String sizing is a safety calculation first. The array must stay below the inverter’s maximum DC voltage on the coldest morning, not on a mild afternoon.
Voltage adds in series; current adds in parallel. Panels in one string share the same current, and strings on one MPPT add their current.
Every layout is tested, not guessed. The calculator enumerates the valid series-parallel arrangements, checks voltage, current, DC input power and DC/AC ratio, then ranks them.
What string sizing actually decides
A solar array is not just a pile of panels connected to an inverter. The panels are wired in strings — groups in series — and those strings are connected in parallel to the inverter’s MPPT inputs. The arrangement determines the voltage and current the inverter sees.
Get it wrong in one direction and the inverter shuts down or is damaged by over-voltage. Get it wrong in the other and the system underproduces, clips heavily or refuses to start in the morning. The calculator’s job is to find layouts that satisfy every hard limit, then show you the trade-offs among the safe ones.
The inputs you need from the datasheets
Inverter limits
- Maximum DC voltage (Vmax): the absolute ceiling for the string’s open-circuit voltage. Never exceed it, even briefly.
- MPPT start voltage (Vmin): the minimum voltage at which the tracker begins working.
- Maximum current per MPPT (Imax): the current limit for each tracker input.
- Number of MPPT trackers: how many independent inputs the inverter has.
- AC rating (kW): used for the DC/AC ratio.
Panel specifications
- Voc: open-circuit voltage at standard test conditions (STC).
- Vmp: voltage at maximum power, used for the operating-voltage check.
- Imp and Isc: maximum-power and short-circuit current.
- Wattage: for total array kWp.
- Record low temperature: the coldest expected cell temperature at the site, which sets the voltage rise.
The four checks
1. Cold-weather voltage
Voltage rises as temperature falls. A string that is comfortably under the limit at 25 °C can exceed it on a cold, clear morning, when the array is unloaded and the open-circuit voltage is highest.
String Voc (cold) = panels in series × Voc × cold factor
The 0.30%/°C coefficient is a planning convention. If you leave the record low blank, the calculator applies a flat 1.10 margin. That is safer than assuming a warm climate, but it is not a substitute for the site’s actual record low. If the site never freezes, you can switch the derating off and judge against the datasheet Voc — but be certain before doing so.
2. Operating voltage window
The string must also be above the MPPT start voltage under normal operating conditions:
Requirement: Vmp(string) > MPPT start voltage
A very short string may pass the maximum-voltage check and still fail to start the inverter. This is why “2 panels on the second MPPT” is often not a valid way to use leftover modules.
3. Current per MPPT
Strings in parallel on the same tracker add their current:
Requirement: current ≤ Imax per MPPT
Isc is shown alongside Imp because the short-circuit current is what the hardware must survive under fault conditions. A layout that is just over the limit is not “close enough”; it is outside the specification.
4. DC/AC ratio
The default healthy band is 1.15–1.35. Below it, the inverter is oversized for the array and spends its life at low load. Above it, the array can produce more than the inverter can convert, so the top of the midday curve is clipped. Some modern inverters allow a higher ratio; that is why the band is editable rather than fixed.
The calculator keeps layouts outside the band and marks them as warnings instead of hiding them. A 1.45 ratio may be a deliberate choice for a cloudy site; it should be a choice, not an accident.
A worked example
Twelve 550 W panels with Voc 49.5 V, Vmp 41.5 V, Imp 13.2 A and Isc 14.0 A. The inverter has a 600 V maximum, a 120 V MPPT start, a 26 A per-MPPT limit, two MPPTs and a 5 kW AC rating. Record low is 0 °C.
10 panels in series: 10 × 49.5 × 1.075 = 532 V → below 600 V ✓
11 panels: 11 × 49.5 × 1.075 = 585 V → below 600 V ✓
12 panels: 12 × 49.5 × 1.075 = 639 V → exceeds 600 V ✗
So no single string can hold all twelve panels. The valid arrangement is two strings of six:
Current per MPPT: 1 string × 13.2 A = 13.2 A → below 26 A ✓
Array: 12 × 550 W = 6.6 kWp
DC/AC: 6.6 ÷ 5 = 1.32 → inside 1.15–1.35 ✓
This is a clean design: two strings, one per MPPT, no parallel current stacking, and a healthy DC/AC ratio. If the same array were paired with an 8 kW inverter, the DC/AC ratio would be 0.825 — still electrically safe, but the inverter would be oversized and the calculator would mark it as a warning.
How the layouts are ranked
The tool enumerates every integer partition of the module count into string sizes inside the valid window, distributes those strings across the MPPT trackers without exceeding current limits, and keeps each distinct arrangement. The recommendation order is:
- Layouts that pass every hard limit.
- Closest to the middle of the DC/AC band.
- Fewer strings (longer series, fewer parallel losses).
- Fewest MPPT blocks used.
- Most balanced wiring across trackers.
“FITS” means every hard safety limit is satisfied. “FITS WITH WARNINGS” means voltage and current are safe but the DC/AC ratio is outside your band. “DOESN’T FIT” means a hard limit is crossed, usually the inverter’s maximum DC input when you entered a datasheet value.
Where the calculation stops
Use it to reject impossible layouts before they reach a roof, and to understand why an installer chose a particular string count. Then take the winning layout to the yield calculator to see what it produces and the roof space calculator to confirm the panels fit.
Frequently asked questions
What is a solar string?
A string is a group of panels wired in series, positive to negative, one after another. Each panel adds its voltage, so a longer string has a higher voltage but the same current. Strings are then connected in parallel to the inverter’s MPPT inputs, where their currents add.
Why does cold weather matter for string voltage?
A solar cell’s open-circuit voltage rises as temperature falls. On a cold, bright morning, an unloaded string can exceed the voltage it shows at 25 °C standard test conditions. If that exceeds the inverter’s maximum DC voltage, the inverter can be damaged. The cold-voltage check uses the site’s record low temperature, not the average winter temperature.
What is an MPPT, and how many strings can one input take?
MPPT stands for maximum power point tracker. It is an independent DC input on the inverter that adjusts voltage and current to harvest the most power. One MPPT can usually accept one or more parallel strings, up to its current limit. Two strings on one MPPT must be electrically similar; otherwise the tracker cannot optimise both independently.
What DC/AC ratio should I aim for?
For most residential systems, 1.15–1.35 is a sensible band. A lower ratio means the inverter is oversized and expensive for the array; a higher ratio means more midday clipping but better use of the inverter in low light. The right answer depends on your climate, tariff and how the system is paid for, which is why the band is editable.
Can I add more panels to an existing string later?
Only if the inverter’s voltage and current limits still allow it, and usually only with panels of similar electrical characteristics. Adding a different panel in series drags the whole string toward the weaker panel’s current. The calculator can test a larger module count, but the final check must use the actual added panel’s datasheet and the existing wiring.
Does the calculator account for shading?
No. It checks electrical limits, not irradiance. Shading is a layout and site problem: it can be reduced with optimisers or microinverters, but it cannot be fixed by string sizing alone. Use the roof space checker to study obstacle shadows, and ask for a site-specific shade survey before committing.