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I don't have exact numbers, but I would expect the difference to be very small. For idle or a very light load (e.g. XviD playback, typing a word doc, etc.) the SU9300 will likely use 0.2-0.7 watts less than the SL9400. This could equate to up to another hour of battery life with an extended battery.
However, the SL9400 offers a very substantial performance difference (55% more MHz, a 33% faster FSB, and 50% more L2 cache) for only a 7w penalty in TDP. This difference is far larger than the spread between the P8600 and T9600 for example (16.7% more MHz, 0% faster FSB, 100% more L2 cache, 10w higher TDP). As such, I would recommend the SL9400 over the SU9300 in almost all circumstances.
An SSD will provide as much, if not more, energy savings as the switch to the SU9300 but will come with a performance increase rather than penalty.
What notebook do you have? Consider this experiment:
1) how long is your battery life now? X hrs
2) what is the capacity of your battery? Y WHr
3) what CPU do you have? if you have the SU and want to simulate an SL, say there is a 1W difference (or 0.5W or whatever)
4) divide 1 into 2 so you get how much power your system is drawing. Y/X Watts
5) add the number of watts from 3 to 4
6) (Y/X + 1) Watts divided by Y WHr -> will give you your new battery life.
Example: 3.5 hours of battery life on a 70 WHr battery, power consumption is 20W. Add 1W, new power consumption is 21W. New battery life is 3.33 hours of battery life. The difference is 10 minutes less.
I would assume the OP is considering either an x200s or x200T (the only ThinkPads that offer BOTH an SU9300 and SL9400). I also assume he does not presently own either of these laptops so comparing it against his current model would be impossible.
Both of these can be extremely energy efficient due to the 12.1" LED display and LV or ULV processor. My x200T tends to operate
between 6w and 10w with a light load depending on screen brightness. The x200s should have nearly identical energy consumption. As such, the 0.2-0.7 watts can make a fair percentage difference in consumption (my SSD made a difference of 0.3 watts at idle and 2.6 watts under continuous access).
The Difference in battery life for the SU9300 will actually be largest for the x200s with 9 cell battery as it is an 84WHr design (compared to 66WHr for the x200T 8 cell, 56WHr for the 6 cell, and 28WHr for the 4 cell). The difference will also be largest at the lowest screen brightness.
He can always use review info to estimate since the battery life and battery capacity are known. Either way, the math isn't difficult and can give at least a ball park figure of what to expect. But as you said, there are many other factors that would give a more significant difference towards battery life than the CPU.
The difference between the two can be seen in the
datasheet from Intel, page 105-106. Depending on how much you're using the CPU, the difference is between 7w and 0.4w, for the different power states. To know which power states it's most often in, I don't know.
I choose the SU9300 because I mostly use my computer to read and browse web pages, programming (typing and editing), and other stuff where the CPU is just waiting for something to do. I believe a SSD does a bigger difference for the average user than a 55 % difference in clock speed.
Problems? See this thread at archive.org.
