A solar module offer can match the requested wattage and still require a different string arrangement. If a quotation checks voltage only at standard test conditions, it may miss the higher open-circuit voltage that occurs in cold conditions.
Before comparing suppliers, request a voltage evidence sheet linking the exact module revision, temperature input, calculation method, series count and equipment limits. The buyer's task is to make the assumptions comparable and visible. The responsible electrical designer must approve the project calculation and installation.
The STC and NMOT datasheet guide explains reference conditions and output claims. This guide focuses on a separate purchasing decision: whether the proposed modules and series count have adequate documented voltage compatibility.
Keep three voltage questions separate
Voc is open-circuit voltage. Vmp is voltage at the maximum-power operating point. An equipment maximum DC input limit and its maximum power point tracking, or MPPT, operating range answer different questions.
A proposal needs evidence for the maximum voltage it can present, evidence for useful operation within the applicable tracking range, and any separate start-up requirements. Passing one check does not establish the others.
SAM's official system-design help treats series module count as a multiplier of module voltage and distinguishes maximum DC input voltage from the MPPT operating limits. Its automatic sizing estimate uses nominal specifications and is described as a preliminary analysis. A nominal sizing result is therefore not, by itself, a completed cold-weather review. SAM system-design help.
Keep the battery nominal voltage in another field when the application includes a charge controller. A battery-system label is not a substitute for the controller's PV input specification.
Ask which cold-temperature method was used
The module's voltage temperature coefficient describes how its voltage changes with temperature within the applicable model or method. The coefficient must be for Voc, not the coefficient for maximum power or short-circuit current.
Canadian Solar's installation manual EN v3.1, copyright November 2025, applies a temperature correction to Voc using the selected module's coefficient and the lowest expected site ambient temperature. Its instructions also require consideration of the module's own permitted system voltage. Those provisions apply within that manual's named module scope; obtain the applicable instructions for another offered product. Canadian Solar manual, sections 3 and 5.
Record the temperature value, its source and the method that makes it applicable. Do not silently replace the specified design input with the annual average temperature, the usual daytime temperature or the temperature printed beside an unrelated rating.
Module cell temperature and site ambient temperature are different quantities. A manufacturer's prescribed ambient-temperature correction method must be identified as such. Do not mix its input with a cell-temperature model without the designer's agreement.
Victron's BlueSolar MPPT 75/10 to 100/20 installation instructions specifically require Voc and its temperature coefficient to be considered when determining panels in series, and warn of higher Voc below 25°C ambient. The manual's voltage limits and array examples belong to those controller models; they are not limits for every controller. Victron installation manual, PV array section.
Build a voltage evidence sheet
Use one row per input, with a document reference and an owner for anything unresolved.
On small screens, swipe or scroll sideways to read every column. Keyboard users can focus the table and use the arrow keys.
| Review item | Evidence to request | Reason to hold the proposal |
|---|---|---|
| Exact module identity | Full model, power variant, datasheet revision and offered configuration | The calculation uses another model or an unidentified family average |
| Reference Voc | Value, units, reference conditions and applicable tolerance | A typical value is presented as a guaranteed upper limit |
| Voc temperature coefficient | Signed value, units and applicable correction method | The supplier uses the power coefficient or omits units |
| Site temperature input | Project value, source and designer's basis | A convenient average replaces the specified cold condition |
| String configuration | Series count and documented arrangement for each input | A drawing and quotation use different counts |
| Equipment voltage limits | Controller/inverter, module and other relevant DC component limits | Only the inverter headline rating is checked |
| Operating requirements | MPPT range, start-up conditions and relevant temperature cases | An upper-voltage calculation is offered as complete operational approval |
A completed sheet should allow another reviewer to reproduce the assumptions without asking the author which spreadsheet cell or product revision was intended.
Distinguish maximum permitted values, nominal values and tolerances. If an upper-bound allowance is needed, ask the designer how the applicable tolerance and method handle it. Do not invent a generic extra percentage or apply the same allowance twice.
A power-output tolerance is not automatically a Voc tolerance; request the parameter-specific basis. Record each applicable component's voltage limit and conditions separately. Meeting an inverter's higher ceiling does not close a lower limit on another component in the proposed arrangement.
