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 insolation:

• Design insolation
• Average daily Watt-hours required

Example 1: A potential off-grid PV system in Puerto Maldonado, Madre de Dios, Peru in the Amazon rainforest with PV source with a tilt of 12 degrees of PV module tilt. Solar resource data shows that despite being relatively near the equator there is significant monthly variation due to seasonal rains. The load evaluation shows that loads will be used more frequently during the rainy season, which is common.

• July (highlighted in red) has the worst ratio of solar resource relative to energy requirement throughout the year. The average insolation value (3.39 kWh/m²) and Average daily Watt-hours required (3000Wh) from this month should be used in the design.

Month	Average daily insolation	Average daily Watt-hours required	Ratio
January	6.06 kWh/m²	2000 Wh	330
February	6.32 kWh/m²	2000 Wh	316
March	6.49 kWh/m²	2000 Wh	308
April	6.42 kWh/m²	2000 Wh	311
May	5.00 kWh/m²	2000 Wh	600
June	3.75 kWh/m²	3000 Wh	800
July	3.39 kWh/m²	3000 Wh	885
August	3.69 kWh/m²	3000 Wh	813
September	4.21 kWh/m²	3000 Wh	713
October	5.17 kWh/m²	3000 Wh	580
November	5.27 kWh/m²	2000 Wh	380
December	5.60 kWh/m²	2000 Wh	357

• 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 daily insolation