The planning tool for district energy systems

Photovoltaics: Calculation and validation

nPro helps to generate hourly resolved power profiles for photovoltaic systems. On this page you learn how these are calculated and validated.

How are photovoltaic profiles calculated?

The calculation of PV generation profiles in nPro is based on the well-known and validated calculation tool PVWatts of the National Renewable Energy Laboratory (USA). Here, the radiation on the tilted module surface is calculated from weather profiles with the global horizontal radiation and diffuse horizontal radiation and as a function of elevation and azimuth. In a second step, various loss coefficients are taken into account, which represent, for example, aging or ohmic losses. The calculation model used in nPro assumes that the direct current of the PV system is converted to alternating current. The inverter model also uses a calculation approach from the PVWatts tool, which is based on analyses of inverter performance data from the California Energy Commission. For the inverter, partial load efficiencies in the low-light phases are modeled in detail, among other factors. The calculation approach is documented in the technical description of the PVWatts tool. In nPro, a cell temperature coefficient of -0.47 %/K is assumed. This describes the decrease in cell efficiency with an increase in cell temperature.

The nPro tool calculates for arbitrary orientations and inclinations the PV generation profile in hourly resolution.

How were the photovoltaic generation profiles validated?

The profiles generated with nPro were compared to PVWatts generation profiles for a variety of different locations and orientations. An excerpt of the validation is shown in the tables below.

Table 1: Comparison of PV generation profiles from nPro with those from the PVWatts tool for Berlin (Germany) for different orientations and a plant capacity of 1000 kWp.
Orientation nPro PVWatts Deviation
Horizontal 789 MWh 790 MWh 0.2 %
30° / South 893 MWh 900 MWh 1 %
90° / South 633 MWh 614 MWh 3 %
45° / West 672 MWh 671 MWh 0.2 %
45° / East 684 MWh 684 MWh 0 %
90° / West 457 MWh 445 MWh 3 %
90° / East 466 MWh 455 MWh 2 %
45° / North 446 MWh 431 MWh 3 %
Table 2: Comparison of PV generation profiles for different locations for a module orientation of 30° / South and a system power of 1000 kWp.
Location nPro PVWatts Deviation
Berlin (Germany) 893 MWh 900.2 MWh 1 %
Riyadh (Saudi Arabia) 1725 MWh 1740 MWh 1 %
Moscow (Russia) 906 MWh 907 MWh 0.1 %
Kiruna (Sweden) 768 MWh 771 MWh 0.4 %
Bangkok (Thailand) 1324 MWh 1334 MWh 0.7 %
Barcelona (Spain) 1309 MWh 1341 MWh 2 %
Cairo (Egypt) 1547 MWh 1562 MWh 1 %
Table 3: Comparison of monthly PV yields calculated with nPro and PVWatts for Munich (Germany) with 30° inclination and south orientation and a plant capacity of 1000 kWp.
Month nPro PVWatts
January 33.3 MWh 34.3 MWh
February 58.0 MWh 58.8 MWh
March 78.7 MWh 79.8 MWh
April 109.8 MWh 110.9 MWh
May 138.5 MWh 139.1 MWh
June 116.8 MWh 117.0 MWh
July 136.5 MWh 137.5 MWh
August 122.4 MWh 123.3 MWh
September 88.7 MWh 88.8 MWh
October 71.0 MWh 70.3 MWh
November 35.2 MWh 35.1 MWh
December 25.8 MWh 26.0 MWh
Table 4: Comparison of monthly PV yields calculated with nPro and PVWatts for Cairo (Egypt) with 30° inclination and south orientation and a plant capacity of 1000 kWp.
Month nPro PVWatts
January 96.8 MWh 97.9 MWh
February 111.4 MWh 112.2 MWh
March 134.5 MWh 135.3 MWh
April 139 MWh 140.1 MWh
May 150.2 MWh 151.8 MWh
June 149 MWh 150.5 MWh
July 153.4 MWh 154.8 MWh
August 149.1 MWh 151.0 MWh
September 145.3 MWh 146.7 MWh
October 117.3 MWh 118.2 MWh
November 101 MWh 101.7 MWh
December 99.8 MWh 101.2 MWh

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