String Test Verifier
Record each string's Voc, Isc and polarity, compare them with temperature-corrected expected values and with sibling strings, and keep the readings on this device.
A field check, not an audit record. Readings are kept only in this browser on this device until you clear them. Nothing is sent to Wattbench.
Site and module datasheet
Conditions at the time of the test
Strings (2 of 24)
String 1
As recorded. No minimum is applied.String 2
As recorded. No minimum is applied.
Sheet summary
Enter the datasheet Voc, its temperature coefficient and the measured module temperature to compare strings.
Nothing kept on this device yet.
Show readings as text
- Compares each string's measured Voc with a temperature-corrected expected value built from the datasheet Voc, its coefficient and a measured module temperature.
- Scales datasheet Isc only when you enter irradiance. Without it, Isc is compared between sibling strings, and in low light the sibling spread is the primary check.
- Labels arithmetic patterns, such as Voc about one module's worth low, as inferences to check, and reads each string as consistent or review against the tolerance you set.
- Readings stay in this browser on this device until you clear them or copy them as text.
How it works
Expected open-circuit voltage (Voc) starts from the datasheet Voc you enter and its temperature coefficient, corrected to the module temperature you measured:
Voc(T) = modules × Voc_datasheet × (1 − |β| ÷ 100 × (T − 25)), where 25 °C is the datasheet reference cell temperature. It is the same correction the string sizing calculator uses, and it is temperature-corrected only.
- Temperature must be measured. Use a back-of-module or cell reading taken at the time of the test. Ambient air temperature is not a substitute. Back-of-module readings run below cell temperature, and no offset is added.
- Voc also falls in low light.No irradiance correction is applied to Voc. With no irradiance entered, or with the light marked low or changing, the string-to-string spread is the primary check: each string's Voc per module is compared with the median of its siblings.
- Short-circuit current (Isc) is scaled only when you enter irradiance:
Isc = Isc_datasheet × G ÷ 1000, the datasheet reference irradiance in W/m². With no irradiance entered it is not scaled at all, and Isc is compared between sibling strings only. - Whole-module patternscome from dividing measured Voc by one module's reference Voc and rounding to the nearest module. A reading closer to 0 V than to one module is labelled an open circuit.
- The tolerance is yours. It starts blank. Without it, deviations are shown but not flagged. Insulation resistance is recorded as entered, with no minimum applied.
Each string reads consistent or review. The pattern labels are arithmetic inferences to check on the roof, not a diagnosis. Bifacial modules and strings with optimisers or microinverters break these expectations, so the sheet warns when either is switched on. Translating measured I–V data to reference conditions is the subject of IEC 60891, which this sheet does not apply.
For where string tests sit in commissioning, see the IEC 62446-1 scope page, the commissioning evidence pack and the commissioning and handover field pack. For a low-production callout, start with the low-output playbook.
Readings are kept only in this browser on this device, under their own record, until you clear them. Copy them as text into your own job system when you need them elsewhere. This is a field check, not an audit record.
Estimates only: for guidance, not a quote or a compliance certificate. Verify against the current AS/NZS standards and a licensed design. The maths runs in your browser. Nothing is sent to Wattbench unless you sign in and deliberately save the result.
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Frequently asked
- Does Wattbench set the comparison tolerance?
- No. The tolerance field starts blank and is yours to fill. Without it, deviations are shown but not flagged, and only whole-module, open-circuit and polarity patterns mark a string for review. No minimum is applied to insulation resistance.
- Why not use the air temperature?
- Voc follows cell temperature, which ambient air temperature does not represent. Enter a back-of-module or cell reading taken at the time of the test. Back-of-module readings run below cell temperature, and the sheet adds no offset.
- Why is Isc sometimes not compared with the datasheet?
- Isc scales with irradiance, so an expected Isc exists only when you enter a plane-of-array irradiance reading. Without one the sheet does not assume a reference irradiance; it compares Isc between sibling strings instead.
- Do these expectations hold for bifacial modules or MLPE?
- Not fully. Rear-side light on bifacial modules adds current a front-side irradiance reading does not capture, and optimisers or microinverters change what string readings represent. Switch either on and the sheet warns you and narrows its comparisons.
- Where are the readings stored?
- Only in this browser on this device, under their own record. Nothing is sent to Wattbench or an account. Clear them with the clear control, or copy them as text into your own job system. This is a field check, not an audit record.
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