Worked comparison: the same wattage, different voltage decisions
The following values describe a fictional quotation review. They are not a SINAWATTS product specification, a design recommendation or a substitute for the applicable manufacturer's calculation.
Assume the review uses a module Voc of 40.0 V at 25°C, a Voc coefficient of -0.25% per °C, and a specified temperature input of -15°C. The adopted illustrative linear method uses the signed coefficient as a fraction:
-0.25% per °C = -0.0025 per °C
The temperature difference is -40°C. The resulting factor is 1 + (-0.0025 × -40) = 1.10, giving an illustrative corrected module Voc of 44.0 V.
For comparison only, assume the equipment upper limit is 500 V:
On small screens, swipe or scroll sideways to read every column. Keyboard users can focus the table and use the arrow keys.
| Offered series count | Reference string Voc | Corrected string Voc | Limited review finding |
|---|---|---|---|
| 12 modules | 480 V | 528 V | Exceeds the assumed 500 V limit in this scenario |
| 11 modules | 440 V | 484 V | Below that limit, but other required inputs remain to be checked |
| 10 modules | 400 V | 440 V | Below that limit; this does not prove MPPT or start-up compatibility |
The first offer illustrates why a reference-condition check can miss a problem. For the second offer, the arithmetic leaves 16 V below the assumed ceiling. That difference is not an approved safety margin: the example has not established the actual temperature basis, module tolerance, all component ratings or required design allowances.
The third offer cannot be selected simply because it gives the smallest voltage. Its operating voltage, equipment start-up behavior and project output requirements still require review. A purchasing comparison should retain those open rows instead of labelling the entire system acceptable.
If the temperature assumption changes, the calculation must be revised. If the supplier changes module type, do not carry over the old module's coefficient because the replacement has similar wattage.
Turn supplier replies into clear dispositions
Separate an evidence gap from a demonstrated mismatch.
- Documented mismatch: the agreed calculation exceeds an applicable limit, or the manufacturer excludes the proposed configuration. Request a revised configuration and its supporting review.
- Unresolved: a model, coefficient, tolerance treatment, design-temperature basis or relevant equipment limit is missing. Request the specific missing input; absence of evidence is not a measured failure.
- Ready for technical acceptance: all required inputs are traceable and the responsible reviewer has accepted the resulting configuration. This status follows the complete review, not one favorable voltage result.
Ask each bidder to mark deviations in the same sheet. A statement that the offer is “suitable for cold climates” is not an answer to a missing coefficient or an inconsistent series count.
Where accessories are included, connect the voltage sheet to the exact component schedule. Use the PV connector compatibility guide for mating and termination evidence. A compatible connector shape does not establish the voltage suitability of the complete arrangement.
Control the quotation and release package
Request separate commercial scope for modules, accessories, drawings, engineering confirmation and any additional evaluation. Identify sample quantities and production quantities, and ask when the required documents will be supplied. No fixed price, MOQ or lead time follows from a wattage or voltage label.
Record the accepted module and equipment revisions with the approved configuration. Changes to module Voc, coefficient, controller type, site assumptions or series arrangement should reopen the affected checks before substitution is released.
For purchasing discussions, the solar accessories page and product catalogue identify relevant categories. They do not certify the voltage compatibility of a proposed system.
Send a PV voltage evidence RFQ with the module datasheet, equipment references, design-temperature basis, proposed string arrangement and unresolved evidence rows.
Buyer FAQ
Can the module wattage determine the number allowed in series?
No. Similar wattage does not guarantee identical Voc or temperature behavior. Request the exact module's voltage data, the adopted correction method and the applicable equipment limits.
Can I use the power temperature coefficient to correct Voc?
Do not substitute it. Use the coefficient and units specified for open-circuit voltage under the applicable method. Ask for clarification if the quotation provides only a power coefficient.
Is a string below the maximum voltage automatically compatible?
No. The complete review must also address relevant tolerances, component limits, operating voltage, MPPT and start-up requirements. A favorable upper-voltage comparison closes only that defined check.
What should the supplier quote if the cold-voltage evidence is incomplete?
Request the missing documents and identify responsibility, cost and timing for any additional calculation or evaluation. Keep technical acceptance separate from a sample order, and do not release an unreviewed substitution on a general assurance.