On the DC side, many people are rightly afraid of voltage. But it is the current that, in practice, causes overheating, connector failure, and even fire. Therefore, checking string current is not an optional calculation but a fundamental safety step.
Here, the question is not "does the system produce power," but rather, can the inverter's DC input handle the current we connect to it?
Two pieces of data to always check: Impp and Isc
On a panel's datasheet, we typically find two current values:
- Impp – the current at which the panel delivers its nominal power.
- Isc – short-circuit current, the maximum current the panel can deliver in an extreme situation.
In normal operation, the system works around Impp.
However, when considering protection, Isc is more important.
At the installer level, this logic is sufficient: Impp for operational checks, Isc for safety margins.
What happens with series connection?
In a series connection, voltages add up, but the current does not change.
If a panel's Impp value is 10.5 A, then:
- 8 panels connected in series → 10.5 A
- 14 panels connected in series → 10.5 A
It doesn't matter how many panels are in series: the string current remains the same.
Therefore, if we connect a single string to an inverter's MPPT input, the current calculation is very simple: string current = panel Impp value.
What changes with parallel connection?
In a parallel connection, the voltage remains unchanged, but the currents add up.
Suppose:
One string's Impp current = 10.5 A
Two identical strings connected in parallel to the same MPPT.
Then at the inverter's input:
10.5 A + 10.5 A = 21 A
And here comes the critical point.
The inverter's input current limit
The inverter's datasheet always specifies the maximum input current, for example: Max. input current per MPPT: 20 A
This is not a recommendation. This is a hardware limit.
If, in the example above, 21 A arrives at an input rated for 20 A, then the inverter may shut down, or continuous overheating may begin, or the input electronics may be damaged in the long run. On paper, it's only a 1 amp difference, but in reality, it's an overload.
Let's look at a specific example
It is very important to clarify what we are talking about here: panel current, string current, or the total current going into the inverter's input. The three are not the same, and most errors arise from confusing them.
Let's take a specific piece of data. The panel's datasheet states:
- Impp = 11 A
- Isc = 11.8 A
Impp is the operating current, so in normal operation, approximately this much current flows through a panel when it is working at maximum power. This is still the current of a single panel.
Now let's connect 12 panels in series into one string. In a series connection, the voltage adds up, but the current does not change. So, even with 12 panels in the series, the string's operating current remains 11 A. Here we are no longer talking about panel current, but string current, but the value is the same.
The situation changes when we connect in parallel.
Suppose we connect two identical strings in parallel to the same MPPT. The first string's current is 11 A, and the second string's current is also 11 A. In a parallel connection, the currents add up, so at the inverter's input:
11 A + 11 A = 22 A
Here we are no longer talking about panel current or a single string current, but the total current arriving at the inverter's specific MPPT input.
Now let's look at the inverter's datasheet. Let's assume it states:
Maximum MPPT input current = 18 A
This is a hardware limit. It's not a recommendation, but the limit up to which the inverter's electronics can be safely loaded.
In this example, 22 A clearly exceeds the 18 A limit. There's no deliberation here: the design is incorrect. It might work for a while, but the inverter will either limit power, start to overheat, or be damaged in the long run. This is typically the kind of fault that doesn't appear on the first day but months later.
In such cases, there are three professionally correct solutions:
- connect the two strings to separate MPPTs (if the inverter allows this),
- reduce the number of parallel strings,
- or choose an inverter with a higher input current limit.
So the bottom line is: in series connection, the current does not increase, but in parallel connection, it does. And when choosing an inverter or designing strings, always compare the total current going into the MPPT with the limit value on the datasheet, and not the current of a single panel or a single string separately.
Why shouldn't you design for 100%?
The current of the panels also depends on irradiance and temperature. In strong sunlight and cold air, the current can be slightly higher than the nominal Impp value. Therefore, it is not advisable to design exactly to the inverter's current limit. If the limit is 20 A, then the calculated value should not be 19.9 A. A small reserve prevents many problems in the long run.
When is a string fuse needed?
If three or more strings are connected in parallel, in case of a fault, the other strings can backfeed into the faulty branch. In such cases, the reverse current can be dangerous, so a separate string fuse is required.
For two strings, many inverters' internal protection is sufficient – but this should not be decided by habit, but based on the datasheet.
Typical mistakes in practice
- Most problems are not calculation errors, but oversight:
- only checking the voltage, not the current
- checking how many connectors are on the inverter, but not reading the current limit
- paralleling strings with different orientations
- confusing Impp and Isc values
