Is it possible to charge my MacBook Air from a powerbank? I recently bought a powerbank that has two USB outlets. It's great for charging my phone and iPad. Is it possible to charge my MacBook Air from the powerbank? If it's not possible to do this "out of the box", would it be possible to build or buy something that would enable me to do this?
Yes, The answer is positive. The power bank capable to charge a laptop needs to supply not only the USB standard charging port but also (or only) a DC output port, or a build-in power inverter with an AC outlet.

Here i collect some power bank that can charge laptop:

1.Anker® Astro Pro2 Multi-Voltage Portable Charger – 20000mAh
Multi-Voltage 5V 12V 16V 19V for iPad Air, Mini, iPhone, Samsung Galaxy, Note, HP, Dell, Acer, Asus, Toshiba Netbooks

2.RAVPower® Xtreme 3-Port Portable External Battery Charger with Multi-Voltage 23000mAh
Multi-Voltage 9V/12V/16V/19V/20V – for Apple Macbook, UltraBook, Digital Cameras, Surface Tablets,Asus Tablets and more

3.Poweradd™ Pilot Pro Monster Capacity Multi-Voltage Portable Charger – 32000mAh
Multi-Voltage 5V 9V 12V 16V 19V 20V for Tablet PC, Netbooks, Notebooks, Laptops, Smart Phones

4.BiXPower 153 Wh (42500mAh) Super High Capacity 12V &amp; 19V External Rechargeable Battery Pack – BiXPower BP160
Made with high quality Panasonic Li-ion battery cells. Compact size can easily fit into a laptop/notebook carry bag

5.Energizer XP18000AB Universal Power Adapter with External Battery for Tablets/Laptops/Netbooks/Smartphones

Energizer XP18000AB Universal Power Adapter with External Battery for Tablets/Laptops/Netbooks/Smartphones

6.Qi-Infinity™ Powergrid External Battery Pack/High Capacity Power Bank charger – 34200mAh
For Laptops, Notebooks, Netbooks, Tablets and Smart Phones

7.Lizone® Extra Pro Super Capacity Portable External Battery Charger 40000mAh
For Apple Books, HP, IBM Lenovo, Tablets, Smartphones and more

8.Intocircuit® Power Castle PC26000 External Battery Pack/High Capacity Power Bank charger – 26000mAh
12 Connectors for Laptops, 10 Connectors for Phones and Tablets, AC Adapter Included

There are power banks available with various plug in connectors for laptops, but they cost around 15,000. I have seen one on ebay india, and the ad says that they’ve even sold 29 units. These are not marketed by laptop manufacturers so I am not sure what your laptop warranty status will be if you purchase one of these banks and use it charge your laptop. Most laptops charges through its adapter only. Powerbanks on the other hand have only usb 2 output interface. Replacing laptop batteries with Extra cell batteries available in the market can help. One desi jugaad is using a UPS to charge a lappy but it wont make it handy and portable.