Factors affecting rooftop solar plant output

The power output of a rooftop solar system is dependent on several factors such as

• Location
• Orientation of the roof
• Panel efficiency
• Ambient temperature

Location

Your location determines the amount of solar insolation (sunlight falling on the panel per day).
Overall, the location of the rooftop is by far the most important factor that determines the solar power plant output. Location determines the DNI (Direct Normal Irradiance). DNI at a location is the amount of solar energy falling per sqm per day at that location.

The DNI for a sunny region is approximately 6 kWh/m2/day. This is the amount of sunlight energy falling on a square meter every day in a good location. The higher the DNI, the higher the electricity produced by a solar cell.

Which regions of the earth have the highest solar DNI?

Northern portions of Africa, large portions of USA and Australia, Central America and South America, Middle East, southern parts of China and India get good solar DNI.

A 2009 study by German Aerospace Center estimated how much open land in each region receives incoming solar energy of more than 2,000 kWh/m²/y (about 5.5 kWh/m2/day). The following were their conclusions:

• Africa – about 13 million sq km
• Australia – about 6 million sq km
• Middle East – 2.6 million sq km
• Central South America – 1.2 million sq km
• China – 1.2 million sq km
• USA – about 1 million sq km
• Other parts Asia – 0.7 million sq km
• Mexico – 0.37 million sq km
• Central Asia – 0.16 million sq km
• India – 0.11 million sq km
• EU 27+ – 0.02 million sq km.

What does the solar DNI depend on?

The solar radiation that reaches on different locations of earth depends on several factors such as geographic location, time, season, local landscape, local weather etc. The Earth rotates around the sun in an elliptical orbit and is closer to the sun during part of the year. When the sun is nearer the Earth, the Earth’s surface receives a little more solar energy. This can cause increases in the DNI. When sunlight passes through the atmosphere, it is subjected to absorption, scattering and reflection by air molecules, water vapor, clouds, dust, pollutants, forest fires etc. These can also affect the DNI in a particular region.

Some highlights on DNI

• While many installations in sunny locations such as California or parts of India are able to generate 4.5-5 kWh/kW/day, the output from colder regions in Germany and elsewhere have reported 3.5kWh/kW/day or less. Ironically, Germany, a leading country in rooftop solar installations receives much less than 4 kWh/kW/day for most of its installations as mentioned earlier!

 

• The approximate solar insolation at your location can be ascertained by entering the latitude and longitude of your location at the NASA website

 

• To be absolutely certain of DNI at a particular site we would have to place sensors on-site that measure the actual insolation received over a period of time. This is both an expensive and time consuming process

 

Orientation

In the northern hemisphere, a south-facing roof is ideal as the sun is always to the south if you are in the temperate zone and predominantly in the south for many parts of the tropical zone. Conversely, for those in the southern hemisphere (for instance in Australia), a north facing roof is more optimal.

If a south-facing or north-facing roof is not available an east-west facing roof could also be considered (as it will cover the sun’s movement across the sky from east to west during the day). As the output of the solar plant reduces in proportion to a horizontal angle greater than 15% from due south, the output for the particular site should be calculated and assessed to understand the impact on power generation from an east-west facing roof.

Solar PV plants are not restricted to flat roofs – they can be mounted on sloped roofs as well, with a correction in the angle of mounting for the slope of the roof.

Panel Efficiency

The efficiency of the panel is calculated as the ratio of the capacity of the panel (KWp) with respect to the size (area) of the panel (m²), expressed as a percentage. This table illustrates the calculation for different panel capacities having the same size:

 

Panel Capacity (Wp)	Panel size (m2)	Panel efficiency [Wp/(1,000*m2)]
200	1.61	12.42%
225	1.61	13.98%
250	1.61	15.53%

Note: Efficiency of a solar panel is calculated with respect to the size of the panel, and therefore the efficiency percentage is relevant only to the area occupied by the panel. If two panels have the same capacity rating (Wp), their power output is the same even if their efficiencies are different.

To illustrate: A 1KW rooftop solar plant will produce the same power output whether it uses lower or higher efficiency panels. The area occupied by the plant with lower efficiency panels will be greater than the area occupied by the plant with higher efficiency panels, but the power output is the same.

 

The efficiency of the panels matters where the rooftop space is limited. As the lower efficiency panels occupy a greater area than higher efficiency panels, we will be able to install fewer panels in the same size roof. Fewer panels mean lower plant capacity and therefore lower power output from the plant.

This is illustrated in this table:

 

Panel efficiency	Area required for 1 KW	Roof area available	Plant capacity that can be installed(Roof area/Area required)
Lower	120 SF	1,000 SF	8.33 KW
Higher	100 SF	1,000 SF	10.00 KW

 

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Ambient Temperature

Solar panel temperature is an often ignored but critical parameter in a hot country like India. Though it might seem counter-intuitive, solar PV panels generate less power in very hot summers as the heat reduces their efficiency (the voltage reduces). In Chennai, the month of January delivers better output than May

Temperature Coefficient

The rated capacity, or power, of a solar panel (e.g. 250 Wp) is measured at 25°C. The effect of temperature on the solar panel’s power is measured by its thermal coefficient, expressed as %/K or %/°C. It denotes the % change in power for 1-degree change in Kelvin or Celsius (both are the same on a unit level) above 25°C. A negative (-) sign indicates the direction of the change.

A temperature coefficient of -0.447 indicates that every 1°C increase in temperature over 25°C will cause a 0.447% decrease in power. Equally, every 1°C decrease in temperature over 25°C will cause a 0.447%increase in power. This is illustrated in this table:

 

Rated panel capacity (Wp)	Temperate (° C)	Temperature Coefficient	Effective panel capacity (Wp)	Change in Wp
250	20	-0.45%	255.59	102.24%
250	25	-0.45%	250.00	100.00%
250	35	-0.45%	238.83	95.53%
250	45	-0.45%	227.65	91.06%

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Approximation of PV plant output

As we have seen, estimating the power output from your rooftop solar plant can be a complex exercise. Luckily we can use a simple heuristic for calculating the power output in India:

 

1 KWp of panel will generate about 1,400-1,600 kWh (units) per year i.e., about 4 kWh per day in regions that have good-excellent sunshine. It is an average calculated over a year. Generation on individual days at your location will vary based on meteorological conditions.

 

PV power plant performance is often denominated as Capacity Utilisation Factor or CUF. CUF is the ratio (expressed as a percentage) of the actual output from a plant to the maximum possible output under ideal conditions if the sun shone throughout the day and throughout the year.

• CUFs can vary widely for different regions. Sunny regions worldwide have reported CUFs in excess of 20% while colder regions with much less sunshine have seen CUFs lower than 15%.

Capacity Utilisation Factor (CUF) = Actual energy from the plant (KWh) / Plant capacity (KWp) x 24 x 365

 

Optimizing rooftop PV plant design to maximize power output

Amongst these 4 factors, location is not usually within our control when setting up a captive rooftop solar plant. Some optimization is possible with the other three factors.

Orientation

We can, to some extent, overcome roof orientation issues using trackers. This will, however, add both to the initial cost and maintenance expenditure of the installation. The cost-benefit of using trackers will have to be carefully analyzed for the particular installation to determine if it is worth the additional investment.

Panel Efficiency

If rooftop space is a constraint we can use panels of greater efficiency to maximize the output from the space available.

Ambient Temperature

Ambient temperature is not within our control, but we can help cool the panels by ensuring that we provide adequate room for air to circulate around and under the PV panels. We have seen plant performance improve significantly when panels that were mounted too close to the roof were raised to allow greater air circulation.

 

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