In the solar business, there is no fixed GST rate like 8.9%. Instead, the government has given a special rule for solar EPC projects where GST must be calculated using a 70:30 ratio.
This means 70% of the invoice amount is taxed at 5% (goods) and 30% is taxed at 18% (services). Because of this mixture, the total GST looks like 8.9%, but this is only an effective rate, not an official GST slab.
Example: If your invoice amount is ₹1,00,000, then GST is calculated like this :
70% of 1,00,000 = 70,000 @ 5% = ₹3,500
30% of 1,00,000 = 30,000 @ 18% = ₹5,400
Total GST = ₹8,900 (which is effectively 8.9%).