massive industrialization of the renewable energy industry at “war-time” production levels, with huge diversions of capital from other social and economic needs. There’s the gap we need to mind.

The current dialog in the UK about getting back to 1990 levels of carbon emissions by 2050 is both realistic and discouraging. Because even at 1990 levels, we will still be seeing global warming, based on carbon dioxide levels in the atmosphere of about 500 parts per million, nearly double the pre-industrial level.

 The political implications for the U.S. Congress and a new President in 2009 are clear.

 Start now to plan a low-carbon future by diverting investment away from carbon-producing technologies and into those that are carbon-reducing.

 Provide the incentives and requirements for a massive shift of capital into the renewable energy sector, so that we can reach these ambitious targets.

 Avoid cheap sloganeering (recall President Jimmy Carter’s 1977 pronouncement during an earlier energy crisis that energy conservation was “the moral equivalent of war,” which had the unfortunate acronym of “MEOW”)

 Get on with the tough task of technology development, appropriate regulation updates, and changes in building design and construction needed to get us out of this fix.

We need to “Mind the Gap” between expectations and reality, between pronouncements and capabilities, between what’s theoretically possible and what actually is happening in the real world of building design, construction and operations. Mind the gap, indeed!