Solar String Design Basics: How Panels Are Grouped

A string is a group of solar panels connected one after the other, so their voltages add up. How many panels go in a string, and how strings are connected to the inverter, is one of the most important decisions in a solar design. It affects safety, output and the life of the equipment.

Key takeaways

• Panels in a string are connected in series, which adds voltage and keeps the current the same.
• A string must stay inside the inverter's voltage range on the coldest morning and the hottest afternoon.
• All panels on one string should have the same direction, tilt and shade as far as possible.
• Parallel strings share one MPPT input and must have equal voltage.

1. Series and parallel

In series, the positive of one panel joins the negative of the next. Voltages add and current stays the same. In parallel, positives are joined together and negatives together, so current adds and voltage stays the same. Many systems use both: several panels in a string, and several strings in parallel.

2. Counting panels per string

The designer divides the inverter's allowed voltage range by the panel voltage, using the Voc at the lowest expected temperature for the upper limit and the Vmp at the highest panel temperature for the lower limit. Cold weather raises the voltage, hot weather lowers it. If you ignore this, the string may exceed the inverter's maximum input on a cold, bright morning or drop out of the tracking range on a hot afternoon.

3. Keep like with like

The weakest panel in a string limits the whole string. If one panel is shaded, dirty or facing a different way, the current of the whole string falls. For this reason panels on different roof faces should be on separate strings, and ideally on separate MPPT inputs. See what MPPT is.

4. Paralleling strings

When two strings share one MPPT input, they must have the same number of panels of the same type and a similar orientation. Different lengths give different voltages, and the tracker then compromises, wasting output. Fuses on each string may be needed when more than two strings are connected in parallel.

5. Optimisers and microinverters

Some systems use power optimisers or microinverters so that each panel works almost on its own. They cost more but help on roofs with a lot of shade or many orientations. For a clear, uniform roof, a plain string design is usually sufficient and cheaper.

6. Practical points

1. Ask for the string layout drawing, showing panels per string and strings per MPPT.
2. Check that the expected cold-day voltage is below the inverter's maximum.
3. Keep cable runs short and tidy to reduce loss and fault points.

Next step

Want a design checked? Send your quotation for review or request a new one through the quotation form.