In this study, the Solar Irradiation received was estimated in Illorin, Kwara , Nigeria, using the maximum and minimum temperature measured by an Arduino -based solar power parameter measuring system. The Extraterrestrial, Global and Diffuse Solar Radiation were estimated monthly using the Hargreaves and Samani model. This model also required the daily temperature, clearness index, and extraterrestrial solar irradiation in its use alongside the maximum and minimum temperatures. In using this model, we resolved the graphs of the three types of Solar Irradiation over ten days. For the s elected days, the estimated extraterrestrial solar irradiation varies from 10.41kWh/m2 to 10.47 kWh/m2, while the mean extraterrestrial solar irradiation is 10.44 kWh/m2. The estimated global solar irradiation varies from 4.42 kWh/m2 to 5.677kWh/m2, while the estimated mean global solar irradiation is 5.19kWh/m2 per day. The estimated diffuse solar irradiation varies from 3.46kWh/m2 to 5.48kWh/m2, while the mean diffuse solar irradiation is per day is 4.61kWh/m2. Keywords: Solar Irradiation , Hargreaves and Samani , Temperature , Clearness Index Autho rs: Yaqub B. Adediji, Ayobami E. Bamigboye, Joel O. Aboderin, Allison L. Onyeije , and Emman uel O. Uzim . 1. Introduction Global energy demand is increasing rapidly due to population and economic growth, especially in emerging economies such as Africa. It is also glaring that th e current energy infrastructures in place cannot handle this demand . The number of people without access to electricity remains unacceptably high (Hannah Ritchie, 2019) . Fossil fuels are resident s’ alternative to the epileptic power supply, leading to higher greenhouse gas emissions, especially carbon dioxide, contributing to global warming. Hence, there is a need for a shift from conventional energy systems towards a more affordable, , and cleaner energy system to combat these problems. The abundance of solar energy in many countries contributes to why it is currently the world ’s most preferred and valued source of alternative energy . The design of a solar energy conversion system requir es precise knowledge regarding the availability of global solar radiation at the location of interest. Since the global solar radiation reaching the Earth’s surface depends upon the local meteorological conditions ;, a study of solar radiation under local climatic conditions is essential (Alsamamra, 2015) . However, for locations where measured values are not available, solar irradianc e may be estimated using empirical models. Therefore, many researchers have explored various method s to estimate, with reasonable accuracy, the solar radiation from other available meteorological data. Environmental parameters most frequently investigated are sunshine, cloud cover, temperature, and precipitation variables . Solar radiation can be easily estimated from sunshine duration using the Angstrom -Prescott models . The models are sunshine -based and have widely applied to estimate global solar radiation (Angstrom, 1924) (Prescott, 1940) . However, data for sunshine and cloud observations are usually not available at most meteorological stations. Global solar radiation estimation models based on measured air temperature are attractive and a viable option in this context . It is necessary to develop a precise solar radiation model which utilizes commonly available parameters such as maximum and minimum temperatures of the geographical location (Hargreaves GL, 1985) . Several methods have been reported using empirical relationships to estimate global solar radiation from commonly measured meteorological variables. Daily solar extraterrestrial radiation, Ho is often included in the relationships. Ho values are calculated in this project using standa rd geometric procedures. 2. Basic Solar Components 2.1 Declination angle (δ) The declination angle of the Sun is defined as the angle between the Equator and the line drawn between the centers of the Earth and the Sun. The declination angle (δ) varies every season due to the Earth ’s rotation around the Sun and the Earth’s movement on its axis of rotation. It should have an angle of zero but for the Earth ’s tilt on its axis of rotation. Earth ’s Equator is tilted 23.45 degrees with respect to the p lane of the Earth ’s orbit around the Sun, so at various times during the year, as the Earth orbits the Sun, declination varies from 23.45 degrees north to 23.45 degrees south. (Abood, 2015) This declination gives rise to the different seasons. At around 21st December , the Earth’s northern hemisphere is tilted 23. 45 degrees away from the Sun, thus, the winter solstice for the northern hemisphere and the summer solstice for the southern hemisphere. In the periods around 21st June , the southern hemisphere is tilted 23.45 degrees away from the Sun, which results in the summer solstice for the northern hemisphere and the winter solstice for the southern hemisphere. 