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.
Quick answer
Max panels = inverter max V ÷ cold Voc. Min panels = MPPT min V ÷ hot Vmp. Cold Voc = Voc × (1 + temp coefficient × (Tmin − 25)).
- 49.5 V Voc panel, −0.27%/°C, −5°C coldest: 53.5 V each, so a 600 V inverter takes at most 11 panels
- 41.5 V Vmp at 65°C cell temperature: 37.0 V each, so a 90 V MPPT minimum needs at least 3 panels
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 decides how many panels to connect in series in one string. The string voltage must always stay within the safe operating window of your inverter or charge controller. A voltage that is too high can damage the equipment. A voltage that is too low moves the inverter out of its efficient tracking range. Panel voltage changes with temperature, so string sizing is a temperature calculation. It 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 voltage that is correct on a mild installation day can go above the inverter's maximum on the first hard frost. It can also fall below the tracking range in a summer heatwave. Neither problem shows in mild weather. That’s why the calculation is built around your local temperature extremes rather than the comfortable conditions of a test bench. With the correct string length, the system works all year for decades. With the wrong length, you can damage the inverter or lose production continuously. 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
Solar panels produce DC voltage in daylight, and you cannot switch them off. A series string can supply hundreds of volts. Cover the panels with an opaque sheet, or do the wiring in the dark.
Panel voltage increases in cold weather. Calculate the string open-circuit voltage (Voc) at the lowest temperature that you expect. If it is more than the maximum input voltage of the controller or inverter, a cold morning can damage the equipment.
- 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.
Learn more
Sources & standards
String sizing is safety-critical — exceeding an inverter's voltage window can damage equipment — so the limits used here follow the electrical codes and array-design standards below rather than rules of thumb.
- NFPA 70 — National Electrical Code (NEC), Articles 690.7 & 690.8 — National Fire Protection Association
Sections 690.7 and 690.8 set how PV system maximum voltage and current must be calculated, including temperature correction for cold-weather open-circuit voltage — the basis for this calculator's cold-limit check. - IEC 62548 — Photovoltaic (PV) arrays: design requirements — International Electrotechnical Commission
The international equivalent of NEC 690 for array and string design, used outside North America to set string voltage and current limits. - IEC 61215 — Terrestrial photovoltaic (PV) modules: design qualification and type approval — International Electrotechnical Commission
Defines how a module's temperature coefficients and open-circuit voltage are measured and reported, the exact figures this calculator asks you to enter. - Photovoltaic system design research — Sandia National Laboratories
Source of widely used guidance on selecting record low-temperature design values for cold-limit string calculations.