How many monitoring systems on a PV solar power plant?

IEC 61724-1:2021 

The 61724-1 standard for PV system performance monitoring has been revised. The latest version, released July 2021, contains a recommendation for the number of monitoring stations at a PV power plant. The numbers suggested are too low. Here is what we have found interviewing Hukseflux users.

Solar radiation measurement on PV solar power plant
Figure 1 Two SR30 Class A pyranometers measuring GHI (Global Horizontal Irradiance) and POA (Plane of Array) in a PV performance monitoring system. How many systems should there be on such a power plant? 


IEC 61724-1: Photovoltaic system performance monitoring – Guidelines for measurement, data exchange and analysis –, includes recommendations for the number of monitoring stations at a PV power plant. See below the IEC recommendations and Hukseflux’s observations of common practice in tables 1 and 2. Hukseflux sees many more stations at PV power plants than recommended by IEC.

In some cases, Class A and Class B monitoring systems are mixed. Class B systems, in such case, contain only the most important sensors; POA, GHI and PV module temperature.

Table 1 How many monitoring systems per PV power plant? Copied from  IEC 61724-1: 2021 table 3. The number suggested by IEC is not a requirement, but “guidance” (IEC 61724-1 Clause 7). 



[MW][ # ]Although not specifically mentioned in IEC 6172401, the suggested numbers are generally understood to be a minimum 
 < 40 2 
 40 to 100 3 
 100 to 300 4  
 > 300 + 1 / every additional 200 MWThe number of 1 / 200 MW is generally considered much (at least a factor 4) too low. See next table for common practices


Table 2 What number of monitoring stations to use? Common practices at utility scale solar power plants as observed by Hukseflux.



[MW][ # ] 

 + 1 / offtaker

+ 1 / VPP source


+ 1 / terrain type

In case of a large power plant with multiple offtakers (buyers of electricity): at least 1 station per offtaker

In case a large power plant consists of smaller sources grouped together (together forming a VPP or Virtual Power Plant): at least 1 station per smaller source

The topography of the terrain may require deployment of additional systems to attain good representativeness.

all+ mixing Class A and Class B systemsIn some cases, some of these added systems are not all Class A. 
Class B or “secondary systems” may be added to Class A systems to reduce costs. IEC 61724-1 Class B  systems contain a limited set of sensors only, at least as POA, GHI and panel temperature. In such cases, the Class B systems are equipped with the same “high quality” sensors POA, GHI and panel temperature as used in the Class A systems.
all+ soiling systemsAll systems contain soiling measurement, however, in case non-uniform soiling is expected over the power plant: add separate systems only equipped with soiling measurements.
all+ albedo / reflected The optional albedo or rearside reflected irradiance measurement is often seen when bifacial systems are used. Typically at every station, however at least at 1 station / 50 MW. 
all+ diffuse irradianceThe optional diffuse measurement is often seen when 1 axis trackers are used. Typically at 1 station / 50 MW. 
 > 10 MW minimum number: 2Users consider use of Class A systems above 10 MW.
20 to 40/50 MW

 minimum number: 3



Above 20 MW Class A systems are commonly required as part of asset management. 
Many - but not all - users prefer a minimum of 3 systems because this guarantees: 
- spatial coverage
- redundancy of data (possibility for cross checks)
- redundancy in case of system malfunction
- redundancy in case sensors are sent away for recalibration
> 40/50 MW

 + additional systems 

Simple rules of thumb that we see used in practice as the starting point for design: 

- minimum 3 Class A, above 40 MW + 1 / 20 MW, all Class A
- minimum 3 Class A, above 40 MW + 1 / 40 MW, all Class A
- minimum 3 Class A, above 50 MW + 1 / 50 MW, all Class A
- minimum 2 Class A, above 50 MW + 2 / 50 MW, all Class A
- minimum 3 Class A, above 50 MW + 2 / 50 MW, (1 Class A + 1 Class B)
- minimum 2 Class A, above 60 MW + 1 / 30 MW, all Class A

Where can I order the standard? 

The standard can be purchased from the IEC Webshop

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Read the full articles here:

IEC 61724-1:2021 How many monitoring systems at a utility scale PV solar power plant?