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
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.[1] The load evaluation shows that loads will be used more frequently during the rainy season, which is common.
- May (highlighted in red) has the worst ratio of solar resource relative to energy requirement throughout the year. The average monthly insolation value (135.47 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
|
147.27 kWh/m²
|
2000 Wh
|
13.58
|
| February
|
140.08 kWh/m²
|
2000 Wh
|
14.28
|
| March
|
166.77 kWh/m²
|
2000 Wh
|
11.99
|
| April
|
161.56 kWh/m²
|
3000 Wh
|
18.57
|
| May
|
135.47 kWh/m²
|
3000 Wh
|
22.15
|
| June
|
157.44 kWh/m²
|
3000 Wh
|
19.05
|
| July
|
149.74 kWh/m²
|
3000 Wh
|
20.03
|
| August
|
178.82 kWh/m²
|
3000 Wh
|
16.78
|
| September
|
172.36 kWh/m²
|
3000 Wh
|
17.41
|
| October
|
170.63 kWh/m²
|
2000 Wh
|
11.72
|
| November
|
161.02 kWh/m²
|
2000 Wh
|
12.42
|
| December
|
164.17 kWh/m²
|
2000 Wh
|
12.18
|
Outputs
| 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
|
Notes/references
