Microsoft spared no expense when it came to keeping the Surface Pro going. They sourced the Cadillac of batteries from LG: an Escalade 42 Wh unit. The battery is rated for 7.4 V and 5676 mAh.
Impressive specs? Note that the iPad 4 has a 43 Wh battery, albeit at 3.7 V.
And even with all this battery juice, the reported battery life of the Surface Pro is less than 5 hours.