Solar PV String Size Calculator
Find the maximum number of PV modules in series so the cold-weather open-circuit voltage stays under the system limit, with the formula and worked example.
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Estimate only. Not engineering or safety advice. Check the result against the applicable standard and the equipment data, and have a competent engineer review it before it is relied on. Terms of use.
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About this calculator
Use this to find how many modules you can connect in series. The open-circuit voltage of a module rises as it gets colder, so the limit is set by the lowest site temperature, not by the standard test conditions on the data sheet.
Limits of this calculator
This sets only the upper limit of the string length. Also check the inverter MPPT window at hot-weather voltage so the string still operates in summer.
Cross-check: 22 reference cases, calculated from textbook formulas coded separately from the engine by the same author, agree with this calculator. This is not an independent review.
Verification status: Engineering reference
Engineering reference. Formula and method documented and tested against separately coded reference calculations. Not yet Verified: the evidence listed on this page is still needed.
What this calculator checks
- The arithmetic of the published formula for the values you entered, in the units shown.
What remains unverified
- That the inputs suit your application. Take ratings, resistances and constants from the manufacturer data sheet or project specification.
Verification register entry
| Calculation author | Rahul Sharma (author) |
|---|---|
| Technical review | No independent technical review has been carried out |
| Individual professional approval | Not recorded |
| Formula reference | not recorded |
| Standard and edition | IEC 62548-1, 2023 + AMD1:2025, consolidated version |
| Standard review state | Public-scope screened; licensed clause review still pending |
| Scope-screening note | PV-array design, DC wiring and protective provisions are relevant to cold-voltage string checks. Confirm module Voc and temperature coefficient, inverter maximum voltage and local code requirements. |
| Cross-check test cases | 22 reference vectors (tests/reference_vectors.json) |
| Review date | not recorded |
| Next review date | not recorded |
| Status | Engineering reference |
Needed before this calculator can be marked Verified: reviewer name, qualification and review date; licensed clause-by-clause standard review; exact formula reference; formula source (textbook or standard); next review date.
Formula
Worked example
| Input | Value |
|---|---|
| Module open-circuit voltage (STC) | 49.5 V |
| Voc temperature coefficient (%/°C) | -0.28 |
| Lowest site temperature (°C) | -5 |
| Maximum system voltage | 1,000 V |
- Voc(cold) = 49.5 × (1 + -0.0028 × -30) = 53.658 V
- N = floor(1,000 / 53.658) = 18
Assumptions
- Standard test conditions are 25 °C
- Linear temperature coefficient
Also check the inverter MPPT window at hot-weather Vmp; this sets only the upper limit.
Inputs and results
| Input | Unit | Allowed or shown | Note |
|---|---|---|---|
| Module open-circuit voltage (STC) | V | greater than 0 V | |
| Voc temperature coefficient (%/°C) | (number) | -1 or more, up to 0 | |
| Lowest site temperature (°C) | (number) | -60 or more, up to 25 | |
| Maximum system voltage | V | greater than 0 V |
| Result | Unit | Allowed or shown | Note |
|---|---|---|---|
| Cold open-circuit voltage per module | V | Shown to 2 decimal places | |
| Maximum modules per string | (number) | Shown to 0 decimal places |
How it is calculated
Each input is converted to the base unit of its quantity, the formula above is applied in the order shown in the worked example, and each result is shown in the unit you choose. Results are held at full precision and rounded only for display, except where a step selects a standard size, as the worked example shows. A value outside its allowed range is refused with a message instead of giving a result.
Typical values
| Lowest site temperature (°C) | Cold open-circuit voltage per module (V) | Maximum modules per string |
|---|---|---|
| -20 | 55.74 | 17 |
| -10 | 54.35 | 18 |
| -5 | 53.66 | 18 |
| 0 | 52.97 | 18 |
| 10 | 51.58 | 19 |
Other inputs are the same as in the worked example above.
Standards and further reading
- IEC 62548 (photovoltaic array design requirements)
Module and battery limits come from the manufacturer data sheet; local grid codes set connection rules. These are named as background reading. Elecmetric does not embed clause text or rating tables, and the reviewer record for this calculator is separate from this list.
Frequently asked questions
What do I need to enter?
Enter module open-circuit voltage (STC), voc temperature coefficient (%/°C), lowest site temperature (°C) and maximum system voltage. Change the unit of a value with its selector and the result stays the same.
Why use the lowest site temperature?
The open-circuit voltage is highest when the modules are coldest and unloaded. The string must stay under the maximum system voltage in that condition.
Where do I find the temperature coefficient?
On the module data sheet, as the temperature coefficient of Voc in percent per degree Celsius. It is negative, so a lower temperature raises the voltage.
Does this check the inverter input window?
No. It checks only the maximum voltage. The minimum number of modules depends on the inverter MPPT window at the hottest operating temperature, which needs its own check.
How accurate are the results?
Every figure comes from the same tested engine as the Elecmetric calculators and is meant for estimating. 22 reference cases, calculated from textbook formulas coded separately from the engine by the same author, agree with this calculator. This is not an independent review. Check design values against the standard and the equipment data you work to.
Formula guides and reference
Longer explanations with worked examples and tables, written for the same topic.
- Solar PV Formulas: Array Size and Strings: Solar PV formulas for array size from daily energy, yearly yield
- Solar Panel Temperature Derating Formulas: Solar panel temperature formulas: cell temperature from air temperature and irradiance with NOCT
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