A billion dollars in renewable energy is huge.
SoftBank will hold majority stake in the joint venture.
With Nikesh Arora's exit from Softbank, India's start-ups have lost one of its biggest supporters.
SoftBank's investments in the past two years include $627 million in online-retailing marketplace Snapdeal and leading a $210 million funding round in taxi-hailing app Ola Cabs.
The lowest bid earlier had come for Madhya Pradesh at Rs 5.05/unit.