• by Gavin O'Malley  @mp_gavin, February 28, 2011

Barring any unforeseen events, U.S. ecommerce spending will reach $278.9 billion by 2015, Forrester Research is predicting.

Forrester's forecast is based on a 10% compound annual growth rate through 2015, as well as the continuation of several key trends, including the ubiquity of consumer Web usage; their increased familiarity with -- and preference for -- online shopping; and ever-better ecommerce user experiences.

Forrester also credits new online shopping models -- and "flash sales" and "daily deal" services in particular -- with continuing to generate excitement among shoppers.

In 2010, U.S. online retail sales grew 12.6% to $176.2 billion, per the company. "This was unusual because Web sales growth had been consistently declining over the years," according to Sucharita Mulpuru, principal analyst at Forrester and lead author of the report, "U.S. Online Retail Forecast, 2010 To 2015."

"The reason for the reversal of the trend was that the growth rates in 2008 and 2009 were deflated by the recession that hit the retail sector particularly hard," Mulpuru added. "2010 was simply a normalization of the trend as consumer behavior returns to normal."

There are additional factors driving ecommerce.

While there are fewer new shoppers online every year, online shopping "holdouts" are increasingly giving in, and giving over credit-card information. Per the study, these are younger "millennials," older "seniors," or less technology-savvy shoppers compared to more "tenured" online shoppers.

In 2010, 5.5 million customers shopped for the first time, on average driving approximately 30% of the overall growth in 2010 ecommerce sales, Forrester estimates.

Also, while new shoppers are one component of the overall growth in Web shopping, a bigger component of ecommerce growth is the increased "wallet share" among existing Web shoppers.

In particular, they are increasing their spend in "traditional" online categories such as books and media products, while increasingly exploring high-touch, high-consideration goods, like furniture and home appliances. As a result, in 2010, 70% of the overall growth in ecommerce sales came from existing shoppers simply buying more online.

Overall, ecommerce penetration of total retail sales is now 8%, according to Forrester. Yet Mulpuru notes, the 8% figure is weighed down by the low penetration of grocery online.

Excluding grocery sales, overall ecommerce penetration is now 11%, while some categories such as PCs and software now have an online penetration level over 50%.