In summary, MacKay says: "First, we electrify transport. Electrification both gets transport off fossil fuels, and makes transport more energy-efficient. (Of course, electrification increases our demand for green electricity.) Second, to supplement solar-thermal heating, we electrify most heating of air and water in buildings using heat pumps, which are four times more efficient than ordinary electrical heaters. This electrification of heating further increases the amount of green electricity required. Third, we get all the green electricity from a mix of four sources: from our own renewables; perhaps from "clean coal"; perhaps from nuclear; and finally, and with great politeness, from other countries’ renewables. Among other countries’ renewables, solar power in deserts is the most plentiful option. As long as we can build peaceful international collaborations, solar power in other people’s deserts certainly has the technical potential to provide us, them, and everyone with 125 kWh per day per person."
Or, in a bit more detail, he also provides a 10-page synopsis.
And here's one of his proposed possible energy plans (plus a diagram summarising some alternative plans).
Some other bits worth looking at:
His calculation of total possible UK renewable energy. His is the column on the left, compared with some more conservative estimates. (He is deliberately ignoring economic, political and practical factors at this point and concentrating only on possible energy output from renewables.)
And if you're wondering what you could do, here's some bits about how he reduced his home energy consumption by two thirds (turning down the thermostat and reducing draughts have the biggest impact), more here (5 pages or so). For buildings, he's particularly into heat pumps, more detail here. And some stuff on gadgets here, and why articles about standby and unplugging mobile phone chargers have the emphasis wrong (though you should still switch off gadgets and unplug chargers anyway).
Oh, he's got a pretty good blog too, which I hadn't realised.