I don't know how some people can squeeze 10+ hrs on a Surface 3... no matter how light I try to keep my usage at, i can only get 7 hrs pretty much guaranteed, but anything beyond 7 is quite difficult.
anyway, some of my unscientific observations are: (when Surface is stable, with no app running)
1. stock charger + stock cable, the charge rate is around 6500mW
2. Mi 10000 mAh battery pack (5V 2.1A) + stock cable, rate is around 5700mW
3. Mi 10000 mAh battery pack + Nexus 7 stock cable, rate is around 4700mW
4. Apple 12W charger (2.4A) + Orico 3A cable, virtually same rate as the MS stock combo
5. A power strip comes with USB socket (rated 2A Max) + stock cable, charge around 5500mW
So, a $19 Apple 12W charger is smaller and cheaper than a MS charger, and you can get a 2.5-3A USB to MicroUSB cable online with very little cost. So, if you need a quality second charger to leave at your office or stuff in your bag, this may be a good choice.