Basic Solar Energy Math

The basic unit of power or electricity is Watt. It is actually a measure of rate of energy. Larger units of power are measured in multiples of 1000. For example,

1000 watts     =       1 kilowatt (kW)

1000 kW        =       1 megawatt (MW)

1000 MW       =       1 Gigawatt (GW)

1000 GW       =       1 Terawatt (TW)

If an electrical appliance consumes 1000 watts for one hour, it has consumed 1 kWh of energy or 1 unit of electricity. So, 1000 watt hours = 1 kilowatt hour (kWh). If you run a 100 watt bulb for 10 hours, it again consumes 1 kWh.

Solar panels are characterized by number of watts (Wp) they can produce under Standard Test Conditions (STC) of 1000 W/m2 irradiation, cell temperature of 25 degree Celsius and air mass of 1.5. This is their peak performance. However, the amount of power they actually produce in outdoor conditions depends upon the amount of sunshine.

Air Mass

Air mass is a measure of the distance traveled by sunlight through the Earth’s atmosphere. Since light intensity is attenuated by scattering and absorption, the more distance it passes through the atmosphere, the greater is the attenuation. Consequently, the sun appears less bright at the horizon (morning and late afternoon) than when at the zenith (noon). An air mass of 1 means the sun is looking straight down on the sea surface when it is directly overhead. At any location with latitude greater than 23.5 degrees, the sun is never directly overhead and so air mass will be always greater than 1. The number 1.5 has been agreed upon for the STC (Standard Test Condition) for testing solar panels.

Solar Irradiance and Solar Constant

Solar irradiance is the amount of sunshine incident on a unit area and is typically expressed in watts per square meter (W/m2) or kilowatts per square meter (kW/m2). Irradiance is measured through an instrument called ‘pyranometer,’ which displays the instantaneous power available from the Sun.

Solar constant is the solar irradiance outside the earth’s atmosphere on a 1 square meter surface oriented normal to the sun’s rays. It is about 1367 W/m2. This is attenuated by the atmosphere and the peak solar insolation on a earth’s surface oriented normal to the sun on a clear day is of the order of 1000 W/m2.

This irradiance of 1000 W/m2 corresponds to Standard Testing Conditions (STC) and is called “peak sun” or “1 sun”. If the incident radiation is concentrated 10 times using a lens or a mirror assembly and the incident power increases to 10,000 W/m2, then the irradiance is called “10 Suns.”

Solar Insolation

Insolation is the amount of solar irradiance that is incident on a fixed area over a period of time, and hence is a unit of energy. It is typically expressed in watt-hours per square meter per day (Wh/m2/day) or kilowatt-hours per square meter per day (kWh/m2/day) or even (kWh/m2/year) for a particular location, orientation and tilt of a surface.

Since 1000 W/m2 is “1 sun”, one hour of this ideal irradiance produces 1,000 watt-hours per square meter (1 kWh/m2). This is also known as “1 sun hour.” Colorful maps of solar potential display solar energy in kWh/m2/day, which is equivalent to the number of full sun hours per day. This is a useful parameter for sizing solar panels in the PV systems. More “sun hours” means more potential for solar power.

Global Horizontal Insolation (GHI): It is the solar insolation received by a fixed flat horizontal surface.

Global Tilt Insolation (GTI): The fixed solar panel or collector is generally inclined at an angle roughly equal to the latitude of its location (facing south in India or any place in the northern hemisphere) to maximize the annual insolation received. The insolation received by such an oriented surface is called the Global Tilt Insolation (GTI).

How much energy does one panel produces?

The unit of electrical energy consumed is generally measured in kilowatt-hours (kWh). If an array of solar panels rated at 1000 Wp produce electricity for 1 hour under good sunshine, they have produced 1 kWh or 1 unit of electricity. The total amount of energy they produce during the day is governed by things like solar latitude which is associated with latitude and season, and atmospheric conditions such as cloud coverage, temperature and degree of pollution apart from panel orientation and shading.

For same sunshine, panels produce more power in cooler climates than under hot temperatures. In India, ideal orientation for solar panels is slight tilt towards true south; in South India placing panels flat (horizontal) will also do.

How much space is required to install 1 kW solar panels?

Under clear skies and good sunshine each square meter is receiving about 1000 watts of solar energy. At typical 15% panel efficiency, a 1 sq m area will generate 150 watts of power. For 1 kW power output about 7 sq m area will be required. After leaving some free space, about 10-12 sq m clear roof area will be required.

