In string design, you define which inverters your system uses and how the panels are wired into strings and connected to the MPP trackers (MPPT). While you work, Eturnity continuously checks whether the wiring is electrically sound.

The automatic design gives you an electrically correct solution in just a few steps. However, because the installation on site rarely follows the calculation model exactly, you can then adjust each string by hand – or draw the entire wiring yourself. The finished string design is the basis for the string plan and the single line diagram, and the inverter costs are included in the bill of materials. The string plan and the single line diagram do not pick up later changes to the string design by themselves: in the string plan, click Update on the roof view; in the single line diagram, click Reload from project data at the bottom right – your manual changes to the diagram are lost in the process.

How to find the page: Open the project > Design group in the left-hand side menu > Inverter.

On the left, you see the Inverter design side menu with your inverters, their MPPTs and strings; on the right, the roof with the panels. Each string appears in its own colour.

The finished strings then appear on the roof. In the Auto-wire window that opens next, you can have them rewired using Horizontal or Vertical, or simply close the window. You can reopen it later with Auto-wire in the toolbar. You then adjust the strings as described below.

Switch polarity, Auto-wire and Generate file are labelled in the toolbar; the other tools are shown as icons – hover over one to see its name (not in very narrow windows up to 768 pixels wide). Some tools only appear when needed: Switch polarity once a selected string has more than one panel, Auto-wire once strings exist, the delete tool once a string is selected, and Cancel as soon as something is selected.

By default, a string only accepts panels of the same type and the same orientation. For strings with optimisers, lift the restriction on different orientations in the bar that appears when you select a string: Allow different orientations. If you don't click an MPPT, Eturnity uses the first free, suitable MPPT. If there is none, the string still ends up on an MPPT, even one that is already in use – in that case, check the assignment in the side menu.

Every change is saved immediately. Use Undo to reverse steps as long as you stay on the page.

At the top of the side menu, a coloured badge shows the result of the check:

A coloured icon on the inverter card, the MPPT and the string shows which string is affected. Hover over it to read the finding. If it says, for example, OPEN-CIRCUIT OVERVOLTAGE, the string is too long for the MPPT. Remove panels or spread them across an additional string.

Click Generate file in the toolbar, tick Compliance report (PDF) and click Generate. You can untick the preselected Export Drawing (DXF). Eturnity stores the files in the project's File manager; Open documents in the message that appears afterwards takes you straight there.

The report first shows a system overview with the panels and inverters and their limits, then the checked values with their result for each inverter and MPPT. What you need to change for a finding is not in the report – it stays here, on the coloured symbols. The report records the state at the time it was created, so generate it again after changes. It is written in the project's correspondence language.

No PDF of the string plan is created here; you create that on the String plan page, which the window refers you to.

The check tells you whether a wiring layout works. The following basic rules explain why – and help you assess a proposal from the automatic design.

In a string, the panel voltages add up, and cold panels deliver a higher voltage than warm ones – for example on a cold, sunny morning before the inverter starts up. Eturnity therefore calculates the open-circuit voltage for cold panels – using exactly the minimum temperature from the settings, −10 °C by default. If it exceeds the permitted input voltage, the inverter can be damaged; that is why this is an Error. Too much DC power, on the other hand, is simply clipped by the inverter. This costs yield but is not dangerous – such findings are Info. If the short-circuit current, including the reserve factor, exceeds the MPPT limit, this violates the IEC 62548 standard; Eturnity reports it as a Warning.

By default, panel temperatures are set from −10 °C to +70 °C. Eturnity does not adapt these values to the site itself – the choice is yours. For systems at high altitude or in very cold regions, set a lower minimum temperature via the gear icon. Bear in mind that a lower minimum temperature increases the calculated open-circuit voltage, which can make a string too long. The automated design uses the same minimum temperature.

The nominal power ratio is the DC power of the panels divided by the AC power of the inverter (DC/AC). A higher value therefore means a smaller inverter. Because they heat up, panels rarely reach their rated power, so an inverter that is slightly smaller than the PV array is usually the most economical choice: at around 110 to 120 %, clipping losses stay low. An oversized inverter is electrically unproblematic but unnecessarily expensive.

The automatic design searches around your target ratio – 110 % by default – and allows combinations from 20 percentage points below to 40 percentage points above it. The target ratio is the setting you are most likely to adjust: systems with an east-west orientation never reach their full power at the same time and can handle a higher ratio.

Panels in a string carry the same current, so the weakest panel holds back the entire string. Therefore, connect panels with different orientations to separate MPPTs – that is why most inverters have several. Different orientations within one string only make sense if every panel in that string has an optimiser, for example on small roofs with several pitches.

Group panels that are shaded at the same time of day – by a tree, for example – into one string. That way, they don't hold back the other strings. The automatic design does not know about shading; this is one of the main reasons to refine its proposals by hand.

Some MPPTs have two DC inputs. Only connect two strings in parallel there if both have the same number of panels of the same type and orientation – otherwise, Eturnity reports UNEQUAL STRINGS DETECTED (different number or different panel type) or DIFFERENT ORIENTATION. The currents of the two strings add up. This way, a larger inverter can also be loaded sufficiently.

To ensure that all projects are designed to the same criteria, administrators set the default values centrally: app launcher in the header > Settings > Projects section > Inverter design. The Electrical design tab contains the panel temperatures and the reserve factor for the check, the Automated inverter design tab includes the Inverter target ratio, among other settings, and the String label naming tab contains the naming scheme for the strings.

The defaults apply to new projects. An additional variant in the same project takes on the values of the most recently edited variant; existing variants keep their values. You can adjust many of these values per variant via the gear icon on the Inverter page, and Load default there restores the company defaults for the open tab.

The naming scheme and the Activate Beta mode toggle take effect immediately in all projects. The toggle is on by default; if it is off, the Inverter page shows an older table view instead of the roof and side menu.