There is a new power bank, BatPower ProE ES9 Power Bank, that seems to provide the best value for the money. You can get two separated chargers, portable battery and wall charger, that compatible with all Surface lines including the Surface RT, Surface Pro 1/2/3/4/(2017), Surface Book and Surface Laptop.
If you choose 36,000 mAh you can get:
Surface Pro 3 (42 Wh), Surface Pro 4 (38 Wh), Surface Pro (2017) (45 Wh) up to 2 times.
Surface Book (69 Wh) up to 1.5 times.
Surface Book with Performance Base (81 Wh) up to 1 time.