Does solar ppa project attract gst ?

I am investing in a Solar system and installing at client's place.

It generates electricity. Customer pays me according to the no of units of power generated.

I bill him every month after taking the generated reading.

Its my responsibility to maintain the system for entire contract period.

At the end of contract period, I am handing over the system (Selling) to customer at agreed cost.

Now, how much GST should I charge on every month's bill ? Someone  told me 18% but the customer's auditor says since you are charging based on the number of units generated, you are selling power and  power is out of GST effect.

Please let me know the correct situation with some supporting

.