Solar String Voc Calculator

Both ends of a grid-tie string: the most modules in series before the coldest morning’s open-circuit voltage passes the inverter’s absolute DC maximum, and the fewest before a hot afternoon drops the array out of the bottom of the MPPT window — with the valid range between them, and a flag when there isn’t one.

Last updated 24 Sep 2026

Grid-tie string length

Module + inverter → the valid string range

Module open-circuit voltage V(oc) at STC (V)From the module datasheet. Use the figures from your part’s datasheet; typical values vary widely between manufacturers.

Module voltage at maximum power V(mp) at STC (V)

Module power at STC (W)

Module short-circuit current I(sc) (A)

Temperature coefficient of V(oc) (%/°C)A negative number, typically −0.24 to −0.35 %/°C.

Temperature coefficient of V(mp) (%/°C)Rarely printed. The coefficient of P(max) is the usual stand-in, because I(mp) barely moves with temperature — −0.29 %/°C on the example module.

Record low ambient temperature at the site (°C)The lowest the site has seen, not the average winter minimum. Use the coldest recorded or an extreme-minimum design figure.

Hot design ambient temperature (°C)The hottest ambient the array works in — a 2% design dry-bulb figure or the site’s record high.

Module NOCT / NMOT (°C)Nominal operating cell temperature, measured at 800 W/m², 20 °C ambient and 1 m/s wind. 42–48 °C on most modules.

Irradiance for the hot case (W/m²)1,000 for full sun. The cell’s rise over ambient scales with this.

Inverter absolute maximum DC input voltage (V)The number that destroys the inverter if you exceed it, not the top of the tracking window.

Bottom of the inverter’s MPPT window (V)Use the RATED MPP voltage range, not the absolute minimum operating voltage: below the rated range the inverter tracks but cannot reach full power.

Top of the inverter’s MPPT window (V)

Inverter maximum input current per MPPT (A)

Inverter rated AC power (W)

Modules in series you are planning

Strings in parallel on this MPPT

A geometry, not a schematic: the modules of one string in series, feeding one MPPT input of a grid-tie inverter. The string's voltage is the sum of the module voltages, so the whole design problem is that this sum has to stay below the inverter's absolute maximum on the coldest morning AND above the bottom of its tracking window on the hottest afternoon. Both figures are printed for the string length you entered; the inverter turns amber when the hot figure falls out of the window and red when the cold figure is over the maximum. Nothing moves, because a geometry has no loop.

10modulesExample

a 590 W module (V(oc) 51.99 V, V(mp) 43.53 V, β(Voc) -0.25 %/°C, β(Vmp) -0.29 %/°C, NOCT 45 °C) on an inverter with 600 V maximum DC and a 220–550 V rated MPPT window; record low -10 °C, hot ambient 40 °C

Both ends of the window

V_oc,cold = V_oc × (1 + (T_min − 25) × β_Voc ÷ 100) → N_max = floor(V_DC,max ÷ V_oc,cold)
T_cell,hot = T_amb + (NOCT − 20) × G ÷ 800 · V_mp,hot = V_mp × (1 + (T_cell,hot − 25) × β_Vmp ÷ 100) → N_min = ceil(V_MPPT,low ÷ V_mp,hot)

β Voc: temperature coefficient of open-circuit voltage, %/°C, negative — so a cold module makes MORE voltage
β Vmp: the same for the operating voltage. Usually taken as the coefficient of P(max), since I(mp) hardly changes
NOCT: nominal operating cell temperature: what the cell reaches at 800 W/m², 20 °C air and 1 m/s wind
800: the irradiance NOCT is defined at, which is what makes the rise scale as G ÷ 800

