Free Calculator
How Many Panels Fit in One String?
Cold weather sets the maximum and hot weather sets the minimum. Pick an inverter, a panel and a climate to see both limits worked out, with the temperature coefficient applied.
| Voc at -10 degC (cold limit) | 53.8 V per panel, up from 49.5 V at STC |
|---|---|
| Max panels before 1000 V is exceeded | 18 |
| Max panels before the 800 V MPPT ceiling | 17 |
| Vmp at 70 degC cell (hot limit) | 36.6 V per panel, down from 41.2 V at STC |
| Min panels to stay above 80 V | 3 |
| String current against the 25 A input limit | 11.5 A is within the limit |
A 1000 V ceiling is the European norm and gives roughly twice the string length of a 600 V unit for the same panel. Most of western Europe, the UK, coastal US. A middle case on both sides.
Why cold sets the maximum, and what this does not check
Panel voltage rises as temperature falls. That makes a clear winter morning at record low the moment of peak DC voltage, not a hot afternoon. Exceeding the inverter's absolute maximum can destroy it, and it is not a warranty claim. In the US, NEC 690.7 makes the cold-side calculation mandatory; the physics is the same everywhere else.
- Uses the record low for the climate you pick, not your site's actual record. Look yours up before building.
- Cell temperature runs 20 to 30 degC above ambient in full sun — the hot figures here already assume that.
- Does not model shading, module-level electronics, or multiple strings sharing one MPPT input.
- Panel figures are representative of each class, not a specific datasheet. Always use your own module's Voc and temperature coefficient.
The mistake this tool exists to prevent
Panel voltage rises as temperature falls. It is counter-intuitive, and it is the single most expensive thing to get wrong in a DIY string design: the dangerous moment for your inverter is a clear, freezing winter morning, not a blazing afternoon. A string sized on the datasheet's standard-test-conditions Voc can sail past the inverter's absolute DC maximum on the first cold sunrise and destroy it, and that failure is not a warranty claim.
The correction is a single number on every datasheet: the temperature coefficient of Voc, a negative percentage per degree Celsius. A panel at minus 25 degrees can sit 12 to 15 percent above its rated Voc. Multiply that by a string of twenty and the margin disappears quickly.
And the quieter mistake at the other end
Too few panels is a real fault too, it just costs energy rather than hardware. As cells heat, Vmp falls, and cell temperature runs 20 to 30 degrees above ambient in full sun. If a short string drops below the MPPT's minimum tracking voltage on a hot afternoon, the inverter stops tracking and the array produces nothing — precisely when the sun is strongest. Nobody notices, because there is no error, only a flat line in the monitoring app.
In the US, NEC 690.7 makes the cold-side calculation mandatory. The physics does not care about jurisdiction, so the same arithmetic applies everywhere.
What to read next
String design is only half the electrical picture. If your array faces more than one direction, or anything shades it for part of the day, module-level electronics change the calculation entirely — our comparison of power optimisers versus microinverters covers when strings stop being the right answer at all.
For three-phase properties, sizing interacts with phase balance: how solar works with three-phase power. And the inverters modelled here have full write-ups in our inverter reviews.