MacKay introduces some - but not enough - realism into the field of renewable energy. As a professor of physics, he does an excellent job of explaining power and the fundamental physical limitations of various technologies. In particular, wind, solar, and biofuels diffuse sources of energy, so MacKay explains the cost of a particular technology in a variety of units, including m2 needed to provide the average person's power demand and the fraction of Great Britain that would need to be devoted to a particular technology to meet total demand. For example, MacKay calculates that covering the windiest 10% of Great Britain with windmills would provide 20 kWh/d, about half the energy we currently expend driving today! This is a necessary rude awakening for many Greens. Some reviews complain that the efficiency of solar panels has improved since the book was written, but they are missing the larger point: There are physical limitations to what is practical. In the ideal world, the online version of this book could be updated with new information from both the author and readers (like Wikipedia).Unfortunately, as an academic and government advisor, MacKay appears to have little practical experience with real systems. I think a sabbatical working with a power distributor or hedge fund investing in alternative energy projects would be valuable. MacKay minimizes the difficulty of meeting variable demand with intermittent renewable power sources. For example, he absurdly equates the difficulty of dealing with unpredictable in wind power output with the difficulty in dealing with the highly predictable daily demand cycle using today's flexible fossil fuel generators (p 189). Given that wind power output varies with the cube of wind speed, a 20% drop in wind speed is a 50% drop in power output. The size of the reserve (currently provided by fossil fuels) needed to meet fluctuations in output depend on forecasting accuracy. If flexible fossil fuel plants are not used, variations in output and demand will need to be met by expensive storage systems, which roughly doubles the cost!Unfortunately, MacKay discusses cost in an isolated chapter, not in the chapters about the technology itself. The cost of photovoltaic electricity is presented in terms of area is the chapter on photovoltaics and the cost in pounds is elsewhere. While MacKay uses clear units for cost in terms of area (10-20 W/m2 or 100-200 m2/person), price is obscure in Table 28.3. We aren't presented with any information about the cost of fossil fuels, including estimates of the "social cost" of carbon. MacKay leaves the reader with the impression that cost is not an important factor when considering how to decarbonize the economy - or whether to decarbonize at all. (Fossil fuel production will peak someday and economic factors will force decarbonization long before complete depletion, but the impetus for decarbonization today is clearly climate change.)Mackay ignores other costs. In Table 20.8, we learn that trains consume only 6 kWh of energy moving one person 100 kilometers, while buses consume 19 kWh doing the same job. However a train requires very expensive tunnels or that contiguous land be dedicated solely for railroad track that is occupied only a small fraction of the day. Buses, cars and other forms of transportation can share the same right of way. Cars and planes consume 68 and 51 kWh doing the same job, but can take one from home to a final destination in far less time and for lower total cost. MacKay is fixated on reporting costs in terms of some useful units (m2 per person), but not other useful units.In reality, many of us and society as a whole can't afford to maintain our current lifestyle using renewable energy. The solution to that problem - conservation - can be found on p228-9: stop flying (save 35 kWh/d), drive at least 50% less (save 20 kWh), change your thermostat setting (20 kWh/d), replace old buildings (35 kWh/d) and even air-drying laundry (0.5 kW/d). Although he doesn't say so explicitly, MacKay would prefer that change be enforced by government edict rather than be driven by a carbon tax and the marketplace.Despite my criticisms, the book (which is also available online for free) is an extremely useful resource.