Rooftop solar PV for a school: layout, yield estimate and payback

Sahyadri Vidya Mandir used 1,10,000 kWh of electricity last year and paid about ₹9.60 per unit once fixed charges, fuel adjustment and duty were spread across the bill. That is roughly ₹10.5 lakh a year, and the trustees want it lower. A vendor has quoted a "100 kW" system without visiting the roof. The trustees do not trust the number and have asked the school's alumni engineering cell, which is you, for an independent design.

What you know: - Two flat RCC roofs: Block A has 480 m² usable and Block B has 260 m², after you leave walkways and stay clear of a 3 m tall water tank on Block A. - Sanctioned load is 95 kW on a three-phase 415 V LT connection, and the discom allows net metering up to the sanctioned load. - The trust board has approved up to ₹38 lakh. They want a simple payback under 5 years. - The school is almost empty in May and during the Diwali break, so consumption drops in those months.

Your job is to work out how much PV really fits, what it will generate in Pune, how to wire it safely, and what it pays back. You will use free tools: PVGIS and PVWatts for yield, a spreadsheet or Python for the calculations, and any drawing tool for the layout and single-line diagram.

Done looks like a short design pack the trustees and the discom engineer can both read: a roof layout, a yield estimate from two tools with the difference explained, string and cable calculations, a single-line diagram with protection, a bill of materials and a payback table with a sensitivity check. If your honest answer is that 100 kW does not fit, say so and show why.