# 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 U _{oc }**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:

U_{OC (-10°C)} = ((1- (35 * (TC_{OC} /100))) * U_{OC 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 V**_{MPP }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:

V_{MPP (70 °C)} = ((1+ (45*(TC_{OC}/100))) * V_{MPP (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 I _{SC}** 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:

U_{SC (70 °C)} = ((1+ (45 * (TC_{SC}/100))) * I_{SC}) = ((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.

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