Methods and analysis for calculating the daily operating time of solar charged off-grid lighting products

Access to clean and affordable lighting services is an important part of addressing issues of equality, development and the environment. Flame based lighting, which is the primary source of light for over a billion people worldwide, is expensive, hazardous for users, and environmentally damaging. In the last decade, off-grid lighting technologies like light emitting diodes (LED) and compact fluorescent lights (CFL) have emerged as a bridge between the traditional flame-based lighting and access to the electric grid. This study includes an evaluation of current empirical and analytical methods for estimating the daily operating time of solar charged off-grid lighting products. The study also explores low cost measures to improve the accuracy of these methods. Experimental tests conducted with a variety of off-grid solar charged lighting products show that a method developed by Fraunhofer ISE for estimating the daily operating time of the products tested is imprecise. Revisions to the Fraunhofer ISE method proposed in this study and an abbreviated version of the PV GAP method (12 hour cycle) resulted in improved estimates of daily operating time. However, neither of these methods provides accurate results for all product types. Future efforts should focus on combining elements of these methods to create an accurate, low cost approach for evaluating daily operating time for off-grid lighting products. This study has also identified a framework to incorporate accessory charging features of lighting products (e.g. mobile phone charging) into an estimate of daily operating time. Future efforts will be directed at validating this new framework for calculating daily operating time.