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 and insolation largely determine the energy production of PV source. Temperature affects the performance of every component of an off-grid PV system, in almost all cases negatively. 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 the weather and solar resource for a location is most accurate when it is obtained from a nearby weather station that has many years of recorded data that can be analyzed 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 months with the worst weather are the most important. Weather and solar resource data readily available on the internet, although there are print copies of this information for some locations. There is rarely local weather and solar resource data for remote locations where off-grid PV systems tend to be installed, 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 computer models. There is no single definitive source for information that covers the entire globe and is available in many languages, although there are some resources that are close. These will be explored below.

There are additional resources available see - Design resources and Country pages.

Temperature

There are several important values that need to be determined to be able to properly design a PV system:

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 multi-year period will suffice. The highest recorded temperature can be used, but may lead to the the unnecessary oversizing of some system components.

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 value during a multi-year period will suffice. The lowest recorded temperature can be used, but it is unlikely that the PV modules would see this temperature and be exposed to sunlight.

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.

Obtaining temperature data

There are two recommended resources for obtaining weather data for sites:

PVGIS

European Union - Photovoltaic Geographical Information System A free and very detailed production modeling, system design tool and source for weather data that works in most locations globally. Also available in Spanish, French, and Italian.

• 

  (1) Select language (2) Select type of data (3) Select location using name, GPS coodinates, or double-clicking (4) Select period - most recent is recommended (5) Select "Dry bumb temperature." (6) Click "View!" to see the data (7) For more detailed information, click ".csv" to export it as a spreadsheet.

• 

  (8) Determine the minimum ambient temperature by using your mouse to locate to the lowes or one of the lowest temperatures on the graph. (9) Determine the maximum ambient temperature by using your mouse to locate to the highest or one of the highest temperature on the graph.

Solar resource

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