Inverter Sizing Guide: How to Size A Solar Power Inverter In A Solar PV system?
Sizing a solar power inverter is not a difficult task if done correctly. This article shows you with the aid of a simple example how to size a solar power inverter.
The inverter sizing comprises two parts, voltage, and current sizing.
During the inverter sizing you need to take into account the different configuration limits (see the table below).
The next example is a solar panel system in the USA with:
- 20 solar PV panels type Trina Solar TSM-PD14 320 W
- One inverter: SMA Sunny Boy 6000-US
How to size the solar power inverter (SMA Sunny Boy 6000-US)?
The next table summarizes the configuration limits, which should be considered when sizing the solar power inverter (Data from the inverter and solar panel data sheets).
| Solar Panel Type: Trina Solar TSM-PD14 320 W | Solar Inverter Type: SMA Sunny Boy 6000-US |
|---|---|
| Open Circuit Voltage VOC = 45.8 V | Max. Input voltage Vmax = 600 V |
| Maximum Power Voltage VMPP (STC) = 37.1 | MPPT voltage range: 250 V – 480 V |
| Temperature Coefficient of VOC = - 0.32%/°C | Max. input current = 25 A |
| Temperature Coefficient of ISC = 0.05%/°C | |
| Short Circuit Current ISC = 9.10 A | |
| The lowest temperature: -10°C The highest temperature: 70°C |
A) Voltage Sizing
At the lowest temperature (location dependent, here -10°C), the open-circuit voltage Uoc of the modules in each string must not exceed the maximum input voltage of the inverter (600 V):
1) Calculation of the Open Circuit Voltage at -10°C:
UOC (-10°C) = ((1- (35 * (TCOC /100))) * UOC STC) = ((1- (35 * (-0.32/100)))*45.8)
= 50.93 Volt
2) Calculation of N the maximum number of modules in each string:
N = Max input voltage (600 V)/50.93 Volt = 11.78 (always round down)
The number of solar PV panels in each string must not exceed 11 modules
Besides, at the highest temperature (location dependent, here 70°C), the MPP voltage VMPP of each string must be within the MPP range of the solar power inverter (250 V – 480 V):
3) Calculation of the maximum Power Voltage VMPP at 70°C:
VMPP (70 °C) = ((1+ (45*(TCOC/100))) * VMPP (STC)) = ((1+ (45 * (-0.32/100)) )* 37.1)
= 31.76 Volt
4) Calculation of the minimum number of modules M in each string:
M = Min MPP voltage (250 V)/ 31.76 Volt = 7.87(always round up)
The number of solar PV panels in each string must be at least eight modules.
B) Current Sizing
The short circuit current ISC of the PV array must not exceed the allowed maximum Input current of the solar power inverter:
1) Calculation of the maximum Current at 70°C:
USC (70 °C) = ((1+ (45 * (TCSC/100))) * ISC) = ((1+ (45 * (0.05/100))) * 9.1)
= 8.89 A
2) Calculation of P the maximum number of strings:
P = Maximum input current (25A)/8.89 A = 2.84 strings (always round down)
The PV array must not exceed two strings
C) Conclusion:
- The PV generator (PV array) consists of two parallel strings, which are connected to the solar power inverter.
- In each string ten solar PV panels, which are connected in series.
Note: With the aid of a photovoltaic planning program you do not need to make these calculations, the program makes it automatically.
