# 7. PV System Sizing Herb Wade Consultant

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7. PV System Sizing Herb Wade Consultant
Solar PV Design Implementation O& M March 31- April 11, 2008 Marshall Islands

7. PV System Sizing Contents 7-1. System Sizing

7-1. System Sizing

Sizing Process Determine the load to be served in Wh/day
Determine the available solar energy on at least a month by month basis Determine the types of equipment that will be used in the system so losses can be estimated Calculate the size of panel that will be needed to meet the required load under the worst month conditions. Calculate the size and type of battery that will be needed to provide needed reliability of power

What is System Sizing System sizing is the process used for determining the minimum panel and battery size needed to deliver the required electrical energy under the solar conditions that exist at the system site. It balances the output from the system with the solar input while taking into consideration losses in the system

We need to know: The solar energy in kWh/m2/day at the site for the lowest solar energy month of the year. The average Wh/day required by the user to operate the desired appliances and any special needs for power that go much beyond the average. The losses that occur in the PV system that reduces the energy available to the user

Estimating the load Determine the Watts required by each of the appliances Estimate the hours per day that each appliance will be used. For each appliance multiply the Watts times hours to get Wh/day Total the Wh/day for all appliances

Problem 4 lights of 11 watts each are installed.
1 light will operate 4 hours per day 3 lights will operate 2 hours per day 1 night light of 1 watt is installed Nightlight operates 10 hours/day 1 Radio of 10 watts is installed Radio operates 9 hours per day How many Wh/day will be needed by the appliances?

The Solar Resource Actual measurements at the site are best but at least one full year is needed and several years is preferred. Measurements taken with instruments tilted at the same angle as the solar panels are best but horizontal “meteorological” measurements are ok. NASA satellite measurements are better than “sunshine hours” recorded for the site “Sunshine hour” measurements indicate the solar variation over the year but are not a good measure of actual solar energy in kWh/m2/day but are better than nothing. Choose the average value of solar for the lowest month as the design basis

Wh/day that needs to come from the panel for systems with batteries
Wiring and connection losses about 10% Losses in the battery about 20% Total losses around 30% so the panel will need to produce enough Wh/day for the load plus enough to cover the losses. So it will have to produce about 130% of the energy required by the load To calculate the Wh/d needed from the panel, multiply the load Wh/d times 1.3

Calculating the panel generation factor (1)
The lowest month kWh/m2/day value is the starting point. (Typically between about 5 and 6 kWh/m2/day) This is the same total energy as would come from the sun shining at 1000 W/m2 each day for the number of hours equal to the kWh/m2/day figure.

Calculating the panel generation factor (2)
Since the Wp of the panel is rated using a value of 1000 W/m2, the number of hours at 1000 W/m2 that we calculated can be directly applied to the Wp of the panel to get the Wh/day the panel would provide under perfect conditions. Suppose the lowest month solar has a daily average of 5.2 kWh/m2/day. That is equivalent to 5.2 hours of 1000 W/m2 sunlight every day. Each Wp of the panel would therefore deliver 5.2 Wh/day if all other conditions were perfect. The conditions are not perfect so we have to correct for the variations from standard conditions.

Calculating the panel generation factor (3)
Corrections include: 15% for temperature above 25 C 5% for losses due to sunlight not striking the panel straight on (caused by glass having increasing reflectance at lower angles of incidence) 10% for losses due to not receiving energy at the maximum power point (not present if there is a MPPT controller) 5% allowance for dirt 10% allowance for the panel being below specification and for ageing Total power = .85 X .95 X .90 X .95X .90 = .62 of the original Wp rating.

Calculating the panel generation factor (4)
To get the panel generation factor (Wh/day per Wp capacity) multiply the daily sun hours times 0.62. For the example, that would be 5.2x0.62 = 3.22 Wh/Wp/day. That is, for every Wp capacity in the panel we can expect to get an average of 3.22 Wh/day during the lowest solar month

Calculating the panel size needed
Divide the Wh/day needed from the panel (1.3 times the load Wh/day) by the Generation Factor in Wh/Wp/day. The result is the minimum Wp of panel needed to meet the design load for the lowest solar month after all losses and corrections have been applied.

Calculating the battery size (1)
The load electricity is provided by the battery. So determining the Ah/day needed by the load will determine the battery capacity that has to be available each day to operate the appliances. For a 12V system, Ah/day = Wh/day/12V Solar design methods usually choose a 20% daily depth of discharge (DOD) for deep discharge batteries. For the modified automotive battery used by AMORE, longer life will be seen if that percentage is reduced to 15% DOD.

Calculating the battery size (2)
The rate of discharge is about 5 hours a day for lights. That represents about a C20 discharge rate if 15% of the battery capacity is used in 5 hours (the discharge rate in Amperes being the capacity of the battery divided by the hours to discharge). So the total battery capacity needs to be the daily Ah at C20 divided by 0.15 if 15% is to be the daily depth of discharge.

Use of automotive batteries
Automotive batteries are quite sensitive to deep discharge so the average percentage of daily discharge should be reduced to 10% to provide longer life and even then the life probably will be less than two years.

Summary of Sizing calculation
Estimate the Wh/day of the load Multiply the load Wh/day times 1.3 Determine the kWh/m2/day of sunlight for the lowest solar month Multiply the kWh/m2/day times .62 to get the generation factor Wh/d/Wp Divide the result of (2) by the result of (4) to get minimum panel Wp. Divide (1) by the battery voltage (12V) to get Ah/day Divide (6) by .2 to get the minimum Ah of the battery at C20.

Estimating the Wh/day that can be used for a particular size of panel
To determine the maximum appliance Wh/day that can be served by a particular size of panel: Multiply the kWh/m2/day times .62 to get the local generation factor Multiply the local generation factor times the Wp of the panel. This will give the estimated Wh/day from the panel Divide the estimated Wh/day from the panel by 1.3 to get the estimated appliance Wh/day that can be served by that panel

Example A 36 Wp panel is installed at a site having a low month solar value of 5.2 kWh/m2/day. What is the maximum Wh/day of appliance load that this panel can serve? Multiply 5.2 x .62 = 3.22 Multiply 36 x 3.22 = 116 Divide 116 by 1.3 = 89 Wh/day of appliance use is possible