Solar panel string sizing, explained simply
By Fotovol·Updated 10 July 2026
1. What a string is
A string is a group of solar panels wired in series, one after another, like bulbs on a Christmas light chain. The plus cable of one panel connects to the minus of the next, and the two ends of the chain run down to the inverter.
The basic rule of series wiring: voltages add up, current stays that of a single panel. Ten panels at 33 V each form a 330 V string, but the current in the cable remains ~13 A — what one panel produces.
This is where the math comes from: the inverter only accepts voltage within a certain window, and the panel count sets that voltage. Too few panels — the inverter starts late and runs inefficiently. Too many — the voltage exceeds the maximum rating, the inverter can fail, and the warranty is void. String sizing is the check of those two limits.
2. Why panels are wired in series, not parallel
Cable losses grow with the square of the current. Ten panels in parallel would stay at 33 V but push over 130 A — finger-thick cables, serious losses, connectors heating up dangerously.
In series, the same panels deliver the same power at 330 V and only ~13 A, which standard 4-6 mm² solar cable carries over tens of metres with under 1% loss. That is why every modern residential system uses series strings.
The price of this efficiency: the whole string's current is limited by the weakest panel — one shaded panel drags down the entire string. More on that in #7.
3. Voc and Vmp — the two voltages on the panel label
Two datasheet voltages matter for string sizing:
- Voc (open-circuit voltage) — the voltage when the panel delivers no current: early morning, sun up but inverter not yet started. The highest voltage the panel can produce.
- Vmp (voltage at maximum power point) — the voltage at which the panel delivers its rated power in normal operation.
Typical values for a modern 440 W panel: Voc ≈ 39.5 V, Vmp ≈ 33 V, current at maximum power (Imp) ≈ 13.3 A.
Voc checks the inverter's maximum limit (worst case, frosty morning); Vmp checks the working window (day-to-day operation). Careful: both values are measured at 25°C — which is where the trap in #5 comes in.
4. The inverter's MPPT window
The inverter constantly hunts for the point where the string delivers maximum power — the function is called MPPT (Maximum Power Point Tracking). Each MPPT input has three voltage thresholds, listed in the datasheet:
| Threshold | Typical value (5-8 kW residential hybrid) | Meaning |
|---|---|---|
| Start-up voltage | ~90-100 V | below it, the inverter won't even start |
| MPPT range | 90-560 V | the window the inverter can work in |
| Max DC voltage | 600 V | above it, risk of damage + voided warranty |
The string must stay inside the window under all conditions: on a −20°C January morning and on an August afternoon with cells at 65°C.
Residential hybrid inverters usually have two independent MPPT inputs — two strings can run at different voltages, each tracked separately. That detail becomes essential on east-west roofs (#7).
5. The winter trap: voltage rises in the cold
Counterintuitive but physically inevitable: the colder it is, the higher the panel voltage. The datasheet lists a "temperature coefficient of Voc" of about −0.25%/°C: for every degree below 25°C, Voc rises by 0.25%.
On a clear January morning at −20°C, the panel sits 45°C below the reference temperature, so Voc rises by ~11%. A panel rated 39.5 V reaches almost 44 V on the roof. On a 14-15 panel string that can push the string past the inverter's 600 V limit.
That is why serious string sizing is done at the minimum design temperature — for Romania usually −25°C (down to −30°C in the mountain basins of Harghita and Covasna). An installer who counts panels using datasheet Voc without the cold correction builds a system that works fine all summer and can fail on the clearest winter morning.
6. Worked example: 10 × 440 W panels on a hybrid inverter
Let's run the numbers for a typical setup: 10 panels × 440 W = 4.4 kWp, one string, residential hybrid inverter with a 90-560 V MPPT range and 600 V maximum.
| Check | Calculation | Result |
|---|---|---|
| Working voltage (Vmp) | 10 × 33 V | 330 V — comfortably inside the MPPT window |
| Voc at 25°C | 10 × 39.5 V | 395 V — under the limit, but not the real test |
| Voc at −20°C | 395 V × 1.11 | ~439 V — under 600 V with a solid margin |
| String current (Imp) | 13.3 A | under the typical 16 A per-MPPT limit |
Conclusion: 10 panels on one string is a correct configuration for this inverter. The same math gives the boundaries:
- Maximum: 600 V ÷ (39.5 V × 1.11) ≈ 13.6 → at most 13 panels per string; the fourteenth exceeds the limit on a frosty morning.
- Minimum: in summer, at 65°C cell temperature, Vmp drops ~12% to ~29 V per panel, so you want at least 4-5 panels to stay comfortably inside the window.
Exact figures vary by model — your panel may sit at 38 V or 41 V Voc, your inverter may accept 550 V or 1,000 V. The formula stays the same: panel count × Voc × cold correction must stay under the inverter's maximum voltage.
7. East-west roofs and shading: where good design shows
The golden rule: never mix orientations on the same string. East panels produce in the morning, west panels in the afternoon; on one string the weak ones cap the strong ones (see #2) and you silently lose 10-20% of production, with no error shown anywhere.
The correct answer for an east-west gable roof: east string into MPPT 1, west string into MPPT 2. Each input tracks its own maximum. How much you lose versus south orientation is covered in panel orientation and tilt.
For point shading — chimney, dormer, antenna, the neighbour's tree — splitting across MPPTs isn't enough, because the shadow travels across one string. That is where power optimizers (per-panel electronics, the SolarEdge approach) or microinverters come in; we compared the approaches in Huawei vs SolarEdge.
8. Questions to ask the installer (and red flags)
You don't have to do the math yourself — but you can verify in five minutes whether the installer did:
- "How many strings, and on which MPPT inputs?" — the answer should come immediately, with numbers.
- "What is the string voltage at −25°C?" — the key test. A shrug, or "that doesn't matter", means the cold check from #5 was never done.
- "Are different orientations on separate strings?" — anything other than "yes, obviously" is a red flag.
- "Does the string current fit the MPPT input limit?" — modern panels run 13-14 A; some older inverters accept only 12.5 A per input, and the difference is clipped silently.
- "Can I see the string layout before installation?" — a written diagram with panel counts per string is the mark of a serious firm.
More vetting criteria — licences, warranties, contracts — are collected in how to choose an installer. To compare proposed string layouts from several verified firms, use the quote request.
9. What this means for your system
Correct string design doesn't beat physics — in Romania a well-oriented system produces 1,150-1,400 kWh per kWp per year, so our 4.4 kWp example delivers between 5,100 and 6,200 kWh annually. What good design does is protect the investment: no silent clipping, no inverter failing at −20°C, no voided warranty.
Two closing points:
- Single-phase or three-phase: 4.4 kWp fits easily on single-phase — the real limit in Romania is 6-9 kW, depending on your main breaker. Details in single-phase vs three-phase inverter.
- Size for consumption: the panel count follows the household's consumption, not the other way around. Start with the capacity calculator, then let the installer split panels into strings — and avoid the classic prosumer mistakes; oversizing remains the most expensive one.