The design process for an off-grid PV system should use conservative, worst-case values to ensure that the system is capable of meeting the energy needs of users throughout the year. There are many locations that have a significant seasonal variance in solar resource due to poor weather or latitude. Many off-grid PV systems will see a significant variance in how loads are used throughout the year, especially in locations that are only seasonally occupied. These two different factors - load usage vs. solar resource - make it important to determine what month to use in the system design as the worst-case scenario. An analysis of loads and usage could be performed on a monthly basis, but the most drastic shift in usage likely occurs between the major seasons in a given region meaning two times per year. Determining the worst-case month can be done using a simple table and a quick calculation using monthly insolation data and Average daily Wh estimates for the two different time periods October - March and April - September. The values and calculation can be performed in Wh or kWh - the ratio is what is important. The two following values used for the design should be chosen from the month with the highest ratio of average daily watt-hours relative to average monthly insolation:
- Design daily insolation
- Design daily watt-hours required
Step 1: Determine monthly ratio of consumption to solar resource
| Month
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Average daily insolation
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[[Load evaluation#Total average daily energy demand|Total average daily energy demand]
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Ratio
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| January
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| February
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| March
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| April
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| May
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| June
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| July
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| August
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| September
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| October
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| November
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| December
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Step 2: Determine design values
| Design daily insolation
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= Average monthly insolation from month with the highest ratio ÷ 30
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| Design daily watt-hours required
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= Average daily watt-hours from month with the highest ratio
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Notes/references
