Abstract— Lack of available diffuse solar radiation data in Nigeria and the variation of commonly used estimation methods with site meteorology called for the need to evaluate models and verify their suitability based on the local environmental conditions before application for the design and development of solar energy systems. This study assesses the performance of six models for estimating the monthly average daily diffuse solar radiation over Nigeria. The models considered utilize clearness index, cloud cover, relative sunshine duration or a combination of these parameters as input, for estimating diffuse solar radiations on a horizontal surface. The performance evaluation of estimation models used data sets for 37 cities with varying meteorology, which spread across the six geopolitical zones in Nigeria, for a period of 22-years. The applicability of considered methods of estimation was determined based on the coefficient of correlation, coefficient of determination, mean bias error, root mean square error (RMSE), mean absolute bias error (MABE) and relative percentage error (RPE). This study finds that the multivariable regression relationship, which uses clearness index and cloud cover as input parameters, performs best in the estimation of diffuse solar radiations on a horizontal surface for Nigeria. It gives the lowest range of error terms, with RMSE (0.0778–0.1981 MJ/m 2 ), MABE (0.0663–0.1623 MJ/m 2 ) and monthly RPE of ± 6%. Index Terms—cloud cover, diffuse solar radiation, estimation model, meteorological parameter, solar energy. I. INTRODUCTION Global solar radiation is one of the most commonly used parameters for the design, optimization and performance evaluation of solar energy systems for any given location. Complete and accurate solar radiation data are of considerable significance for research and applications in agriculture, agrology, architecture, ecology, environment, hydrology, limnology and oceanography [1]. Global solar radiation consists of the direct normal irradiation, which is Manuscript received May 9, 2014. This work is supported in part by the Ambrose Alli University through the Education Trust Fund for Academic Staff Training and Development (ETF AST&D) Intervention under Grant AAU/REG/ETF.560/475. M. S. Okundamiya, Ph.D., is with the Electrical and Electronic Engineering Department, Ambrose Alli University, P. M. B. 14, Ekpoma, Nigeria (e-mail:msokundamiya@aauekpoma.edu.ng; msokundamiya@yahoo.com). J. O. Emagbetere, Ph.D., is with the Electrical/Electronic Engineering Department, University of Benin, Benin City, Nigeria (e-mail: joyokumo@yahoo.com). E. A. Ogujor, Ph.D., is with the Electrical/Electronic Engineering Department, University of Benin, Benin City, Nigeria (e-mail: Emmanuel.ogujor@uniben.edu, oguemma@yahoo.com). required for concentrating solar power projects, as well as the diffuse solar radiation on a surface. The diffuse solar radiation is the prime parameter, which directs the quality of both global solar radiation and direct normal irradiation [2]. It gives a view of the atmospheric changes that affect the available solar radiation, such as clouds, smoke, dust or pollen [3]. In addition, knowledge of the long-term average daily diffuse fraction and its contributions to the global solar radiation is of fundamental importance as it enables the efficient and economic utilization of solar energy systems on a tilted plane [4]. In spite of the significance of diffuse radiation, ground-based measured data are not readily available in Nigeria [5]. For such regions where surface measurements (of diffuse solar radiation) are sparse or nonexistent, a common practice is to rely on available methods of estimations as well as to develop new methods. Several methods in [2], [4], [6]–[11] have been proposed for estimating the daily average diffuse solar radiation on a horizontal surface. A commonly used approach is on the basis of meteorological parameters, correlating the diffuse solar radiation as a function of either the clearness index or cloud cover (also referred to as cloud fraction or cloud amount). The advantage of using this approach is that only one observed or measured input (i.e., global solar radiation or cloud cover) is required [10]. A second-order polynomial model deduced in [4] was used for estimating the daily diffuse solar radiations on a horizontal surface for three sites, each selected from the northern, central and southern part of Nigeria. The results indicate the significance of model calibration for a given location. In this study the daily meteorological data sets of 37 cities, which spread across the six geopolitical zone of Nigeria, for a period of 22-years were applied to:  calibrate and validate five existing diffuse solar radiation models;  develop a new multivariable regression model to estimate daily diffuse solar radiations from clearness index and cloud cover; and  compare estimates from the newly developed and five existing radiation models with the measured values. The objective is to determine the most suitable model for estimating the average daily diffuse solar radiation on a horizontal surface for Nigeria. A better estimation tool will ensure a more accurate prediction of future or past values of daily diffuse solar radiations on a horizontal surface in Nigeria and other regions with similar climatic conditions. Assessment of Six Daily Diffuse Solar Radiation Models for Nigeria Michael S. Okundamiya, Joy O. Emagbetere, and Emmanuel A. Ogujor The 4 th International Workshop on Computer Science and Engineering – Summer, WCSE 2014; Dubai; UAE; pp. 13 – 21, August 22 – 23, 2014.