Intel has boasted that dual-core Haswell parts will be available with SDP ratings as low as 6W. As it turns out, there may be an even more power-efficient version of the chip with a 4.5W SDP. Seeking Alpha found evidence of the unnamed Y-series model in an Intel datasheet detailing mobile U- and Y-series parts. That datasheet no longer appears on Intel's site, though one covering higher-wattage M- and H-series chips is still online.
Otherwise known as Scenario Design Power, SDP refers to power draw during sustained workloads. More traditional TDP ratings gauge power usage during shorter bursts of activity. SDP ratings have apparently been around for a while, but Intel only started discussing them publicly when it revealed its Y-series Ivy Bridge CPUs earlier this year. Those chips have 13W TDP and 7W SDP ratings.
According to the datasheet seen by Seeking Alpha, Y-series Haswell processors will have maximum TDP ratings of 11.5W, down from 15W for the U-series chips. Y-series models will be available with 6W and 4.5W SDPs, and peak CPU frequencies will range from 1.3-1.4GHz. Interestingly, the Y-series chips appear to have a special low-power mode that takes the CPU down to 600MHz. 800MHz is listed as the lowest possible frequency for the U-series models.
Seeking Alpha speculates that a 4.5W Haswell part could easily slip into 11.6" tablets. That territory is also being targeted by processors based on Intel's next-gen Silvermont Atom architecture, and the author suggests Intel is trying to squeeze ARM from both sides.
The situation reminds me a little of what happened with CULV processors a few years back. Faced with upmarket netbooks that combined cheap Atom processors with competent Nvidia Ion graphics, Intel released a series of lower-power CULV chips that enabled Core-based ultraportables to reach down to $400. Tablets based on 4.5W Haswell chips probably won't be that cheap. However, they could narrow the gap between Core- and Atom-based systems—if not on price, then at least on size and battery life.