Inverter Sizing Guide: How to Size A Solar Power Inverter In A Solar PV system?

The inverter size includes two parts: voltage and current size. When adjusting the size of the inverter, different configuration restrictions need to be considered

Solar panel type: TSM-PD14 320 W solar inverter type: SMA Sunny Boy 6000-US

Open circuit voltage VOC = 45.8 V maximum. Input voltage Vmax = 600 V

Maximum power supply voltage VMPP (STC) = 37.1 MPPT voltage range: 250 V-480 V

The temperature coefficient of VOC = -0.32%/°C maximum. Input current = 25 A

ISC temperature coefficient = 0.05%/°C

Short circuit current ISC = 9.10 A

Minimum temperature: -10°C

Maximum temperature: 70°C

Voltage adjustment

Inverter Sizing Guide: How to Size A Solar Power Inverter In A Solar PV system?(图1)

At the lowest temperature (depending on the location, here is -10°C), the open circuit voltage U oc of the modules in each string must not exceed the maximum input voltage of the inverter (600 V):

1) Calculation of open circuit voltage at -10°C:

U OC (-10°C) = ((1-(35 * (TC OC / 100))) * U OC STC) = ((1-(35 * (-0.32 / 100))) * 45.8)

= 50.93 volts

2) Calculate the maximum number of N modules in each string:

N = maximum input voltage (600 V) / 50.93 volts = 11.78 (always rounded down)

The number of solar photovoltaic panels per string must not exceed 11 modules

In addition, at the highest temperature (depending on the location, here is 70°C), the MPP voltage V MPP of each string must be within the MPP range of the solar inverter (250 V – 480 V):

3) Calculate the maximum power supply voltage VMPP at 70°C:

V MPP (70°C) = ((1+(45 * (TC OC / 100))) * V MPP (STC)) = ((1+ (45 * (-0.32 / 100))) * 37.1)

= 31.76 volts

4) Calculate the minimum number of modules M in each string:

M = lowest MPP voltage (250 V) / 31.76 volts = 7.87 (always rounded up)

The number of solar photovoltaic panels in each string must be at least eight modules.

B) Current size

The short-circuit current I SC PV array must not exceed the maximum allowable input current of the solar energy converter:

1) Calculate the maximum current at 70°C:

U SC (70°C) = ((1 + (45 * (TC SC / 100))) * I SC) = ((1 + (45 * (0.05 / 100))) * 9.1)

= 8.89A

2) Calculate the maximum number of strings of P:

P = maximum input current (25A)/8.89 A = 2.84 string (always rounded down)

PV array must not exceed two strings