The green alga Scenedesmus sp. was out door scaled-up within three cement open ponds with a final capacity of 15m³ for each one. The used nutrient solution was composed of tap water enriched by macronutrients (NPK) from commercial sources. Dilution of cultures was done according to growth rate and nutrients supplementation as growth reached their maximum dry weight. Continuous centrifugation was carried out at the end of each batch and the obtained biomass was calculated. A comparison was performed; during long term successive season; as growth was employed under the recommended amount of NPK. It was found that electric power represented the maximum cost and the net cost was found in contrast with the volume of growth unit. In addition, enriched growth medium by extra amount of urea and micronutrients led to yield maximization. Maximum cost was observed during inoculums preparation and the lowest cost was found with cultures grown in large scale.