It would have been interesting to have been in the room when the marketing folks revealed how they were going to spin the battery life of e-readers. Was the headline 'battery lasts for weeks, not hours' greeted with a standing ovation? Was the small print 'based on a half hour of reading per day' met with hushed reverence? Did the engineers bury their heads in their hands? (Quotes from Amazon.com's Kindle Paperwhite Touch page.)
Whatever the initial reception, there can be no doubt that it worked. In their comparison of e-readers and tablets, the respected technology site CNET says:
The other big advantage of e-ink readers is battery life, which is measured in weeks, not hours. Instead of using a reading app on a phone or tablet that will cut into the battery life you might need for other tasks, you can read as long as you'd like on an e-ink reader, and keep the phone ready for phone calls, email, or web browsing instead.
Hook, line, and sinker. This review is now quoted on Wikipedia as an authoritative source for the idea that e-readers have better battery life than tablets, but it is entirely based on marketing hype.
The Amazon Kindle Paperwhite claims a battery life of up to eight weeks, with wireless off and the brightness turned down. The Google Nexus 7 (2013 model) tests as having up to 9 hours of battery life in mixed-use tests at trustedreviews.com. Surely there should be no contest? I decided to test the Nexus 7 under similar conditions to the Kindle to see what I would find...
The test setup was quite simple. I charged the tablet fully, turned off wireless, turned down the screen brightness to a point where I could still read comfortably, and read until the battery hit 90%. I used two epub readers for the test - Aldiko and Moon Reader+, both available for free from the Google Play Store. The OS was Android 4.3 (the 4.4 update that is supposed to improve battery life hasn't reached me yet). One mistake was that a weather app I had used earlier turned out to still be trying to check its server in the background; this was responsible for 5% of battery usage during the test period.
My first finding was that the screen dominates battery usage, taking about 85% of the power. The Nexus 7, like virtually every modern device, powers down unused parts of the CPU, so I was running on a single core at a low clock speed. The epub apps and the OS took about 10% of the power between them. Battery usage did not seem to depend significantly on which e-reader app I used.
The big shock, however, was how long the test lasted: I ended up taking the tablet to bed with me instead of the Kindle. To use up just 10% of the battery took 3.25 hours, implying a total life of 32.5 hours. Or 9 weeks, if it were marketed like an e-reader.
I was surprised. I had known that the Nexus would be a lot closer to the Kindle than the official figures implied, but that it actually had better battery life was astonishing. It may be that I got lucky - but 'up to 8 weeks' for the Kindle implies the best possible result is 8 weeks, so I don't feel I'm being unfair. Another potential problem may be that battery usage would change as the charge level dropped, but I've not noticed any such variation in general usage of the Nexus and have no reason to suspect that the battery meter isn't reporting correctly. It might also be suggested that I shouldn't have turned down the screen brightness - but I adjust the screen brightness to suit the conditions when using the Kindle, and in anything other than bright sunlight would have it on a higher setting than the one Amazon uses for their test.
In conclusion, the idea that e-readers have longer battery life than tablets is due to marketing hype and their different patterns of normal usage - comparing apples and oranges. When the Nexus 7 was used solely as an e-reader - comparing apples to apples - it actually had better battery life than the Kindle.