Solar String Sizing Calculator
Work out how many panels you can safely wire in series per string. The key risk is cold weather: panel voltage rises as temperature drops, and exceeding your inverter's maximum input voltage can destroy it.
1 Panel specs — from the datasheet
A negative number from the datasheet (typically −0.25 to −0.30%/°C). Voltage rises as it gets colder.
2 Temperature range
Record low at your site. Cold sets the maximum string length (highest voltage). Use a genuinely low value to be safe.
Panels run far hotter than the air — 65–75°C cell temperature is typical. Heat sets the minimum string length (lowest voltage).
3 Inverter / controller window
The absolute maximum on the inverter or MPPT charge controller spec. Cold-weather string voltage must stay below this.
The lowest voltage the MPPT tracker operates at. Hot-weather string voltage should stay above this so the array keeps producing.
Estimates only. Always verify against your exact inverter datasheet and local temperature extremes, and follow local electrical code. Exceeding the inverter's max DC voltage can permanently damage it and void warranties.
How string sizing works
Voc at cold =
Voc × (1 + tempCoef% × (Tmin − 25)).Max panels per string =
floor(inverter max V ÷ cold Voc) — keeps you under the limit even on the coldest morning.Vmp at hot =
Vmp × (1 + tempCoef% × (Tmax − 25)).Min panels per string =
ceil(MPPT min V ÷ hot Vmp) — keeps the string voltage high enough to track in summer heat.STC ratings are at 25°C. Always size from real local extremes and confirm against the inverter datasheet.
How to size a solar string correctly
String sizing means deciding how many panels to wire in series into one string, so that the string's voltage always stays within your inverter's or charge controller's safe operating window — never too high (which can damage the equipment) and never too low (which pushes the inverter out of its efficient tracking range). Because panel voltage changes with temperature, this is fundamentally a temperature problem, and getting it right is one of the most important steps in a safe, efficient design.
String sizing is where a lot of otherwise-good DIY systems quietly go wrong, because the failure mode is invisible until the worst possible moment. A string that’s comfortable on the mild day you installed it can climb past the inverter’s voltage ceiling on the first hard frost, or sag below its tracking floor in a summer heatwave — and neither problem shows up in fair weather. That’s why the calculation is built around your local temperature extremes rather than the comfortable conditions of a test bench. Get the string length inside the safe window and the system simply works, year-round, for decades; get it wrong and you risk either a damaged inverter or chronically lost production. A few minutes with the numbers below is cheap insurance.
| Limit | Set by | Determines |
|---|---|---|
| Maximum panels per string | Coldest-day open-circuit voltage vs inverter max | Upper string length |
| Minimum panels per string | Hottest-day operating voltage vs inverter min MPPT | Lower string length |
The two limits every string must respect
- Maximum voltage (cold limit). Panel open-circuit voltage rises as temperature falls. The string's open-circuit voltage at the coldest expected temperature must stay safely below the inverter's maximum input voltage. This sets the most panels you can put in a string. Exceeding it on a freezing, sunny morning is the classic way to destroy an inverter.
- Minimum voltage (hot limit). Panel voltage falls as temperature rises. The string's operating voltage at the hottest expected temperature must stay above the inverter's minimum MPPT voltage, or the inverter drops out of its efficient window. This sets the fewest panels you can put in a string.
Your valid string length is the range between those two limits. The calculator above works it out from your panel's voltage and temperature coefficients and your local temperature extremes.
Why temperature coefficients matter
Every panel has a temperature coefficient of voltage — typically around −0.3% per °C — telling you how much its voltage rises or falls per degree away from the standard 25°C test condition. On a −10°C morning, a panel's open-circuit voltage can be several percent above its nameplate figure; on a 65°C rooftop in summer, its operating voltage sags well below it. Sizing a string to the nameplate value alone ignores exactly the conditions that cause failures and dropouts. Always use your site's record cold and realistic hot cell temperatures, not the test-condition numbers.
String sizing and the rest of the system
String length interacts with everything else. Longer (higher-voltage) strings reduce current, which means thinner wire and lower voltage drop, and they suit MPPT equipment — but they're capped by the cold-voltage limit. Shorter strings are safer on voltage but carry more current per watt. On larger arrays you build several equal-length strings and wire them in parallel; keeping the strings identical ensures their voltages match so none is dragged down. This is why string sizing, inverter choice and wire sizing are designed together, not in isolation.
Common string-sizing mistakes
- Using nameplate voltage instead of cold-temperature voltage. The single most dangerous error — it can let a string exceed the inverter's limit in winter.
- Ignoring the hot-temperature minimum. A string that's too short sags below the MPPT window on hot days, quietly losing output.
- Mixing string lengths in parallel. Unequal strings waste energy; keep them identical.
- Using mismatched panels in one string. The weakest panel limits the whole series string's current.
Frequently asked questions
As many as keep the string's coldest-day open-circuit voltage below your inverter's maximum input voltage, and at least enough to keep the hottest-day voltage above the inverter's minimum MPPT voltage. Enter your panel and temperature figures above for the exact minimum and maximum.
Because panel voltage rises as temperature drops. The highest voltage your string ever produces is on the coldest, sunniest morning — and that peak must stay within the inverter's limit, or the inverter can be damaged.
On hot days panel voltage falls. If a string is too short, its voltage can drop below the inverter's minimum tracking voltage, pushing the inverter out of its efficient range and losing production. The minimum string length prevents that.
For strings wired in parallel to the same input, yes — keep them equal and use identical panels, so their voltages match and none is dragged down by another.