Worked example

a 590 W module (V(oc) 51.99 V, V(mp) 43.53 V, β(Voc) -0.25 %/°C, β(Vmp) -0.29 %/°C, NOCT 45 °C) on an inverter with 600 V maximum DC and a 220–550 V rated MPPT window; record low -10 °C, hot ambient 40 °C
Cold V(oc) = 51.99 × (1 + (-10 − 25) × -0.25 ÷ 100) = 56.54 V per module
N_max = floor(600 ÷ 56.54) = 10 modules — and the top of the tracking window allows 11, so the DC maximum is what binds
Hot cell temperature = 40 + (45 − 20) × 1,000 ÷ 800 = 71.25 °C
Hot V(mp) = 43.53 × (1 + (71.25 − 25) × -0.29 ÷ 100) = 37.69 V, a fall of 13.4%
N_min = ceil(220 ÷ 37.69) = 6 modules
So 6 to 10 modules in series work. The planned 8 sits inside it: 452.3 V open-circuit at -10 °C against a 600 V limit, and 301.5 V at maximum power when hot against a 220 V floor

Every string length, tested at both ends

Computed by this page’s own arithmetic at its defaults. Eleven modules fail on the cold open-circuit voltage: 621.9 V against a 600 V inverter. Five fail at the hot end, sitting below the bottom of the tracking window on a summer afternoon.
Modules	V(oc) cold	V(mp) cold	V(mp) hot	String at STC	Inside the window?
6	339.2 V	287.7 V	226.1 V	3.54 kW	yes
7	395.8 V	335.6 V	263.8 V	4.13 kW	yes
8	452.3 V	383.6 V	301.5 V	4.72 kW	yes
9	508.9 V	431.5 V	339.2 V	5.31 kW	yes
10	565.4 V	479.5 V	376.9 V	5.90 kW	yes
11	621.9 V	527.4 V	414.6 V	6.49 kW	no

What you assume about cell temperature moves the hot end

The three fixed adders are the ones the industry uses for different mounting arrangements; the last row is the NOCT model this page computes, which lands between the rack-mounted and flush-mounted figures for a 45 °C NOCT module in full sun. Flush-mounted modules run hottest and therefore need the longest minimum string.
Hot-case assumption	Rise over ambient	Cell temperature	Module V(mp)	Fewest modules
Ground or pole mounted	+25.00 °C	65.00 °C	38.48 V	6
Roof mounted, rack type with air behind	+30.00 °C	70.00 °C	37.85 V	6
Roof mounted, parallel to the roof	+35.00 °C	75.00 °C	37.22 V	6
NOCT model at 1,000 W/m², NOCT 45 °C	+31.25 °C	71.25 °C	37.69 V	6

A string has to fit a window, not just clear a limit

A grid-tie inverter states two DC numbers that are easy to confuse. One is the absolute maximum DC input voltage — exceed it and the inverter is damaged. The other is the MPPT tracking window, the voltage range over which it can actually find the array’s maximum power point. A string has to stay under the first at its coldest and inside the second at its hottest, and those two conditions pull in opposite directions.

The cold end. A photovoltaic module’s open-circuit voltage rises as it cools, by the temperature coefficient on its datasheet — about −0.25%/°C on a modern n-type module, which means +0.25%/°C going down. The dangerous moment is a cold clear morning before the inverter has connected: the array is at open circuit, the cells are at ambient, and the string is at its highest voltage of the year. IEC 62548 clause 7.2 and NEC 690.7 both size the maximum array voltage exactly this way, from V(oc) corrected to the lowest expected temperature. Use the site’s record low, not an average winter minimum — the whole point is the worst case.

The hot end. The operating voltage falls as the module heats, and cells run far hotter than the air around them: 30 °C or more above ambient in full sun. The usual model is the nominal operating cell temperature, measured at 800 W/m² and 20 °C, scaled to the irradiance you care about — a 45 °C NOCT module in 1,000 W/m² sun at 40 °C ambient sits at 71 °C, and its V(mp) has dropped 13.4%. If the string is too short, that fall takes the array below the bottom of the tracking window at midday in July, and the inverter cannot deliver its rated power at the very moment the array is producing most. Use the inverter’s RATED MPP range for this, not its absolute minimum operating voltage: between the two the inverter runs, but derated.

The coefficient nobody prints. Datasheets give temperature coefficients for P(max), V(oc) and I(sc), but rarely for V(mp), which is what the hot end needs. The standard substitution is the P(max) coefficient, because I(mp) is almost independent of temperature, so essentially all of the power coefficient is voltage. On the example module that is −0.29%/°C against −0.25%/°C for V(oc): using the V(oc) figure for both ends would understate the hot-end fall and give a minimum string length that is one module too short at the margin.

