Straightforward String Sizing

When designing PV systems, it is essential to ensure that the voltage output of each series string is within the acceptable range for the system. If the voltage is too low, the inverter won’t kick on and the array won’t produce power. If the voltage is too high, the inverter could be damaged, potentially starting a fire or other hazards. (This is why we’re much more conservative on the high end of voltage considerations, using the Voc and the record low temperature rather than the Vmp and the average low temperature).

The minimum input voltage varies depending on the inverter model. Some inverters have input voltages above 350V. This will have minimal effect on system performance but can constrain the string sizing by requiring a high minimum number of panels on a string. Many newer inverter models have minimum inputs not much higher than 100V, greatly increasing the flexibility of the string sizing.

The formula for determining the minimum input voltage is as follows:

(Vmp) x (# Panels in series) / (Temp Correction Factor) > Min Input Voltage

In the example diagram, the given values are:

30.2VDC x (# Panels in series) / 1.25 > 150VDC

We find that at least seven panels are required on each string to produce a voltage that meets the 150VDC requirement of the inverter.

The maximum input voltage for all US PV systems is either 600VDC or 1000VDC. NEC Article 690.7 limits one- and two-family dwellings to 600V, though multi-tenant dwellings over two units and commercial sites may be eligible for 1000V. The higher voltage of the 1000V systems allows for larger string sizes and fewer materials/labor associated with running the attendant wires, combiner boxes, and other balance of system components.

The formula for determining the maximum input voltage is as follows:

(Voc) x (# Panels in series) x (Temp Correction Factor) < 600VDC

In the example diagram, the given values are:

37.4VDC x (# Panels in series) x 1.3 < 600VDC

We find that no more than 12 panels can be put on a string in this system, as 13 or more panels will lead to a voltage in excess of 600V.

Take a look at the attached diagram for a visual representation of this example system.

Happy string sizing!