Step 1: Find Your Peak Sun Hours
Peak sun hours compress the day's changing sunlight into equivalent hours of full-strength sun (1000 W/sq. m.). Indian cities receive between 3.5 and 5.8: Jodhpur 5.8, Jaipur 5.5, Ahmedabad 5.3, Delhi and Hyderabad 5.0, Nagpur, Chennai and Bengaluru 4.8, Mumbai 4.5, Kolkata and Kochi 4.2, Shillong 3.5. Use your city, not a national average, because the spread changes output by up to 40%.
Step 2: Apply the Output Formula
The 0.8 is the performance ratio, which absorbs real-world losses (Step 3). Worked examples:
| Setup | City | Calculation | Daily Output |
|---|---|---|---|
| 1 × 550W panel | Nagpur (4.8) | 0.55 × 4.8 × 0.8 | 2.1 units |
| 3 kW system | Delhi (5.0) | 3 × 5.0 × 0.8 | 12 units |
| 5 kW system | Jaipur (5.5) | 5 × 5.5 × 0.8 | 22 units |
| 10 kW system | Kolkata (4.2) | 10 × 4.2 × 0.8 | 33.6 units |
Multiply daily output by 30 for monthly and 365 for annual figures, or let the solar generation calculator do it for your exact city.
Step 3: Understand the Losses Inside the 0.8
- Temperature: 8 – 12% on hot Indian roofs (cells run 55 – 65°C)
- Soiling: 3 – 8% with monthly cleaning, far worse if neglected
- Inverter conversion: 2 – 4%
- Wiring and mismatch: 2 – 3% (keep cables sized with the wire size calculator)
- Tilt/orientation: up to 10 – 15% if far from the ideal set by the tilt angle calculator
Step 4: Adjust for Season and Age
Monsoon months produce 30 – 40% below the annual average and summer above it, so judge a system on annual output. Panels also degrade about 0.5% a year: a system producing 4,200 units in year 1 still produces about 3,700 in year 25. For revenue projections, both effects matter; the savings calculator applies them automatically.
Month-by-Month Output Variation in India
The formula gives an annual average; real months orbit around it. Indicative pattern for north and central India (percent of the average month):
| Month | vs Average | Month | vs Average |
|---|---|---|---|
| January | ~95% | July | ~65% |
| February | ~105% | August | ~70% |
| March | ~115% | September | ~90% |
| April | ~118% | October | ~105% |
| May | ~115% | November | ~100% |
| June | ~90% | December | ~92% |
Coastal and eastern regions dip harder in monsoon (down to 55 to 60%), while Rajasthan flattens the curve. Judge any calculated monthly figure against the season it belongs to before concluding anything about the system.
Checking a Real System Against the Calculation
Read your inverter's daily kWh log or your net meter, average over a full month, and compare with the formula result. Consistently landing more than 15% below the estimate points to soiling, new shading, an under-performing string or an inverter issue worth a service call. Panel-level specs can be sanity-checked with the efficiency calculator and the wattage calculator.