Understanding your string voltage numbers
Why STC voltage rarely matches real-world voltage
Standard test conditions assume a cell temperature of exactly 25 degrees Celsius, a condition your panels hit only briefly during spring or fall mornings. In practice, your string spends most winter mornings colder than STC and most summer afternoons much hotter. That's why this calculator shows all three figures side by side instead of just the STC number printed on the datasheet. The Department of Energy's PV system design basics cover these array design principles in more general terms.
What the cold Voc number tells you
Cold Voc is your worst-case voltage spike, the number to check against your inverter's absolute maximum input voltage rating. A string that looks fine at STC can climb well past a 600V or 1000V inverter limit on a genuinely cold morning in places like Burlington, Vermont or Winnipeg, Manitoba, where record lows can drop below -25C. The max panels per string calculator works this limit backward to find your safe panel ceiling.
What the hot Vmp number tells you
Hot Vmp is your worst-case voltage floor, the number to check against your inverter's minimum MPPT voltage. Rooftop cell temperatures commonly reach 60 to 70 degrees Celsius on a sunny summer afternoon, even in cooler climates, since dark panel surfaces absorb heat well above the surrounding air temperature. A string sized too short can dip below the inverter's tracking range on exactly the sunny days you want peak production. The min panels per string calculator finds the safe floor for this exact limit.
How panel count changes everything proportionally
Because string voltage is a straight multiplication, every voltage figure scales in direct proportion to panel count. Adding one panel to a 10-panel string increases all four voltage figures by exactly 10%. This linear relationship is why installers usually settle on a specific panel count early and stick with it, since even small changes ripple through every voltage check, which is exactly what the string length vs panel count calculator helps settle before finalizing a design.
Comparing multiple strings on the same inverter
If your design uses more than one string per MPPT input, every string wired in parallel on that input should have matching or very close voltage figures, or the mismatched strings can pull down each other's output, a real loss the string mismatch loss calculator can help quantify. The parallel strings current calculator covers the current side of that same parallel wiring check, and NREL's PVWatts calculator can model the production side of a well-matched multi-string design.