Weather and solar resource evaluation

Map of global horizontal irradiance (GHI) in kWh/m² which is equivalent to Peak Sun Hours (PSH). GHI does not account for tilt and orientation of PV modules.

Temperature effects the performance of every component of an off-grid PV system, in almost all cases negatively. Lead acid batteries exhibit higher capacity at higher temperatures, but higher operating temperatures greatly shorten the life of the batteries. Temperature and insolation largely determine the energy production of PV source. When designing an off-grid PV system the conditions under which a system will be operating will be factored into the design to make sure that the system can still deliver power when necessary even under the worst case weather and solar resource conditions.

Information about weather is most accurate when it is obtained from a weather station near the site that has many years of recorded data that be examined at the level of days. Weather and solar resource can vary quite significantly from year to year and in the case of off-grid PV system design the worst periods of weather during the year are the most important. Monthly averages and yearly averages can be deceiving as they may tend to hide these periods of particularly bad weather. Weather and solar resource data is most readily available on the internet, although there are print copies of this information for some locations. In many locations globally there is no available local weather information which means that information must be used from the nearest site – which can be quite far away – or be obtained from an organization that provides estimates based upon satellite observation. There is no single definitive source for information that covers the entire globe and is available in many languages at this point, although there are some resources that are close. This means that for a given location various sources of information should typically be explored to find the most accurate resource. See Design resources and Country pages for more information.

Maximum ambient temperature

For PV source and charge controller sizing and selection, Wire and overcurrent protection sizing and selection, and energy storage sizing and selection it is necessary to have a maximum ambient temperature value that these components are likely to be exposed to. It is not necessary to use the highest temperature ever recorded - the highest value during a given one year or two year period will suffice. It may be easiest to find a maximum recorded temperature for a location, but this number may be somewhat extreme and could throw off some design calculations.

Minimum ambient temperature

For PV source and charge controller sizing and selection it is necessary to have a minimum ambient temperature value that these components are likely to be exposed to. It is not necessary to use the lowest temperature ever recorded - the lowest value during given year or two year period should suffice. The lowest recorded temperature can be used, but it may be too extreme and throw off some design calculations.

Minimum indoor temperature

energy storage sizing and selection it is necessary to have a minimum indoor temperature for the location where the system will be stored. This will have to be an estimate decided upon by the designer based upon how well the batteries are protected from low temperatures. Even if the batteries are in a space without heating, they are unlikely to experience the minimum ambient temperature.

Solar resource (insolation)

For the purpose of PV system design, the solar resource for a given location is expressed as insolation in kWh/m². The insolation value for a location will vary each day throughout the year. For the purpose of sizing the PV source for an off-grid PV system the most relevant value is the monthly average insolation value for a location as compared to the monthly average energy requirement - this process is covered in the Load and solar resource comparison. This value will be used to size the PV source to ensure that system can generate enough energy to meet the demands for all loads and to recharge the battery bank during periods of poor weather. It is very important to use a conservative insolation value for PV array sizing calculations.

For determining the days of autonomy for a PV system it can be worthwhile to look at data that is even more detailed than monthly averages. There may be weeks in place like the Amazon rainforest, where it rains continuously for a week straight and the solar resource is very minimal. A system in a location like this may require a larger PV source, additional storage, or another form of power generation.

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

Month	Average daily insolation
January	6.06 kWh/m²
February	6.32 kWh/m²
March	6.49 kWh/m²
April	6.42 kWh/m²
May	5.00 kWh/m²
June	3.75 kWh/m²
July	3.39 kWh/m²
August	3.69 kWh/m²
September	4.21 kWh/m²
October	5.17 kWh/m²
November	5.27 kWh/m²
December	5.60 kWh/m²