Inverters & equipment

String sizing tool

Enter your panel's Voc, Vmp, and temperature coefficient, your site's record low and expected hot cell temperature, and your inverter's voltage window. Get a safe min and max panels per string.

Your numbers

1 Project details Optional
Shown on your exported report. Leave blank to skip.
2 Panel electrical specs
Open circuit voltage, from your panel's datasheet.
Voltage at maximum power point, also on the datasheet.
Typically -0.25% to -0.35% for standard silicon panels. Enter as a negative number.
3 Site temperature extremes
NEC 690.7 requires the lowest expected ambient temp for your site. Check ASHRAE design temps or your local weather service extreme record.
Cell temperature runs well above air temperature. 65C is a common design assumption for rooftop arrays.
4 Inverter voltage window
Absolute limit, from the inverter datasheet. Common residential values are 600V and 1000V.
Often slightly below the absolute max input voltage.
Subtracted from the max panel count as a buffer against voltage measurement tolerance.

How it works

Three steps to a safe string size

Step 1

Pull your panel's electrical specs

Get Voc, Vmp, and the Voc temperature coefficient from your panel's datasheet. These three numbers drive the whole calculation, so double check them against the exact model you're installing, not a similar one.

Step 2

Find your site's record low

NEC 690.7 wants the lowest expected ambient temperature for your site, not just a typical winter low. The temperature derate calculator breaks down how much voltage a single cold snap can add.

Step 3

Match against your inverter's window

Your max panel count comes from the cold Voc limit, and your min panel count comes from the hot Vmp limit. Check both against the MPPT voltage window calculator for your exact inverter model.

Understanding your string size

Why cold weather sets your maximum panel count

Solar panel voltage rises as temperature drops. A panel rated for 49.5V Voc at the standard 25C test condition can climb past 58V on a -10C morning. String too many panels together and that cold-morning voltage spike can exceed your inverter's maximum input rating, which trips a fault or, in a worst case, damages the inverter. This is why installers use the site's record low temperature, not the average winter temperature, when sizing a string. The max panels per string calculator isolates this cold-weather limit on its own if you already know your target inverter, and the string length vs panel count calculator covers the broader practical layout question this maximum feeds into.

Why hot weather sets your minimum panel count

The opposite problem shows up in summer. Panel operating voltage drops as cells heat up, and rooftop panels regularly run 30 to 40 degrees Celsius hotter than the surrounding air on a sunny afternoon. If a string is too short, its hot-weather voltage can fall below your inverter's minimum MPPT voltage, and the inverter loses its ability to track the array's peak power point. The result is quiet underperformance rather than a fault code, which makes it easy to miss without checking the numbers directly. The min panels per string calculator isolates this hot-weather floor if you want to check it separately from the cold-weather max. The Department of Energy's PV system design basics cover these layout fundamentals in more general terms.

Why the temperature coefficient matters more than most specs

Standard silicon panels have a Voc temperature coefficient between roughly -0.25% and -0.35% per degree Celsius. A panel at the milder end of that range tolerates a longer string in cold climates, while a panel at the steeper end needs a shorter string for the same site. This single spec, easy to overlook next to headline numbers like wattage and efficiency, can shift your safe max panel count by a full panel or two per string.

Regional differences across the US and Canada

A site in Minneapolis or Winnipeg with a record low near -30C to -40C needs meaningfully shorter strings than a site in Phoenix or Miami, where record lows rarely drop below freezing. Conversely, a hot desert site like Phoenix pushes cell temperatures higher in summer, which tightens the minimum panel count from the other direction. Always use your actual site's numbers rather than a national average, and NREL's PVWatts calculator can help confirm your site's typical production conditions alongside its record temperature extremes.

What this tool doesn't check

String sizing also has a current limit set by your inverter's maximum input current per MPPT input, separate from the voltage limits calculated here. Once you have a safe panel count range, run the parallel strings current calculator to confirm your combined string current stays within the inverter's rating too. This tool also assumes every string in your design matches; if that's not the case, the string mismatch loss calculator covers the output cost of stringing mismatched panels together.

Questions

Frequently asked

It depends on your panel's cold-weather open circuit voltage and your inverter's maximum input voltage. A panel with a 49.5V Voc and a -0.29%/C coefficient hits about 58.8V at -40C, so a 600V inverter tops out at 10 panels per string once you factor in a safety margin.

Panel voltage rises as temperature drops, the opposite of how most people expect electronics to behave in the cold. On the coldest morning your site ever records, a long string can push voltage above your inverter's maximum input rating and trip a fault or damage the inverter.

The string's hot-weather operating voltage needs to stay above your inverter's minimum MPPT voltage, or the inverter can't track the array's power point and output drops. A string that is too short underperforms on hot summer afternoons even though it never trips a fault.

Both numbers are on the panel's datasheet, usually in a table labeled electrical characteristics at STC. Look for Voc in volts and the temperature coefficient of Voc in percent per degree Celsius, typically between -0.25% and -0.35% for standard silicon panels.

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Max panels per string calculator Min panels per string calculator String length voltage calculator String length temperature derate Parallel strings current calculator MPPT voltage window calculator MPPT count vs array layout All inverters and equipment tools