Load and solar resource comparison

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 to four times per year. Determining the worst-case month can be done using a simple table and a quick calculation. 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	Average daily insolation	Average daily watt-hours required	Ratio
January
February
March
April
May
June
July
August
September
October
November
December

• Month: The month of the year.
• Average daily insolation: Solar resource data obtained for the location from Weather and solar resource data sources.
• Average daily watt-hours required from load evaluation.
• Ratio = Average daily watt-hours required ÷ Average monthly insolation

Step 2: Determine design values

Design daily insolation	= Average monthly insolation from month with the highest ratio ÷ 30

Design daily watt-hours required	= Average daily watt-hours from month with the highest ratio