Current is a separate question. This page is about voltage; it reports the array current with the 125% factor for reference, but sizing conductors, fuses and the inverter’s current inputs is its own calculation. For an off-grid array on a charge controller rather than a grid-tie inverter, the solar charge controller calculator does the controller’s current rating and the same cold-voltage check; for the array size itself the solar panel size calculator, and for the tilt that array should sit at, the solar panel tilt angle calculator.

What this leaves out. Bifacial rear-side gain, which raises current more than voltage but is not nothing. Module-level power electronics, which change the problem entirely. Degradation, which lowers voltage slightly over decades. Partial shading, which can leave a string operating on a local maximum well away from the voltage computed here. And any national wiring rule that imposes its own correction factors in place of the datasheet coefficients — NEC Table 690.7(A) is one such, and where a code requires it, it wins.

Frequently asked questions

How many solar panels can I put in one string?

As many as keep the string’s cold open-circuit voltage below the inverter’s absolute DC maximum, and no fewer than keep its hot operating voltage above the bottom of the MPPT window. For the example 590 W module on a 600 V inverter that is 6 to 10 modules.

Why does panel voltage go up when it is cold?

Because a solar cell’s open-circuit voltage has a negative temperature coefficient — around −0.25%/°C. Cooling the cell from 25 °C to −10 °C raises V(oc) by nearly 9%, which is why string sizing is done at the site’s record low.

What temperature should I use for string sizing?

The lowest ambient the site has recorded for the maximum voltage, because the array sits at open circuit at ambient temperature before the inverter connects. For the minimum string length, use a hot design ambient plus the cell’s rise over it — typically 25 to 35 °C depending on the mounting.

What happens if the string is too short?

The inverter cannot track the array’s maximum power point when the modules are hot, so it loses power exactly when the sun is strongest. Nothing is damaged; the system simply underperforms every summer afternoon.

What happens if the string is too long?

On a cold sunny morning the open-circuit voltage exceeds the inverter’s absolute DC maximum and the inverter can be destroyed. It is not a derating, it is a failure, and it usually voids the warranty.

Which temperature coefficient do I use for V(mp)?

The coefficient of P(max), if the datasheet does not give one for V(mp). Current at maximum power hardly changes with temperature, so almost all of the power coefficient is voltage.

Related calculators

References

IEC 62548:2016. Photovoltaic (PV) arrays — Design requirements, clause 7.2 “PV array maximum voltage”: the array’s maximum voltage is the open-circuit voltage corrected to the lowest expected cell temperature at the site.
NFPA 70 National Electrical Code, Article 690.7: the maximum PV source and output circuit voltage is the sum of the modules’ rated open-circuit voltages corrected for the lowest expected ambient temperature, using the module manufacturer’s temperature coefficient where one is available.
JinkoSolar. Tiger Neo N-type 72HL4-(V) 570–590 W datasheet, model JKM590N: 590 W, V(mp) 43.53 V, I(mp) 13.5 A, V(oc) 51.99 V, I(sc) 14.7 A, 22.5% efficient, temperature coefficient of P(max) −0.29%/°C, of V(oc) −0.25%/°C, of I(sc) +0.045%/°C, NOCT 45 ± 2 °C. The page’s module defaults are this sheet.
SMA Solar Technology. Sunny Boy 3.0-US / 3.8-US / 5.0-US / 6.0-US / 7.0-US / 7.7-US datasheet: for the 5.0-US, maximum DC voltage 600 V, MPPT operating range 100–550 V, rated MPP range 220–480 V, minimum/start voltage 100 V / 125 V, maximum input current 10 A per MPPT, 5,000 VA AC. The page’s inverter defaults are this sheet, using the rated MPP range for the window.
Sandia National Laboratories PV Performance Modeling Collaborative. NOCT cell temperature model: cell temperature rises above ambient in proportion to plane-of-array irradiance divided by 800 W/m² and to (NOCT − 20 °C), the conditions NOCT is measured at.