Tag Archives: solar power in monsoon

Solar Generation During Monsoon

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Solar Electricity Generation During the Monsoon

A frequently asked question for rooftop solar power is: “Will my rooftop solar PV system be useful during the monsoon season?”. I did some theoretical analysis as well as practical measurements to answer this question. The analysis pertains to Pune, India and the measurements were for a rooftop solar PV system of capacity 12.2 kW, installed in Pune. Based on this analysis, a prediction will be made for other locations in India too. The discussion assumes a PV system with net-metering.

The monsoon in Pune is characterised by extremely overcast conditions in the month of July. The month of August is very similar to that of July. June and September are the other two months of the monsoon season, characterised by more sunshine as compared to July and August. The average solar insolation in Pune in May is 6.34 kWh/m2/day for a surface facing south and tilted at an angle of 18 (latitude) degrees. The corresponding number for July is only 3.46 kWh/m2/day (Source: NASA Data). In other words, the solar insolation in July is only about 55% of that in May. Most of this solar insolation is diffused in nature, with very little direct sunlight. The question is whether this diffused solar radiation generates sufficient electricity. The monsoon was very active in Pune in July 2018. It rained almost every day. In fact, it was reported that, on average, there was just one hour of clear sunshine per day in this month and the insolation averaged 3.23 kWh/m2/day .

Our 12.2 kW rooftop solar system generated about 53 kWh/day in May, while in July, the generation averaged 31 kWh/day. Thus, the generation in July was about 58% of that in May. The observed results are quite close to the prediction based on NASA data. There were a few days in July when it rained very hard and the sky had a thick cloud cover. One such day was July 15th, when the solar generation was only 9.6 kWh. However, the average generation for July was slightly higher than predicted values.

Compared to Pune, most cities in India report a higher insolation in July:

  1. Mumbai: 5.06 kWh/m2/day (72% of May)
  2. Bangalore: 4.44 kWh/m2/day (74% of May)
  3. Delhi: 4.75 kWh/m2/day (81% of May)
  4. Kolkata: 3.84 kWh/m2/day (71% of May)
  5. Nagpur: 3.78 kWh/m2/day (62% of May)

(Source: NASA Data)

A solar PV system in these cities will perform even better than that in Pune in July. In other months during the monsoon season, the insolation is higher than that in July. I measured the generation in August. It was 37 kWh/day, higher than that in July. The generation in September was even higher, at 53.6 kWh/day. However, this September was considerably dry. The average generation in the monsoon season (June to September) was 43.3 kWh/day and considering that this is a 12.2 kW system, the average generation per kW per day is calculated to be 3.55 kWH. Thus, any concerns regarding very low generation during the monsoon season are misplaced. The only days when the generation is extremely low (< 25% of average) are the days of continuous heavy rain throughout the day. It is expected that most rooftop solar systems will be in a net-metering configuration. For such systems, the grid acts as storage. On the days that generation is very low, electricity will be imported from the grid.

However, for an off-grid system, the monsoon season can be very challenging. It is rather difficult to size the solar panels and batteries for uninterrupted operation during the monsoon season. There is a significant chance that days of heavy rainfall are consecutive along with several days of extreme cloudiness. I will cover this in detail in another blog-post.