Solar Panel String Calculator
Find how many panels fit in a string for your inverter and your local cold and heat, and get a wiring layout for your panel count.
Wire too many panels in series and a cold morning pushes the voltage past what the inverter can take. Wire too few and the string drops out of the inverter’s working range on a hot afternoon. This calculator finds the safe number of panels per string for your panel, your inverter and your ZIP code.
It’s for installers, designers and homeowners who have the panel and inverter data sheets in front of them. Type in the numbers from the sheets, add your ZIP code, and it shows the shortest and longest string that works. Add your panel count and it suggests how to split them into strings.
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Planning estimates only. Check the final design against the panel and inverter data sheets, the NEC and your local rules. A licensed electrician or designer has the last word. Data as of October 2026.
How this calculator works
Two things change a panel’s voltage: temperature and sunlight. The tool uses temperature, because it moves the voltage the most. A panel’s open-circuit voltage (Voc) rises as it gets colder. Its working voltage (Vmp) falls as it gets hotter.
The longest string
The tool takes the lowest air temperature for your ZIP code and raises the Voc by the panel’s temperature coefficient. It then divides the inverter’s maximum DC voltage, or your system limit if that’s lower, by that cold Voc and rounds down. A 400 W panel with a 37.2 V Voc climbs to about 40.3 V at 21 °F, the typical winter low in Austin. Fifteen of them would reach 605 V, so 14 is the most on a 600 V inverter.
It also checks the cold Vmp against the top of the MPPT range. A string can sit above that range on a cold morning without any danger, but the inverter then gives less power until the panels warm up. The tool shows both limits, so you can decide.
The shortest string
On a hot day the panels run well above the air temperature, so the Vmp falls. The string has to stay above the bottom of the MPPT range then, or the inverter shuts the string out. The tool uses the hot-day air temperature for your ZIP code plus a panel temperature rise, 30 °C by default. Flush roof mounts run hotter than open racks, so raise the number for a tight mount.
Layouts
Strings on the same MPPT input need the same length. If your panel count doesn’t divide evenly, the tool puts strings of different lengths on separate inputs when the inverter has enough of them. It also checks the current against each input’s limit and against the short-circuit current rating, when you enter one.
Tip: if the layout comes out uneven, ask whether one panel more or fewer gives two equal strings. Equal strings are simpler to wire, to monitor and to troubleshoot.
What it doesn’t do
- It isn’t for optimizers or microinverters. Those systems have their own string rules, and the maker’s design tool gives the answer.
- It uses an estimate of the cold low. Your inspector may ask for the local design temperature from ASHRAE tables. If you have it, type it in and it replaces the estimate.
Where the numbers come from
- NEC (NFPA 70) section 690.7 for maximum voltage, including the 600 V limit on one- and two-family dwellings, and section 690.8 for circuit current and the 125% sizing rule.
- The cold low is the average of each year’s coldest daily minimum from 2006 to 2025. The hot-day value is the 98th percentile of summer daily highs. Both come from the NASA POWER daily data for your ZIP code area.
- Panel and inverter limits come from the data sheets you enter. The calculator has no built-in product list.
- NASA POWER, daily data 2006–2025 at the center of each 3-digit ZIP area, data as of October 2026.
Questions
How many solar panels can I put in a string?
It depends on the panel’s Voc, the inverter’s maximum voltage and how cold your area gets. For a typical 400 W panel on a 600 V inverter in Austin, the limit is 14 panels. In a colder place it’s lower. Enter your own data sheet numbers to get your limit.
Why does the voltage go up when it’s cold?
Solar cells make more voltage at low temperatures. The data sheet gives the rate as a negative percentage per degree. A panel rated at -0.27% per °C gains about 8% in Voc when it’s 31 °C below the 25 °C test temperature.
What happens if the string voltage goes over the inverter’s maximum?
The inverter can be damaged, and the warranty usually doesn’t cover it. The cold-morning Voc is the number that matters, because that’s when the open-circuit voltage is highest. The tool never suggests a string above that limit.
What is the minimum number of panels in a string?
Enough to keep the hot-day voltage above the bottom of the MPPT range. The inverter can’t track power below that voltage. If a string is too short, it can drop out on the hottest afternoons.
Can I mix strings of different lengths?
Yes, but only on separate MPPT inputs. Strings sharing an input must be the same length. The tool does this for you and marks layouts with unequal strings.
What does the DC to AC ratio mean?
It’s the panel power divided by the inverter’s AC rating. A ratio of 1.1 to 1.3 is common. Above that, the inverter clips the midday peak, which costs a little energy but often costs less than a bigger inverter.
Can I use this for a 1000 V or 1500 V system?
Yes. Pick the system voltage limit and enter the inverter’s maximum DC voltage. Those systems are for commercial and utility arrays. The NEC keeps one- and two-family homes at 600 V.
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