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.

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 solar resource and loads from this month should be used in the design.

Month	Average daily insolation	Total average daily Wh requirement	Ratio
January	6.06 kWh/m²	2.00kWh	.330
February	6.32 kWh/m²	2.00kWh	.316
March	6.49 kWh/m²	2.00kWh	.308
April	6.42 kWh/m²	2.00kWh	.311
May	5.00 kWh/m²	3.00kWh	.600
June	3.75 kWh/m²	3.00kWh	.800
July	3.39 kWh/m²	3.00kWh	.885
August	3.69 kWh/m²	3.00kWh	.813
September	4.21 kWh/m²	3.00kWh	.713
October	5.17 kWh/m²	3.00kWh	.580
November	5.27 kWh/m²	2.00kWh	.380
December	5.60 kWh/m²	2.00kWh	.357

• Month: The month of the year.
• Average daily insolation: Solar resource data obtained for the location from Weather and solar resource data sources.
• Total average daily Wh requirement = Total average daily DC Watt-hours + Total average daily AC Watt-hours
• Ratio = Total average monthly Wh requirement ÷ Average daily insolation