7. Evaluation Method for Photovoltaic Systems¶

This chapter describes the calculation logic for photovoltaic electricity generation and the corresponding reduction in primary energy consumption.

7.1 Introduction¶

7.1.1 Scope¶

This chapter covers grid-connected photovoltaic systems installed on buildings. Hourly electricity generation is calculated from the array azimuth, tilt angle, system capacity, solar-cell type, and installation method.

7.1.2 Calculation Procedure¶

The photovoltaic calculation is performed in the following order.

Load hourly weather data corresponding to the climate zone and annual solar radiation regional category.
Calculate irradiance on the array plane from the array azimuth and tilt angle.
Calculate the overall system design coefficient from the solar-cell type, installation method, module temperature, and power-conditioner efficiency.
Calculate hourly electricity generation for each photovoltaic system.
Sum generation over all systems and hours, and convert it to primary energy.

7.1.3 Inputs and Outputs¶

The main inputs and outputs used in this chapter are as follows.

Table 1. Inputs
Variable Name	Description	Unit	Input Sheet
\(Region\)	Climate zone	-	Form 0: (5) Climate zone
\(AnnualSolarRegion\)	Annual solar radiation regional category	-	Form 0: (11) Annual solar radiation regional category
\(\eta_{IN,R,i}\)	Rated-load efficiency of the power conditioner for photovoltaic system i	-	Form 7-1: (2) Power-conditioner efficiency
\(CellType_i\)	Solar-cell type of photovoltaic system i	-	Form 7-1: (3) Solar-cell type
\(ArraySetupType_i\)	Array installation method of photovoltaic system i	-	Form 7-1: (4) Array installation method
\(P_{p,i}\)	Array system capacity of photovoltaic system i	kW	Form 7-1: (5) Array system capacity
\(P_{\alpha,i}\)	Panel azimuth of photovoltaic system i	°	Form 7-1: (6) Panel azimuth
\(P_{\beta,i}\)	Panel tilt angle of photovoltaic system i	°	Form 7-1: (7) Panel tilt angle

Table 2. Outputs
Variable Name	Description	Unit
\(e_{p,i,d,t}\)	Hourly generation of photovoltaic system i at hour t on day d	kWh/h
\(E_{PV}\)	Annual reduction in primary energy consumption due to photovoltaic generation	MJ/year

7.2 Irradiance on the Array Plane¶

The irradiance \(I_{S,i,d,t}\) [W/m²] on the array plane of photovoltaic system i at hour t on day d is calculated.

Builelib transforms the direct normal irradiance and horizontal diffuse irradiance in the weather data to the array plane according to its azimuth and tilt angle. The azimuth is 0° for south, 90° for west, 180° for north, and 270° for east. The tilt angle is 0° for horizontal and 90° for vertical. In the calculation, the entered tilt angle is rounded to the nearest 10°.

Direct irradiance on the array plane, \(I_{OD,slp,i,d,t}\), is calculated as follows.

\[ I_{OD,slp,i,d,t} = I_{OD,d,t} \left[ \sin h_{d,t}\cos P_{\beta,i} + \cos h_{d,t}\sin P_{\beta,i} \cos\left(P_{\alpha,i}-A_{d,t}\right) \right] \]

Diffuse irradiance on the array plane, \(I_{OS,slp,i,d,t}\), is calculated using the isotropic-sky model.

\[ I_{OS,slp,i,d,t} = I_{OS,d,t} \frac{1+\cos P_{\beta,i}}{2} \]

The irradiance on the array plane is therefore calculated as follows. When the sun is behind the array plane, direct irradiance is set to zero and only diffuse irradiance is included.

\[ I_{S,i,d,t} = \max\left(0, I_{OD,slp,i,d,t}\right) + I_{OS,slp,i,d,t} \]

Here, \(I_{OD,d,t}\) is direct normal irradiance [W/m²], \(I_{OS,d,t}\) is horizontal diffuse irradiance [W/m²], \(h_{d,t}\) is the solar altitude, and \(A_{d,t}\) is the solar azimuth.

7.3 Photovoltaic Module Temperature¶

The weighted-average photovoltaic module temperature \(\theta_{CR,i,d,t}\) [°C] of photovoltaic system i at hour t on day d is calculated as follows.

\[ \theta_{CR,i,d,t} = \theta_{A,d,t} + \left( \frac{f_{A,i}}{f_{B,i}V^{0.8}+1} +2 \right) I_{S,i,d,t}\times10^{-3} -2 \]

Here, \(\theta_{A,d,t}\) is outdoor air temperature [°C], and \(V\) is wind speed at the array plane [m/s]. Wind speed is fixed at 1.5 m/s regardless of region, day, or hour. The values of \(f_A\) and \(f_B\) are selected from Table 3 according to the installation method.

