Solar Panel Output Voltage Explained — Voc, Vmp, And Nominal Volts For 2026 Panels

I built a 6 kW PV array on my own house in Slovenia in 2024 using 14 LONGi Hi-MO 6 410 W panels in two strings of 7. The first thing I had to do — before ordering anything — was figure out the string voltage at both ends of the operating envelope: cold morning and hot afternoon. This article is the foundation for that math.

The Three Voltages On A Solar Panel Datasheet

Open any modern Tier 1 datasheet and you will see (at minimum) these voltage entries in the STC electrical block:

The first two are measured on the production-line flash tester (see STC In Solar Panels for how this works). The third is calculated using the Voc temperature coefficient. The fourth is a legacy label that almost never appears on modern grid-tie residential panels because it has no engineering meaning in a grid-tie context.

Voc — The Open-Circuit Voltage

Voc is the voltage you measure across the panel terminals with no load connected. It is the highest voltage the panel can produce. Mathematically, it is the right-edge intercept of the I-V curve (where current = 0). Physically, it is set by the diode equation Voc = (n·kT/q)·ln(I_L / I₀ + 1) — see the dedicated Voc derivation article for the full physics.

Voc is what the inverter sees on a cold morning before the panels are loaded. It is also what you measure with a multimeter on the panel terminals in sunlight. It is not the operating voltage — as soon as you connect a load, the voltage drops.

Vmp — The Maximum Power Voltage

Vmp is the voltage at which the panel delivers its maximum power. It is always lower than Voc — typically 80–90 % of it for a healthy c-Si cell. The exact ratio Vmp/Voc is set by the cell's fill factor:

Vmp / Voc ≈ 0.83 to 0.86 (modern Tier 1 c-Si)

The inverter's MPPT (Maximum Power Point Tracking) algorithm continuously adjusts the operating voltage of each string to keep it sitting at Vmp as conditions change. When a cloud reduces irradiance, both Vmp and Voc shift slightly; the MPPT tracks the new peak.

Vmp is the voltage that actually flows in normal operation. It is what your kWh production hangs off of. If your inverter is configured wrong and parks the string off-Vmp, you can lose 5–15 % of yield.

Nominal Voltage — A Legacy Off-Grid Term

Older articles (and the original version of this one) talk about "12 V solar panels" or "24 V solar panels." That terminology comes from off-grid solar where the panel charges a 12 V or 24 V lead-acid battery bank through a charge controller. The panel itself produces about 18 V (Vmp) so the controller has enough voltage headroom to push current into a 14-volt charging battery.

For grid-tie solar — which is virtually every residential installation in 2026 — nominal voltage is meaningless. You don't have a battery bank dictating system voltage. You have a string of panels feeding an inverter — whether a string inverter or microinverter — with a 100–600 V MPPT input range, and the goal is to fit as many panels in series as the inverter can accept without exceeding cold-corrected Voc. Nominal voltage doesn't enter the equation.

So when you see a 410 W LONGi Hi-MO 6 panel with no nominal voltage on the datasheet, that is correct and intentional. It is a grid-tie panel; nominal voltage doesn't apply.

How Many Volts Per Cell? (The 0.58 V Number Is Outdated)

A solar panel is a series-connected stack of individual photovoltaic cells. The module Voc is just the per-cell Voc multiplied by the number of cells in series.

The "0.5 to 0.6 V per cell" figure that older articles cite is based on 1990s-era p-type aluminum-back-surface-field cells. Modern cells produce noticeably more, because n-type architectures with proper passivation have lower I₀ and therefore higher Voc.