21st March and 21st September are the fall and spring equinoxes when the Sun is directly overhead the Equator. Note that the Tropics o f Cancer and Capricorn mark the maximum declination of the Sun in each hemisphere (Abood, 2015) . Figure 1: Maximum and minimum value of declination angle Declination is calculated with the following formula: 𝛿=23.45 ×sin (360 365)×(284 +𝑁) Where, 𝛿 = declination angle, N = day number, January 1 = day 1 2.2 Hour Angle ( w) The angle between two planes is the hour angle of a point : one containing the Earth’s axis and the zenith (the meridian plane), and the other containing the Earth’s axis and the given point. The angle is negative east of the meridian plane and positive west of the meridian plane , or it can be positive westward zero to 360 degrees. Th e angle may be measured in degrees or in time, with 24 hours which equals 360 degrees exactly (Bhatia, 2014) . The hour angle is paired with the declination to fully specify the position of a point on the celestial sphere. Again, this is the way we represent the apparent displacement of the Sun away from solar noon. An hour angle of zero degrees indicates that the Sun is directly above, and the sign of the hour angle is determined by occurring either before noon (negative) or afternoon (positive) (Bhatia, 2014) . ω=𝑐𝑜𝑠−1(−tanδ tanϕ) tanδ tanϕ≤1 ω= π tanδtanϕ> 1 Where, ω = hour angle, ϕ = latitude, δ = declination angle Figure 2: Hour angle ( ω) for point Q 2.3 Latitude (ф) Latitude is an angle that ranges from 0° at the Equator to 90° (North or South) at the poles. Lines of latitude refer to arcs or circles that span huge distances. Lines of constant latitude , or parallels, run east - west as circles parallel to the Equator. Latitude is used together with longitude to specify the precise location of features on Earth’s surface . Figure 3: Lines of latitude 2.4 Global Solar Irradiation (H g) Solar irradiation is the amount of electromagnetic radiation from the Sun per unit area (usually square meters). In other words, it ’s the amount of solar power detected by a measuring instrument. When these data are integrated over time, the information is called solar irradiation, insolation, or solar exposure (Garner, 2018) . Figure 4 Solar Radiation System The total radiation is composed of the following three components (Garner, 2018) : 1. Direct radiation ” is also sometimes called “beam radiation ” or “direct beam radiation ”. It is used to describe solar radiation trave lling on a straight line from the Sun down to the surface of the Earth. 2. Diffuse radiation ”, on the other hand, describes the sunlight that molecules and particles have scattered in the atmosphere but that has still made it down to the surface of the Earth. 3. Reflected radiation (the radiation reflected back by the lake, seas, and oth er water bodies). The total ground reflection is a sum of all the above three. Figure 5 Solar Radiation Components 2.5 Hourly Extraterrestrial solar radiation on the horizontal surface (Ho) Extraterrestrial solar irradiation is the irradiance available outside the Earth’s atmosphere. Measurement of extraterrestrial solar radiation intensity is difficult because it varies with the distance between the Earth and Sun. The following arithmetical relationship gives the monthly average of daily extraterrestrial radiation (Duffie, Beckman, & Worek, 2003) . 𝐻𝑜=12 𝜋𝑠(1+0.033 cos(360 𝑑 365))[𝑐𝑜𝑠φcosδsinω+ωsinφsinδ] Where ‘s’ is the solar constant , and its value is 1.367 KW/m2, ‘d’ is the day of the year, ‘ω’ is the sunshine hour angle, ‘φ’ is the latitude angle and ‘δ’ is the declination angle. ω=𝑐𝑜𝑠−1(−tanδ tanϕ) /tanδ tanϕ/≤1 ω= π tanδtanϕ> 1 Solar declination angle (Rani, 1982) is given as δ = 23.45sin [2π (284 + d) 365] 2.6 Daily Extraterrestrial Solar Radiation on the horizontal surface (Ho) Extraterrestrial solar irradiation is the irradiance available outside the Earth’s atmosphere. Measurement of extraterrestrial solar radiation intensity is difficult because it varies with the distance between the Earth and Sun. The following arithmetical relationship gives the monthly average of daily extraterrestrial radiation (Duffie, Beckman, & Worek, 2003) . 𝐻𝑜=24 𝜋𝑠(1+0.033 cos(360 𝑑 365))[𝑐𝑜𝑠φcosδsinω+ωsinφsinδ] Where ‘s’ is the solar constant and its value is 1.367 KW/m2, ‘d’ is the day of the year, ‘ω’ is the sunshine hour angle, ‘φ’ is the latitude and ‘δ’ is the declination angle. ω=𝑐𝑜𝑠−1(−tanδ tanϕ) /tanδ tanϕ/≤1 ω= π tanδtanϕ> 1 Solar declination angle (Rani, 1982) is given as δ = 23.45sin [2π (284 + d) 365] Extraterrestrial irradiation can be used to find the value of the clearness index, KT , which is the ratio of global solar irradiation and extraterrestrial solar radiation. 𝐾𝑇=𝐻𝑔 𝐻𝑜 Where Hg is the global solar irradiation and Ho is the extraterrestrial radiation. If ω <81.4° and 0.3<KT <0.8, The diffused solar radiation (Rani, 1982) , Hd is given as 𝐻𝑑 𝐻𝑔=1.391 −3.650𝐾𝑇+4.189 𝐾𝑇2+2.13𝐾𝑇3 If ω >81.4° and0.3<KT <0.8 , 𝐻𝑑 𝐻𝑔=1.311 −3.022 𝐾𝑇+3.427 𝐾𝑇2+1.821 𝐾𝑇3 3. MATERIALS AND METHODS 3.1 Test Location The University of Ilorin is located in Ilorin, the state capital of Kwara State, in Northcentral Nigeria. It is located on latitude and longitude 8.4912°N, 4.5950°E respectively. It is situated at an elevation of 320 meters above sea level , and th e average annual temperature is 26.5°C. 3.2 Measurement Procedure In this work, BMP 108 sensor was incorporated into a designed Arduino -based solar power parameter measuring system using Arduino Uno. This system measures different solar PV parameters , including the real-time temperature and real -time atmospheric pressure. The measuring system was designed and constructed using Arduino Uno and multiple sensors. The system was proficient in measuring different solar PV parameters, including ac tual-time temperature and real-time. The measured temperature parameters were simultaneously logged into a personal co mputer (PC) for future analysis or