Table 3. Coefficients by Array Installation Method
Array Installation Method	\(f_A\)	\(f_B\)
Open-rack installation	46	0.41
Roof-mounted installation	50	0.38
Other	57	0.33

7.4 Overall System Design Coefficient¶

The overall system design coefficient \(K_{p,i,d,t}\) for photovoltaic system i at hour t on day d is the product of the correction factors representing the system losses.

\[ K_{p,i,d,t} = K_{HS,i} \times K_{PD,i} \times K_{PT,i,d,t} \times K_{PA,i} \times K_{PM,i} \times K_{IN,i} \]

The shading, degradation, array load-matching, and array circuit correction factors and the maximum-power temperature coefficient are given in Table 4.

Table 4. Correction Factors by Solar-Cell Type
Coefficient	Crystalline Silicon	Other than Crystalline Silicon
Shading correction factor \(K_{HS}\)	1.00	1.00
Degradation correction factor \(K_{PD}\)	0.96	0.99
Array load-matching correction factor \(K_{PM}\)	0.94	0.94
Array circuit correction factor \(K_{PA}\)	0.97	0.97
Maximum-power temperature coefficient \(\alpha_{p,max}\) [1/K]	-0.0041	-0.0020

The inverter circuit correction factor \(K_{IN,i}\) is calculated as follows.

\[ K_{IN,i}=\eta_{IN,R,i}\times0.97 \]

If the rated-load efficiency \(\eta_{IN,R,i}\) of the power conditioner is not entered, 0.927 is used.

The temperature correction factor \(K_{PT,i,d,t}\) is calculated from the difference between module temperature and the standard test-condition temperature of 25°C.

\[ K_{PT,i,d,t} = 1 + \alpha_{p,max,i} \left(\theta_{CR,i,d,t}-25\right) \]

7.5 Hourly Electricity Generation¶

The hourly generation \(e_{p,i,d,t}\) [kWh/h] of photovoltaic system i at hour t on day d is calculated as follows.

\[ e_{p,i,d,t} = P_{p,i} \times \frac{I_{S,i,d,t}}{\alpha_p} \times K_{p,i,d,t} \times10^{-3} \]

Here, \(P_{p,i}\) is the array system capacity [kW], and \(\alpha_p\) is the reference irradiance, set to 1.0 kW/m². The factor \(10^{-3}\) converts irradiance from W/m² to kW/m².

The hourly generation \(e_{PV,d,t}\) [kWh/h] of the entire photovoltaic installation is obtained by summing the generation of all photovoltaic systems.

\[ e_{PV,d,t} = \sum_{i=1}^{n_{PV}} e_{p,i,d,t} \]

7.6 Annual Reduction in Primary Energy Consumption¶

Annual photovoltaic electricity generation \(E_{PV,e}\) [kWh/year] is obtained by summing hourly generation over one year.

\[ E_{PV,e} = \sum_{d=1}^{365} \sum_{t=1}^{24} e_{PV,d,t} \]

The annual reduction in primary energy consumption due to photovoltaic generation, \(E_{PV}\) [MJ/year], is obtained by converting annual electricity generation using the primary energy conversion factor for electricity.

\[ E_{PV} = E_{PV,e} \times f_{prime,e} \times10^{-3} \]

Here, \(f_{prime,e}\) is the primary energy conversion factor for electricity [kJ/kWh]. The default value is 9,760 kJ/kWh; when another value is specified in the calculation settings, that value is used. The resulting \(E_{PV}\) is deducted as a reduction when calculating design primary energy consumption as described in Chapter 1.

7.7 References¶

Building Research Institute, “Technical Information on the Evaluation of Energy Consumption Performance in Accordance with the 2016 Energy Efficiency Standards (Residential Buildings), Chapter 9: Natural Energy Utilization Systems, Section 1: Photovoltaic Systems,” Ver.12, October 2025.
9-1_251001_v12.pdf
JIS C 8907:2005, Estimation Method of Generated Electric Energy by Photovoltaic Power System.
JIS C 8961:2008, Measuring Procedure of Power Conditioner Efficiency for Small Photovoltaic Power Generating Systems.