Here are the real per-cell Voc numbers for 2026 cell technologies:

For a 60-cell-equivalent panel (108 half-cells in modern terminology), you multiply the per-cell number by 60:

LONGi Hi-MO 6 (HPBC, 0.694 V/cell × 54 full cells) = 37.50 V Voc
REC Alpha Pure-R (HJT, 0.730 V/cell × 54 full cells) = 39.40 V Voc
Trina Vertex S+ (TOPCon, 0.719 V/cell × 54 full cells) = 38.80 V Voc

For a 72-cell-equivalent panel (144 half-cells):

JinkoSolar Tiger Neo 72HL4-V (TOPCon, 0.699 V/cell × 72 full cells) = 50.30 V Voc

What Are Half-Cut Cells And Why Modern Panels Have "108" Or "144" Cells

Walk through any installer's stockpile in 2026 and the panels say "108 cells" or "144 cells" — not 60 or 72. They are still 60- and 72-cell panels electrically. The cells have just been laser-cut in half.

Half-cut cells became standard around 2018–2020 because they have three real engineering advantages:

1. Half the current per cell. I²R losses in busbars and ribbons scale as current squared. Halving the current per half-cell quarters the resistive loss. This adds ~2 % to module efficiency for free.
2. Better partial shading tolerance. With the panel split into upper and lower halves wired with a separate bypass diode arrangement, shading the bottom row of cells doesn't kill the top half.
3. Lower hot-spot risk. Lower current per cell means lower power dissipation if a cell is damaged or shaded.

The electrical equivalence is preserved because the half-cells are wired in parallel pairs of two halves, then in series. So:

• 60 full cells in series → 108 half-cells (54 pairs × 2 in parallel within each pair, 54 pairs in series) → still 60-cell-equivalent → same Voc
• 72 full cells in series → 144 half-cells → still 72-cell-equivalent → same Voc

Per-cell Voc of a half-cell equals per-cell Voc of a full cell — they have the same physics, just less area each (which halves the current proportionally).

So when you read "144-cell panel," mentally convert to "72-cell-equivalent" and the voltage math works as before.

2026 Tier 1 Voltage Chart

Here are the actual STC voltage numbers for the modules you can buy in 2026, with the calculated Vmp/Voc ratio so you can verify the datasheet is internally consistent:

A few things to notice:

1. Vmp/Voc lives in 0.82–0.84 for all modern Tier 1 c-Si. Anything below 0.78 means a problem with the cell (high series resistance, recombination, low fill factor).
2. Vmp × Imp = Pmax within rounding. If a datasheet's Pmax doesn't match the product of its Vmp and Imp, something is wrong with the datasheet. I've personally caught this on Tier 3 panels.
3. More cells → higher Voc but the per-cell physics is the same. The 580 W Jinko isn't a different kind of panel from the 440 W Trina; it's just longer and wider with more cells in series.
4. Voc is always larger than Vmp. The gap is what defines the fill factor. See the efficiency article for how FF, Voc, Isc, and Pmax all interlock.

How Voltage Behaves In Real Conditions

The two most important things to know about panel voltage in the field:

1. Voltage Barely Changes With Irradiance

Voc depends on irradiance only logarithmically. A panel at 200 W/m² has:

Voc(200) ≈ Voc(1000) + (kT/q) · ln(200 / 1000)
        ≈ Voc(1000) − (kT/q) · ln(5)
        ≈ Voc(1000) − 42 mV per cell

For a 60-cell panel that is about −2.5 V — so Voc drops from ~37.5 V at full sun to ~35 V at one-fifth sun. Less than 7 % drop for a 5× irradiance change.

This is one of the most important facts in PV: voltage is nearly insensitive to irradiance. Current scales linearly. So when clouds dim the sun by half, your string still operates at nearly the same voltage, but the current is halved — which means the inverter still sees a usable bus voltage and can keep tracking.

2. Voltage Changes Strongly With Temperature

Voc decreases by 0.24–0.32 % per °C as temperature rises. Vmp moves at almost exactly the same rate. So:

• On a hot summer afternoon at 60 °C cell temperature: Voc is −10 to −11 % below STC
• On a cold winter morning at −15 °C cell temperature: Voc is +10 % above STC

The cold-morning case is the dangerous one. A 7-panel string of LONGi Hi-MO 6 at −15 °C:

Voc_cold per panel = 37.5 × (1 + (−0.0025) × (−15 − 25))
                   = 37.5 × 1.10
                   = 41.25 V
String Voc_cold = 7 × 41.25 = 288.75 V

That is the number that has to fit inside the inverter's maximum DC input voltage. NEC 2023 Article 690.7 requires sizing against this temperature-corrected number, not the STC nameplate. See the Voc article for the full string-sizing worked example.

