GST on Solar Product

Consider the following points first

1. Difference in Transaction Values: It's not that tough to calculate but one thing you are missing here is that there will differnce in values of the Components and the Final product due to simple reason of margins, so While you will be paying A gst of 5% on the Final product value of Rs.1,000(Let's say) but the value of assemled products shall be quite lower maybe Rs400 to Rs 500. 
2. Carry Forward of Input Credit: Yes you will get the Input credit carried forward of the excess amount if any provided you qualify the preconditions

• where the said amount of credit is not admissible as input tax credit under this Act; or
• where he has not furnished all the returns required under the existing law for the period of six months immediately preceding the appointed date; or
• where the said amount of credit relates to goods manufactured and cleared under such exemption notifications as are notified by the Government.