Does supplemental lighting make sense for my crop?-empirical evaluations

An empirical approach to conduct crop-specific cost and return analyses is presented in order to assist greenhouse growers to make better decisions regarding investing in appropriate supplemental lighting (SL) technologies. Electricity cost of lighting was estimated to obtain the target daily light integral (photosynthetically active radiation, PAR) considering factors including lamp photon efficiencies (μmol J^-1). Possible heating fuel cost offsets by SL were also considered in the analyses. Crop specific efficacy of lighting was expressed as dollar return (gross profit) per mole of PAR, estimated from crop productivity (g mol^-1 PAR), wholesale prices, and gross margin. The crop productivity was obtained from linear regression of cumulative yield (kg m^-2) of selected crops (lettuce, tomato, and strawberry) versus cumulative PAR (mol m^-2) obtained in previous studies in Arizona and New York, in which we assumed that crop-specific productivity (g mol^-1) was the same under natural light and under SL. In these example analyses, the highest productivity (14 g mol^-1) was found for cluster-type tomatoes. Lettuce productivity varied largely (2.8-6.9 g mol^-1). The least productive crop was strawberry (1.5-2.1 g mol^-1). For strawberry, even at a high wholesale price of $ 10 kg^-1 and a relatively low electricity price ($ 0.09 kW h^-1), SL was not profitable for most cases according to our analysis. Lettuce and tomato efficacy ranged between $ 0.011-0.041 and 0.010-$0.018 mol^-1 respectively, depending on the growing conditions and cultivar. For lettuce and tomato, profitability from supplemental lighting was dependent on the lamp photon efficiency, electricity price, lamp photon utilization factor and heating fuel cost offset. The approach presented here is easily adoptable by growers who have access to their historical yield data and daily light integrals and the growing conditions (such as environmental set points and utility rates).