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 of average monthly insolation for Puerto Maldonado, Madre de Dios, Peru in the Amazon rainforest with a tilt of 12 degrees of PV module tilt. Despite being relatively near the equator there is significant season variation.

Month	Average daily insolation	Total average monthly 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