StringCalculator

A panel's voltage rises as the temperature drops. The Voc value on the datasheet is measured at 25 °C — but on a clear winter morning at −10 °C, each panel can deliver 10 to 15% more. Multiplied by the number of panels in series, this rise can exceed the inverter's maximum allowed voltage (Vdc max) and damage it permanently — a failure mode that is usually not covered by the warranty.

That's why the calculator uses your city's 20-year cold record: always size for the worst winter morning, not the average.

Conversely, in hot weather (a roof in midsummer can reach 60–70 °C), panel voltage drops. If the string voltage falls below the inverter's MPPT lower threshold, it drops out or underperforms — right when sunlight is at its peak.

The rule: the string must stay within the MPPT range both hot and cold. If the recommended range shown by the calculator is narrow or non-existent, change the number of panels or the panel model rather than forcing it.

In series, voltages add up and the current stays that of a single panel. In parallel, it's the opposite: voltage stays the same but currents add up — and each MPPT has a current (Imp) and short-circuit (Isc) limit to respect.

Slightly exceeding the max Imp current isn't destructive (the inverter just clips output, losing a bit of production), but exceeding Isc max is dangerous for the electronics. Also check that the number of parallel strings doesn't exceed what the MPPT input can physically accept.

Every meter of cable loses a bit of voltage, and therefore production — continuously, for 25 years. Good practice is to stay under a 1% drop on the DC side. The calculator gives you the standard cross-section to buy (4, 6, 10 mm²…), accounting for the round trip of the current.

Only use double-insulated solar cable (H1Z2Z2-K), UV-resistant, with MC4 connectors of the same brand on both ends — mixing brands is the leading cause of overheating and electrical arcing on the roof.

• Mixing different panels in the same string: the current aligns with the weakest one, so you lose the difference.
• Putting two orientations on the same MPPT (east + west): each orientation deserves its own MPPT, otherwise the power point is a poor compromise.
• Sizing with the standard temperature (25 °C) instead of your local cold record.
• Ignoring partial shading: a chimney shading a single panel penalizes the whole string — consider optimizers only on the affected panels.
• Under-sizing the cable to save a few dollars, and losing 2–3% of production for 25 years.

Hardware side: an accessible DC disconnect switch, a DC surge arrester (required depending on cable length and lightning-strike risk in your area), and fuses per string once you have 3 or more strings in parallel.

Paperwork side (requirements vary by country — the following applies in France): prior notification to the local council in most cases, a self-consumption agreement or grid connection request with the local grid operator (Enedis in France), and an official pre-connection inspection certificate (Consuel in France) for any grid-tied installation. This calculator's results help you prepare the file, but they don't replace a professional's sign-off — check your own country's equivalent requirements.