Advancing the effective use of light for society and the environment 
Volume 9 Issue 3

July 2006

What is the process to determine the appropriate size of PV panels for a particular application?Two examples are given below to demonstrate the process involved in determining the size of photovoltaic (PV) panels for a particular PV lighting system. The first is a parking lot luminaire that provides approximately 10lux illuminance on the ground. The second is a posttop luminaire that will provide about 0.5lux illuminance (assuming a luminaire that has a single light source and an optical efficiency of 50%, which uniformly distributes all light output on a circular area with a radius equal to the pole height). These are the same examples used in the previous section (see "How does solar radiation vary by location?"); however, in this example they are specified for a particular location: San Diego, California. Example 1:
The wattage of this example system (13 W), was determined through lighting calculation, considering parameters such as the required illuminance on the ground, size of area to be lighted, and luminaire efficiency. This same calculation is needed for gridpowered lighting systems. A CFL was selected for this application because it is the most efficient light source, considering the required light output, to produce the illuminance needed on the ground from the mounting height selected. Once the light source wattage has been determined, the following steps should be performed: Step 1  Calculate the daily energy consumed by the light source in watthours.E_{Daily Consumed} = Lamp Wattage × Daily Operating Hours= 13 watts × 8 hours/day = 104 watthours/day Step 2  Calculate the electric energy that the PV panels need to produce each day.Assume the battery capacity of the system is large enough to allow necessary charging and discharging for powering the lamp.E_{PV Produced} = E_{Daily Consumed} / (Electronics Efficiency × Battery Charge/Discharge Efficiency) = (104 watthours/day) / (80% × 60%) = 217 watthours/day Step 3  Calculate the amount of solar radiation that the PV panels need to collect each day.E_{Solar Radiation Needed} = E_{PV Produced} / (PV panel conversion efficiency)= (217 watthours/day) / 10% = 2170 watthours/day Step 4  Find the average daily solar radiation at the location for the seasons in which this lighting system will be used, or for the season with the lowest amount of solar radiation if designing for use yearround.In December, the 30yearaverage of monthly solar radiation on a horizontal, flat panel in San Diego is 2900 watthours/square meter/day. For a flat panel tilted to an angle of latitude plus 15 degrees (facing south), it is 5000 watthours/square meter/day.Step 5  Calculate the size of the PV panels needed.If the PV panel is in a horizontal position:Size of PV Panels = E_{Solar Radiation Needed} / Daily Solar Radiation = (2170 watthours/day) / (2900 watthours/square meters/day) = 0.75 square meters = ~8.2 square ft If the PV panel is tilted with an angle of latitude plus 15 degrees (facing south): Size of PV Panels = E_{Solar Radiation Needed} / Daily Solar Radiation = (2170 watthours/day) / (5000 watthours/square meters/day) = 0.43 square meters = ~4.6 square ft It is advantageous to tilt the PV panel to an angle of latitude plus 15 degrees (facing south). Therefore, this lighting system in San Diego in December requires 4.6 square ft of PV panels (tilted as described above), in order to collect enough solar energy to power this parking lot luminaire, providing approximately 10 lux (100 moonlights) on the pavement throughout the night for one year. Example 2:
The wattage of this system was again determined through lighting calculation, considering the same parameters as those in Example 1. In this case, an LED is the most efficient light source able to provide the required light output to produce the desired illuminance on the ground from the mounting height selected. CFLs are not available in small enough lumen packages (i.e., with low enough total light output ratings) to be used in this application. Step 1  Calculate the daily energy consumed by the light source in watthours.E_{Daily Consumed} = Lamp Wattage × Daily Operating Hours= 1.5 watts × 8 hours/day = 12 watthours/day Step 2  Calculate the electric energy that the PV panels need to produce each day.Assume the battery capacity of the system is large enough to allow necessary charging and discharging for powering the lamp.E_{PV Produced} = E_{Daily Consumed} / (Electronics Efficiency × Battery Charge/Discharge Efficiency) = (12 watthours/day) / (80% × 60%) = 25 watthours/day Step 3  Calculate the amount of solar radiation that the PV panels need to collect each day.E_{SolarRadiationNeeded} = E_{PV Produced} / (PV panel conversion efficiency)= (25 watthours/day) / 10% = 250 watthours/day Step 4  Find the average daily solar radiation at the location for the seasons in which this lighting system will be used, or for the season with the lowest amount of solar radiation if designing for use yearround.Using the same information as shown in Example 1, the amount of daily solar radiation is 2900 watthours/square meter/day for a horizontal, flat panel. For a flat panel tilted to an angle of latitude plus 15 degrees (facing south), it is 5000 watthours/square meter/day.Step 5  Calculate the size of PV panels needed.If the PV panel is in horizontal position:Size of PV Panels = E_{Solar Radiation Needed} / Daily Solar Radiation = (250 watthours/day) / (2900 watthours/square meters/day) = 0.09 square meters = ~1 square foot If the PV panel is tilted with an angle of latitude plus 15 degrees (facing south): Size of PV Panels = E_{Solar Radiation Needed} / Daily Solar Radiation = (250 watthours/day) / (5000 watthours/square meters/day) = 0.05 square meters = ~0.5 square foot Again, it is advantageous to tilt the PV panel to an angle of latitude plus 15 degrees (facing south). Therefore, 0.5 square foot (0.45 square meter) of PV panels are needed (tilted as described above) for this lighting system in San Diego in December, in order to collect enough solar energy to power this 0.5lux (5 moonlights) illuminance posttop luminaire. If designing for reliable yearround operation, then the appropriate size of PV panels for this system should be at least 0.5 square foot (0.45 square meter). 


