What are the factors that affect the inverter DC input and the Inverters max DC input current?

We know that inverters can convert the DC power to AC power, there are two types of electrical power going through the inverters.

Today, we are going to analyze how to match your panels with your inverters which makes your house get the maximum efficiency from the solar system, what role the inverter DC input current plays in it, and is vital to your choice of the inverter brand?

One of the main challenges a PV developer faces when designing a PV system is making the right decisions about the DC/AC ratio of their solar system. It is crucial to know how to choose the right ratio and which factors you need to take into account.

In a photovoltaic system, with the development and maturity of technology, the module and inverter capacity ratio (DC/AC) is constantly changing, and is no longer 1:1. Due to light conditions, installation angle, wire losses and other factors, the efficiency of the photovoltaic panels cannot reach 100%, most of the time they may only output 70% of the rated power. Even if the weather is very good it can only reach 90% of the rated power, the inverter power cannot be fully utilized, some of the power will be wasted. So in many countries, the solar panel power will exceed the power of inverter by 20% or 30%.

One may be curious about what DC/AC ratio is. To put it simply, photovoltaic systems have two very important elements, one is photovoltaic modules, which convert solar energy into direct current and the other is inverters that converts the direct current input from the solar panels into alternating current.

When considering how to design a photovoltaic system, you need to think about two questions:

- How much direct current can solar panels generate?
- How much AC power can the inverter convert and output?

The capacitance ratio is the relationship between the DC power of the solar panels and the AC power of the inverters.

Usually we will use STC power as a standard for calculation and is a peak power of photovoltaic modules under standard test conditions (irradiance of 1000W/m2, battery temperature of 25°C), or called “nominal power”. Then we should choose the number of panels that can be installed according to the roof area, and at this time you can roughly get a total power value of power generation. Take Jinko 415W as an example, 415W*10 = 4150watt. Then 4.15/1.13≈3.67, so we can choose a machine around 3.6KW. Then there are some factors that may affect the efficiency of the generation.

As environmental conditions such as temperature and irradiance change, the corresponding parameters of the panels change. At lower temperatures and more sunlight, they generate more electricity, and the short-circuit currents of the two is proportional because the open circuit voltage of the module is inversely proportional to the temperature. The open-circuit voltage drops sharply when the temperature rises, and its downward trend far exceeds the trend of current rise, and the power is equal to the current * voltage, then when the temperature rises, the power of the module will decrease.

When the capacity ratio of the modules and the inverters is selected, the factors that affect our access to solar energy are the aforementioned ambient temperature, obscuration, and the hot spot effect, the inverter’s tracking of the maximum power point efficiency of the module, and the maximum short circuit current and maximum DC input current of the inverter. Here we want to talk about the maximum DC input current.

What’s the inverter max DC input current?

The maximum current that can be allowed to pass through the DC cable.

OHM LAW has institute a formula ：I(Current·A)=U(Volt·V)/R（Resistance·Ω）

When the Voltage is stable, the resistance and current are inversely proportional.

In a series circuit, the current across each resistor is equal, because the sum of the voltages at the end of each resistor is equal to the total voltage of the circuit. Meanwhile, the sum of the resistance in this series circuit is equal to the sum of each circuit in this whole series circuit. The current therefore would be the same in each circuit and the whole series circuit.

Firstly, let’s dive into the spec of some well-known panels. i.e. Jinko 415:

You may notice there are two types of specification standards, STC and NOCT. All these specifications are measured in two different conditions which are instituted by the IEC institute(IEC61215,IEC61730).

One of these parameters is called: Maximum Power Current (IMP)

This parameter represents the maximum current allowed to be input to the inverter, i.e. the current selected for the PV module cannot exceed this value. If it exceeds it, it means that the PV module can generate more power than the inverter can convert, meaning it will lose some of the power. This module parameter corresponds to the maximum DC input current (IDC) of the inverter.

The maximum input current of the module ≤ the maximum input current of the inverter.

Of course, the PV panel also has the maximum input voltage and the maximum short-circuit current. The maximum input short-circuit current is the maximum current that the inverter allows to pass after the PV panels connected to the short circuit. If the PV panels short-circuit current exceed this value, the inverters will burn, which does not only affect the capacity, but also affects its service life.

So let’s take a look at some mainstream brands now, such as Sungrow, Goodwe, Huawei, Fronius. Whether single-phase or three-phase grid-connected machines, the maximum DC input current is between 11-13A. We can see above that Jinko 415 Imp is 13.48A, so to a certain extent it will cause a loss and waste of electric energy.

For all products of Each Energy, the maximum DC input is 18A, far more than the current popular Imp of most modules. In the future development trend, the power of the module will definitely become larger and larger, and its Imp will also become larger and larger. Therefore, it is particularly important to choose an inverter that can adapt to the advanced module in 5-10 years.