kindlefirehdJeff Bezos says that Amazon wants to make money when Kindle owners use their devices to consume media and make purchases, not when they buy the devices in the first place.

And the strategy appears to be working, at least to some extent, according to new estimate from market research firm Consumer Intelligence Research Partners.

Owners of Kindle e-readers and tablets in the U.S. spend an average of $1,233/year, about 55 percent more than the average of $790/year spent by Amazon customers who don’t own Kindle devices, according to the results of the research firm’s survey. The increased spending results primarily from a higher frequency of purchases.

The survey of Amazon customers in the U.S. found that about 40 percent own a Kindle device, and 9 percent own both a Kindle Fire and a Kindle e-reader.

Amazon.com is notoriously secretive about the number of Kindle tablets and e-readers it sells, but based on the U.S. survey, the research firm estimates that Amazon’s installed base is currently 20.5 million e-readers and tablets.

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