**Why doesn't PVGIS work???, or, PVGIS doesn't let me make a calculation!!**

**Why doesn't PVGIS work???, or, PVGIS doesn't let me make a calculation!!**

In 99% of the cases, PVGIS actually works just fine. If PVGIS doesn't allow you to make a calculation because the visualize or download buttons don't work, it is almost always because you have not chosen the location for which you want to make the calculation. You can choose the location by:

- Clicking on the map
- Entering an address (or country or city name)
- Typing in latitude and longitude coordinates

**Where do I enter the PV module efficiency?, and/or, where do I enter the module area?**

**Where do I enter the PV module efficiency?, and/or, where do I enter the module area?**

In PVGIS you should enter the *nominal power* of the PV system (a.k.a. *peak power* or *rated power*). If you know the nominal power of your system, you don't need to know the efficiency, except to calculate the area of the modules. This is why:

The nominal power or *peak power* is the power claimed by the manufacturer of the module or system. It is the power output of the module(s) measured at 1000 W/m^{2} solar irradiance (and a module temperature of 25° C and a solar spectrum corresponding to an air mass of 1.5. These conditions are known as *Standard Test Conditions* (STC)). This means that if your modules were 100% efficient, you would need 1 m^{2} to get a system with a peak power of 1 kW.

Since the modules are NOT 100% efficienct you need a bigger area. If you have 10% efficient modules you will need 10 m^{2} to have a 1 kW_{p} system. We will call the module efficiency at Standard Test Conditions as *eff _{nom}*.

In other words, if *P _{pk}* is the nominal power and

*A*the area of the module(s), we have

*P _{pk}= A*eff_{nom}* (1)

**The different PV losses do not add up!**

**The different PV losses do not add up!**

We calculate the losses as *multiplicative*. For instance: if the angle-of-incidence losses are 3.7% and the temperature/irradiance losses are 7.2%, then we first lose 3.7% reflected away at the surface and then we lose 7.2% **of the rest **of the energy because of high temperature or low irradiance. Finally the system losses may be 14% which is calculated on what is left after the temperature/irradiance losses. The total loss will then be 100*(1-(1-0.037)*(1-0.072)*(1-0.14))=23.1%.

## Methods used for the PVGIS calculations

We often receive questions about the methods used to calculate the PV output or the solar radiation data that PVGIS provides. We have a web page describing the methods here. It is very likely that the answer to your questions can be found there. However, there are a number of questions about the calculation methods that are especially common.

**Do the PVGIS calculations take into account the effect of clouds?**

**Do the PVGIS calculations take into account the effect of clouds?**

Yes, PVGIS uses solar radiation data based on satellite images which are used to identify the presence and the thickness of clouds. A short overview of these methods is found in the page about methods.