SF The ARM was conceived as a processor for a tethered desktop computer, where ultimate low power was not a requirement. We wanted to keep it low cost, however, and at that time, keeping the chip low cost meant ensuring it would go in low-cost packaging, which meant plastic. In order to use plastic packaging, we had to keep the power dissipation below a watt—that was a hard limit. Anything above a watt would make the plastic packaging unsuitable, and the package would cost more than the chip itself.
We didn't have particularly good or dependable power-analysis tools in those days; they were all a bit approximate. We applied Victorian engineering margins, and in designing to ensure it came out under a watt, we missed, and it came out under a tenth of a watt—really low power.
I found the interview of him I was making my comments from, here's the critical 2 paragraphs (http://queue.acm.org/detail.cfm?id=1716385):
SF The ARM was conceived as a processor for a tethered desktop computer, where ultimate low power was not a requirement. We wanted to keep it low cost, however, and at that time, keeping the chip low cost meant ensuring it would go in low-cost packaging, which meant plastic. In order to use plastic packaging, we had to keep the power dissipation below a watt—that was a hard limit. Anything above a watt would make the plastic packaging unsuitable, and the package would cost more than the chip itself.
We didn't have particularly good or dependable power-analysis tools in those days; they were all a bit approximate. We applied Victorian engineering margins, and in designing to ensure it came out under a watt, we missed, and it came out under a tenth of a watt—really low power.