The sun as a resource: irradiance, peak sun hours and Indian solar maps

Every rooftop quote starts with one question: how much sunlight lands on this roof in a year? Get that number right and everything else (system size, savings, payback) follows. Get it wrong and you either disappoint the customer or undersell the job.

This lesson gives you the words and numbers engineers use to talk about sunlight, and shows how to turn them into units (kWh) a customer understands.

Irradiance and irradiation: power and energy

Two words sound alike but mean different things.

• Irradiance is the power of sunlight falling on a surface at one moment, in watts per square metre (W/m²). On a clear Indian noon it is close to 1,000 W/m² on a surface facing the sun. At 8 am it might be 300 W/m²; under monsoon cloud it can drop below 100 W/m².
• Irradiation (also called insolation) is the energy collected over a period, in kWh/m² per day or per year. It is irradiance added up over time.

Think of a water tap. Irradiance is how fast the water flows right now; irradiation is how many litres filled the bucket by evening.

Above the atmosphere the sun delivers about 1,361 W/m². Air, dust, water vapour and cloud absorb and scatter part of it, so we rarely see more than about 1,000–1,100 W/m² at ground level. That is why module ratings use 1,000 W/m² as their test condition.

GHI, DNI and DHI

Solar data sets split sunlight into three parts:

A tilted module receives plane-of-array (POA) irradiance, which software calculates from GHI, DNI and DHI. Tilting towards the south usually collects a little more than a flat surface in most of India.

Peak sun hours: the daily number you will use most

Sunlight rises in the morning, peaks around solar noon and fades in the evening. Engineers squash that curve into a simple number: peak sun hours (PSH), the number of hours at 1,000 W/m² that would deliver the same energy as the real day.

If a roof receives 5.3 kWh/m² in a day, that day had 5.3 peak sun hours. It does not mean the sun shone for only 5.3 hours; it means the day's energy equals 5.3 hours of "full sun".

The yellow area (the real day) and the dashed block (5.7 hours at 1,000 W/m²) hold the same energy. That is all PSH means.

Rule of thumb for Indian sites

Most of India receives about 4.7 to 5.5 kWh/m²/day of GHI averaged over the year. Global Solar Atlas data for a few cities (yearly GHI ÷ 365):

Yields are Global Solar Atlas PVOUT values reduced by 5% for typical rooftop losses (Sunlectric benchmark set, October 2026).

Notice that Chennai has slightly more GHI than Pune but a slightly lower yield. Heat is the reason: hot cells make less power, as you will see in lesson 4.

Specific yield: the number customers really need

Specific yield is how many kWh one kWp of modules produces in a year at a given place. It already includes the tilt and the normal system losses, so it is the fastest way from "sunlight" to "units on the bill".

Worked example: a 3 kWp system in Pune

• Pune specific yield: 1,513 kWh/kWp/yr
• Yearly generation: 3 × 1,513 = 4,539 kWh (units)
• Per month on average: 4,539 ÷ 12 ≈ 378 units
• Per day on average: 1,513 ÷ 365 ≈ 4.1 units per kWp

The same 3 kWp system in Lucknow gives 3 × 1,390 = 4,170 units, about 8% less. Same modules, same installer, different sky.

Seasons matter

Yearly totals hide big monthly swings:

• Monsoon (June–September) cuts output sharply in Maharashtra, Gujarat and Karnataka. A Pune system makes about 81 kWh per kWp in July against 158 in March, roughly half.
• Winter haze and fog in the Indo-Gangetic plain (Lucknow, Kanpur, Patna) drags December–January output down.
• The north-east monsoon (October–December) gives Chennai and coastal Tamil Nadu their cloudiest months.

When a customer complains in July that "the panels have stopped working", the monthly profile is your answer. Show it to them at the time of sale, not after.

Indian solar maps and where to get data

You do not need to measure sunlight yourself. Use these free, reliable sources:

1. Global Solar Atlas (World Bank, Solargis data). Click a location and read GHI, optimum tilt and PVOUT (yield). Good for any Indian address.
2. NIWE Solar Radiation Resource Atlas of India, from the National Institute of Wind Energy, based on its ground station network.
3. MNRE and NISE publications. NISE has estimated India's solar potential at about 748 GWp, using 3% of wasteland.
4. Design software such as PVsyst, PVGIS or Helioscope pulls in hourly weather files for detailed design.

From peak sun hours to units: the quick formula

When you only know PSH, use:

Daily units ≈ kWp × PSH × PR

where PR (performance ratio) is the share left after real-world losses, typically 0.75–0.80 for a good Indian rooftop.

Example, Jaipur: PSH on the tilted plane ≈ 5.6, PR 0.76, system 5 kWp. Daily units ≈ 5 × 5.6 × 0.76 = 21.3 kWh, about 640 units a month.

Cross-check with specific yield: 5 × 1,546 ÷ 12 = 644 units a month. The two methods agree, which is what you want.

Key points

• Irradiance is power (W/m²); irradiation is energy (kWh/m²). Clear-noon irradiance in India is about 1,000 W/m².
• Peak sun hours = daily kWh/m²; most Indian sites see 4.7–5.5 kWh/m²/day on a flat surface.
• Specific yield (kWh/kWp/yr) is the fastest way to estimate generation: roughly 1,380 (UP) to 1,660 (western Rajasthan).
• Monsoon, winter haze and the north-east monsoon change monthly output a lot; show the customer the monthly profile.
• Use Global Solar Atlas or NIWE data for the actual site, not a national thumb rule.

Check yourself

1. A day delivers 4.8 kWh/m² on the panel plane. How many peak sun hours is that?
2. Estimate yearly generation for a 4 kWp system in Ahmedabad (1,616 kWh/kWp/yr).
3. Why does a Pune system produce much less in July than in March?