What a “string” is
Solar panels are connected in series into a string, and strings feed the inverter (often through a combiner box). Because a string is a chain, its output is limited by its weakest link — one bad panel or connection pulls the whole string down. Most commercial systems have many strings, which is what makes “which string?” the key question.
Why a bad string hides
Here is the trap. If you have twenty strings and one drops by half, your plant total falls by only about 2.5% — small enough to blame on a cloudy week. So the inverter app, which shows the total, looks basically fine, and nobody investigates. Meanwhile that string bleeds energy every sunny day. This is precisely why string-level faults have become the single largest category of solar energy loss across monitored fleets.
The signs of a failing string
A failing string usually shows one or more of these:
- One string consistently reads lower current than its identical neighbours in the same conditions.
- A sudden step-down in a string’s (or the plant’s) daily output that never recovers.
- A string that reads zero — an open circuit, blown fuse, or disconnected connector.
- A string that lags only at certain times of day — often new shading.
| Sign | Likely cause | What to check |
|---|---|---|
| One string reads lower current than its twins | Weak panel or connection | Panels and connectors down that string |
| A string reads zero | Open circuit, blown fuse, or disconnection | Combiner fuse, connectors, DC isolator |
| Step-down in output that never recovers | A panel failed or a joint went | Compare strings; inspect the lagging one |
| Lags only at certain hours | New shading | Trees, walls, water tanks, poles near the array |
| Plant total looks fine but the bill is high | A hidden weak string | String-level comparison (the app can’t see it) |
Finding it by hand
If you are doing it manually, the method is comparison. Identical strings in the same sun should behave identically, so you look for the outlier: compare each string’s current and voltage at the inverter or combiner, ideally around solar noon on a clear day. The string that reads low is your suspect; from there it is a physical check — connectors, fuses, and each panel’s output down the chain. It works, but it is slow, needs a clear day and a technician on site, and only catches what is failing at that moment.
Finding it automatically
The faster, continuous version is exactly what a string-level inspection platform does for you: it watches every string, compares each against its neighbours around the clock, and flags the one that has been underperforming — with a plain-language verdict and what to check. Instead of scheduling a site visit to hunt for a fault, your technician arrives already knowing which string and why.
That is what Solar Performance Cloud is for. If you would rather not chase strings with a multimeter, an inspection platform turns the whole exercise into an alert you can act on.
