Note that the SZ5x series and the CR2x series are the first from Sony to adopt a new type of battery. Gone are the VGP-BPS2C and in comes VGP-BPS9A/B. A quick find on ebay for the 2 different types of battery makes the answer very clear. There are lots of compatible and OEM selling the BPS2C at about SGD$50 while the newer battery VGP-BPS9A/B cost in excess of $300 from Sony. I am sure if you go to one of the MEGA electronic mall in China you could pick up a compatible BPS2C for $40 WITHOUT even bargaining.
Some differences about the SZ series laptop is the Type of display.
Some comes with (Clear Bright LCD: White LED) which saves on battery and is tinner and to me more beautiful. A comparisons of VGN-SZ483N/C, and VGN-SZ453N/B show that this 2 laptop differ only by the type of display. Both uses the same CPU chipset and bus speed. The harddisk size should not affect the battery. The image above show the difference in thickness between the 2 types of display. Also one of the most difficult part to swap for a laptop upgrade.
And from the spec it could be seen that the White LED type display enhance the battery life by 1 hour on the VGP-BPS2C and 1.5 hours on the VGP-BPL2C battery.
So it seem that laptop consume 8.8W for an White LED system and
While the Normal display system consume 10.4 W under the same condition. The power saving from the LED display alone is a 1.6W which is really substantial for a laptop.
On another note comparing the different CPU the new Penryn is expected to save 0.7 W at the same frequency. during low power mode. That is worth about 20 min of usage time in most cases.