Microsoft has evolved its power supplies since the first Surface RT device, but until Surface Pro 3, the devices and power supplies were all interchangeable. Not anymore: The power supply that comes with Surface Pro 3 retains the basic design from previous Pro devices, but when you look at it closely, you find that everything, in fact, has changed.
Here's what's happening with the Surface Pro 3 power supply.
New connector. With previous Surface devices, Microsoft used a magnetic power connector that originally didn't work very well at all. (See Surface with Windows RT: Master the Power Connector for more information.) For the original Surface Pro, Microsoft improved this connector by adding more magnets and slightly altering the physical shape of the connector. And then it improved things again with Surface 2. Now, Surface Pro 3's power connector is all new yet again. This time around, it still uses magnets, but the connector piece is actually a thin tab that actually inserts into a new port on the side of the device, rather than resting on the outside. Oddly, it's still possible to misconnect the power connector, but it's much harder than with any previous Surface devices.
New Surface Pro 3 power connector (left) and Surface Pro/Pro 2 connector (right)
Not interchangeable. Because of the new connector design, you cannot use the Surface Pro 3 power supply with previous Surface devices. Likewise, you cannot use the power supply from previous Surface devices to charge Surface Pro 3.
Multi-piece design with a power brick. If you're familiar with previous Surface devices, you know that Surface RT and Surface 2 utilized a single piece power supply with a small brick by the power plug, while Surface Pro and Pro 2 provided a two-piece, PC-like power supply with a power brick. Surface Pro 3 uses a two-piece design like other Pro devices, and in a similar configuration. But it's smaller in each dimension and thus a bit lighter.
Smaller, thinner Surface Pro 3 power supply (left) and Surface Pro/Pro 2 on the right
Model number. The model number of the new Surface Pro 3 power supply is 1625, where the previous version was 1536.
36-watts. The smaller new Surface Pro 3 power supply is 36-watts, compared to 48 watts for previous Pro power supplies and 24-watts for Surface RT and Surface 2.
USB port. As with previous Pro power supplies, the Surface Pro 3's power brick includes a USB 2.0 port for charging devices like phones or tablets. (So this feature hasn't really changed.)
A few other Surface Pro 3 hardware features related to power are perhaps worth mentioning.
Power port. The new power port on Surface Pro 3 is quite different from the connector on all of the previous Surface tablets. It looks like a full-sized SD card slot, and as noted above helps to secure the power supply's connector more reliably than before. But this port will also be how the coming Surface Pro 3 Docking Station connects to the device. (With Surface Pro and Pro 2, connectivity occurred through the USB and mini-DisplayPort ports.)
Power button. Surface Pro 3's power button is on the upper left of the device (when viewed in the normal landscape orientation). This is a first: On all previous Surface devices, the power button was on the upper right.
Pen. Despite the new power port design on Surface Pro 3, you can still magnetically connect the new Surface Pro 3 pen to the side of the device around this port, as you could with Surface Pro 2 and its pen. That's because each device still employs magnets. Obviously, this isn't any kind of permanent recepticle, and you shouldn't travel with the pen attached in this fashion. But it does still work.]