It is not often that a physics book becomes a publishing phenomenon; and when it does happen, the book is usually about strings or cosmology. But David MacKay’s Sustainable Energy – Without the Hot Air is different. It tackles a down-to-earth but controversial subject in a remarkably even-handed manner; it has been lauded by a number of national and international publications; and it remains high in the bestseller lists even though it is free to download.

Despite the trendy word “sustainable” in its title, this is avowedly not a book about climate change, nor is it a political polemic. Instead, its primary purpose is to give decent numerical estimates of the UK’s future energy production and consumption after the oil runs out. Such estimates are normally only found together in think-tank reports, where they can appear surprisingly frail: their underlying physics is seldom revealed, and it is difficult to judge their validity. The mainstream media, on the other hand, rarely seem to use large numbers competently. This book, with its slogan “numbers not adjectives”, is an attempt to bridge that gap.

MacKay, a physicist at Cambridge University who has just been named as the UK government’s scientific advisor on energy and climate change, begins by choosing a single unit for power consumption: kilowatt-hours per person per day, rather than the usual muddle of kilowatt-hours, litres, BTUs, barrels, tonnes and so on. Though at first this seems clumsy, it turns out to be rather effective. Figures mostly fall in the range 1–100, and results are easily translated into personal terms. With this technical detail firmly in hand, MacKay then builds up two stacks: a red stack for energy consumption and a green stack for sustainable energy generation.

If one accepts a need to balance current energy consumption with generation, then conclusions emerge quite clearly, and the book summarizes these in a straightforward manner. Renewable energy takes up a lot of land, and, for the UK, only wind over our coastal seas and solar power from other people’s deserts can provide us with significant amounts of it. Nuclear power may be problematic, even dangerous, but it could meet much of our needs; compared with the alternatives, the volume of waste it produces is small. On the domestic side, building insulation needs to be improved and heating needs to be electrified, probably by the extensive use of air-source heat pumps.

MacKay uses an additional unit – kilowatt-hours per 100 passenger-kilometres – when discussing transport, and this illuminates his comparison of the energy demands of different modes of travel. Here again, the likely advantages of electrifying as much of our transport infrastructure as possible become clear; in the absence of a persuasive “green” alternative like hydrogen cells, no other option offers the flexibility of electricity. The author is at pains not to let politics intrude on his message, and the book concludes with a range of “energy plans” to suit all political tastes – except head-in-the-sand failure to acknowledge that there is a problem.

However, setting energy policy aside, the book also works on a deeper level, bringing the concept of energy home to physics through “back-of-the-envelope” calculations. MacKay shows how a physicist initially approaches a problem by identifying processes and constraints, and getting numerical estimates of them.

Naturally, such estimates need to be refined, but they make a good starting point – as many physicists will know from their own lunchtime conversations. For example, we recall a discussion about recycling in which we needed to know the conversion rate of hydrocarbon fuel mass into energy. For an immediate rough estimate, we used the calorie content of butter – and indeed this same calculation appears alongside a picture of a pat of butter on page 29 of the book. The butter-derived figure proves to be about 20% too low – but since one of us developed instruments to measure the moisture content of butter, we are able to improve the estimate by eliminating this moisture mass (which, of course, contributes no energy) from the calculation.

Of course, some estimates in the book are intended only to be good to within a factor of two or so, and occasionally somewhat poorer, since MacKay’s goal is often just to put a rough upper bound on a potential source of energy. His estimate of tidal-stream power, for instance, combines physical arguments with a perusal of the tidal charts in a nautical almanac, and seemed to us (as yachtsmen) to rather underestimate the ubiquity of strong tides around the UK’s coast. Such an approach does open the book to criticism, particularly from specialists; indeed, it could easily become a victim of its own success if policymakers and others begin to regard its figures as accurate final judgments. That would be a shame, for it would traduce one of the book’s intentions, which is to enable readers to do their own calculations – in fact, its original working title was “You go figure it out!”.

To this end, MacKay is very keen that the book should be digestible. It is well illustrated, and all algebra is confined to the technical chapters (in effect appendices) that make up the last 90 or so pages. The physicist will find much that informs (and occasionally amuses) in these chapters, particularly in those on cars and planes. The former runs a long way on very little algebra, and in the latter the basic physics of a jumbo jet’s energy consumption is shown to scale down quite well to fit an albatross.

In the main text, however, all that is needed is basic numeracy and familiarity with exponential notation. For this reason the book would be a good way of introducing teenagers to how real physicists work – all the more so because MacKay’s treatment of energy is much more positive and empowering than either the school physics curriculum or most environmental literature. By exposing both the data from which his book’s conclusions are drawn, and the methods and arguments by which they are reached, MacKay in effect says “You need take no one’s word for it. You can work it out for yourself, and this is how.” Various reviewers have said something along the lines of “every policy maker should read it”. We would rather advocate it as a book every budding physicist should read – and perhaps also as the one every working physicist would like to have written.