Reading a Solar Panel Datasheet: The Five Numbers That Decide the Install
Wattage is the least useful number on the sheet. These are the ones that determine whether your string fits, your conductor sizes, and whether the inspector signs off.
STC wattage is measured at 25°C cell temperature, 1,000 W/m² irradiance and AM1.5 spectrum - conditions that occur for a few minutes a year at best. Two modules rated 400W can behave very differently when the roof is at 65°C and the string is 300V. Design decisions are made from the other numbers on the sheet.
The module you can actually buy matters as much as the spec: lead time, pallet integrity and warranty terms are part of the purchase, which is why we publish real availability on every solar panel page rather than a catalogue PDF.
| Spec | What it decides | Common mistake |
|---|---|---|
| Voc (open-circuit voltage) | Maximum string length against inverter DC limit at record-low temperature | Using STC Voc and ignoring the cold-temperature multiplier |
| Temperature coefficient of Voc | How much Voc rises as temperature falls | Assuming 25°C is the design case (it never is) |
| Isc / Imp | Conductor ampacity and fuse rating (1.25× Isc) | Sizing conductors from Imp only |
| Maximum series fuse rating | Whether the string can be protected properly | Exceeding it with high-current modules in parallel |
| Mechanical dimensions & load rating | Rail spacing, clamp zones, wind/snow spans, lift plan | Checking module length after the racking is ordered |
Cold raises voltage; heat raises current and lowers output. A module with a Voc temperature coefficient of -0.29%/°C gains roughly 6% Voc at -10°C versus STC. On a 1,000V inverter that is the difference between a 12-module and an 11-module string, and it is the single most common reason a designed string fails the inverter's maximum input check.
Use record-low ambient for the site, not an average winter day. For most of the continental US, -15°C to -20°C is the defensible design low.
-
1
Normalise to the same string voltage
Compute Voc at design-low temperature for each candidate, then see how many modules fit the inverter window.
-
2
Compare cost per usable watt, delivered
Add freight per pallet to the module price, divide by the modules on the pallet. Landed cost per watt is the only comparable number.
-
3
Check the connector and frame
MC4 vs MC4-Evo, frame thickness and clamp zone all decide whether the racking you already own will accept the module.
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4
Read the warranty terms, not the years
Product vs performance warranty, degradation curve, and who performs the claim are the parts that matter at year eight.
Is a higher-wattage module always the better buy?
What is a normal temperature coefficient for Voc?
Do bifacial gains change string sizing?
Price a module against your string design
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