The 12V/24V Off-Grid Voltage Chart (For Battery Systems)

If you are doing off-grid PV with a battery bank, the legacy nominal-voltage classification still applies. Here is the chart for battery-charge solar:

These nominal classifications make sense only for direct battery charging through a charge controller. For grid-tie installs, ignore them — you size strings against the inverter's MPPT input range, not against any nominal battery voltage. For the practical details of series vs parallel panel wiring, see how to wire solar panels.

Worked Example — Voltage Through A Real String

Take my install: 14 × LONGi Hi-MO 6 410 W panels arranged as 2 strings of 7 in series.

The inverter (a Fronius Symo 6.0-3-M, 600 V max DC input) sees its maximum string voltage of about 289 V on the coldest expected morning — a comfortable 311 V of headroom below the 600 V limit. Its operating Vmp range is 154–600 V, so the string sits well inside the MPPT window in every condition.

This is the voltage envelope a real install lives inside. The whole point of knowing Voc, Vmp, and the temperature coefficients is to keep the string inside that envelope all year.

Common Misreadings

1. "My panel produces 12 volts because that's the nominal rating." No — it produces ~18 V (Vmp) and ~21 V (Voc). The "12 V" is the battery-bank classification for off-grid systems, not what the panel itself outputs.
2. "Per-cell voltage is 0.5 V." Outdated. Modern n-type c-Si cells produce 0.71–0.75 V per cell. Use the right per-cell number for the cell technology — see the table above.
3. "Voltage doubles when irradiance doubles." No — voltage is nearly insensitive to irradiance (logarithmic dependence). Current doubles when irradiance doubles. Voltage barely budges.
4. "108 cells means more voltage than 60 cells." No — "108 cells" is a half-cut 54-cell panel and is 54-cell-equivalent (≈60 with traditional accounting). Same Voc as the older 60-cell version.
5. "My inverter's max DC input is the only voltage limit I need to check." Check both the maximum (against cold-morning Voc, NEC 690.7) and the minimum (against hot-afternoon Vmp, the lower MPPT bound). A string that drops below the MPPT minimum at 60 °C will operate off-MPP and lose energy.
6. "Voc and Vmp are the same thing." They aren't. Voc is no-load (highest); Vmp is operating point (always lower). The ratio Vmp/Voc is an indicator of cell quality (fill factor).

Bottom Line

A solar panel has three voltages, not one. Voc is the no-load maximum. Vmp is the operating voltage at the maximum power point. Nominal voltage is a legacy off-grid label that doesn't apply to grid-tie panels at all. For modern 2026 Tier 1 panels, Voc lives between 37 V and 55 V, Vmp lives between 31 V and 46 V, and the per-cell Voc is 0.69–0.75 V depending on cell technology.

If you only remember one thing: the voltage on the multimeter (Voc) is different from the voltage that flows in operation (Vmp), and both are different from the legacy "nominal voltage" label. All three are legitimate; they just answer different questions.

Keep Reading

If you found this useful, these guides go deeper into related topics:

• Open Circuit Voltage Of A Solar Cell — Formula And Cold-Morning String Sizing
• How To Calculate Solar Panel Efficiency
• STC In Solar Panels — The Foundation Of Every Datasheet
• STC vs NOCT (NMOT) — Temperature Math And Modern Datasheet Comparison
• NMOT In Solar — The Faiman Thermal Model Explained
• How Many Amps Does A 100 Watt Solar Panel Produce
• Standard Solar Panel Sizes And Wattages
• Average Peak Sun Hours By State
• Solar Panel Calculator — Full Energy Estimate