How much power a 1 kW solar PV system will annually produce in Delhi?

New Delhi has average daily sunshine of 5.5 hours. If we assume loss of 30 days due to rains and clouds every year, then total annual sun hours are 5.5 x 335 = 1843. Ideally 1843 kWh of energy can be produced. But the actual performance will be less than 100 percent because the outdoor conditions are different from standard test conditions of the panels. So, for a 80 percent system performance the annual power production will be 1474 kWh (ie 1843*0.8). In locations where there are shadows and panel tilt is not towards true south, it is often advisable to consider loss of another 10-15%.


About Goodpal

I am a firm believer in healthy people (mind and body both), healthy societies and healthy environment. Please feel free to comment, share and broadcast your views -- I like rational and intellectual discussions. Thanks for stopping by. Have a Good Day!
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23 Responses to Basic Solar Energy Math

  1. nayanam says:

    Thank you so much for this wonderful and simple explanation.

  2. Goodpal says:

    Thanks for your visit.

  3. Amit says:

    Great explanations in sort. Thanks for provide such a basic and most important informations.

  4. sanjeevgrover says:

    I would like to install this instrument in our home kindly suggest and given full details of the cost and effective cost to our home

  5. V says:

    Clear and concise !!

  6. Solar power in coming time is the main source of electricity. Its is the renewable source of electricity. It reduces the cost of the electricity and make our environment clean.

  7. Explained in a very simple way. Thanks.
    I have a question that for residents rooftop PV panels is any government permissions are required?

    • Goodpal says:

      Thanks Devang, for reading and sharing.

      To my knowledge there is no requirement of govt approval for root-top solar systems. But if you want to claim rebate, check with any MNRE approved installer or MNRE website.

  8. Venkatesh Pulakhandam says:

    Nice explanation

  9. Goodpal says:

    Thanks Venkatesh. Do let me know if you wanted some other useful information to be included.

  10. amritananda sandy says:

    I am looking for the Right Advice to Purchase a Solar System to Run my 220 Volts a.c. Ceiling Fan for a maximum of 20 hours per day. thanks.

    • Goodpal says:

      Please read other articles on this blog. you have all information to estimate your daily energy requirement. If still confused contact any solar vendor.

  11. Pingback: Is it practical to generate all US power by solar PV? | Page 9 | Physics Forums - The Fusion of Science and Community

  12. Goodpal says:

    I am happy to publish this communication received recently through email. Pl note that we have no connection or business relation with this website.

    We recently created an animated info-graphic showing how much power solar panels produce and how many panels are required for various activities / places.

    I came across this Solar Power for Ordinary People page on your website:

    Since you have similar information on your website, I wanted to share it with you.

    The link to the resource is:

    We would love to be included as an additional resource.
    You may also find this companion resource useful on how solar panels works:
    My name is Mitch James and I’m the Communications Director for Let me know if you find this useful. Thank you for your time and consideration.

    I look forward to hearing back,

    Mitch James
    Communications Director
    6860 North Dallas Parkway, Suite 228 | Plano, TX 75024

  13. Pradeep singh says:

    I want to use AC, Tv.and other much it cost.and how many hrs I can use it on daily much wattage l can spend……
    I want to use projector also for teaching purpose.

  14. Goodpal says:

    Hello Pradeep, Please go through other articles on this site. You can easily estimate your power requirement and then size of the system you need. Good Luck.

  15. Mukesh chand Meena says:

    What is maximum ratting of solar panel in (Wp) is available in market and price for installation of 100KW solar power plant connect to grid without batter back only but including other equipment like inverter, controller cables etc

  16. amit kumar says:

    how many unit produced from 1 kw system in a day.

  17. Fortunecp says:

    Thank You for posting you blog/article, and we same product suplayer Solar power systems

  18. Arun Namjoshi says:

    I intend to install solar pannel for power generation. Our roof has a end to end area of about 15*30=450 sq. Foot. What max. area of solar pannels can be installed to get a power equivalent to about 1.5 to 2 KW ?

  19. sanjay says:

    I recently bought 150w solar panel from connected watt meter to Shows reading of ,560Ah in upper left corner. 12.60 in the upper right corner, 11.67vm,3.68Ah,43Wh,12.3Wp,1.14Ap in the lower left corner and 11.4w in the lower right question is if the panel is correctly to150w specifications and giving power according that specs?

  20. Anil says:

    Hello good pal, can you guide me how can i calculate the GHI for a day in kWh/sqM, when i have the per minute readings of irriadiance (kW/sqM) from pyranometer.

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