Solar Panel Specs Explained: Voc, Vmp, Isc, Temp Coeff.

Solar panel specs printed on the back of every module tell you how much voltage and current it makes and how those numbers shift with heat and cold, and NEC sections 690.7 and 690.8 use them to set safe string voltage and wire sizes. Read them correctly and your charge controller or inverter stays within its limits for decades. Here’s every line on a typical spec sheet, with worked cold- and hot-weather math.

Electrical work can be dangerous: follow the manual, fuse and size every cable correctly, and use a licensed electrician for anything connected to household wiring.

The Key Solar Panel Specs

Spec	Meaning	Used for
Pmax (W)	Rated power at STC	System size
Voc	Open-circuit voltage, no load	Max string voltage
Vmp	Voltage at max power	MPPT operating window
Isc	Short-circuit current	Fuse and wire sizing
Imp	Current at max power	Expected operating current
Temp. coefficient of Pmax	% power change per °C	Hot-weather output
Temp. coefficient of Voc	% or mV per °C	Cold-weather max voltage

STC (Standard Test Conditions) is 1,000 W/m² sunlight at 25°C cell temperature. Real panels rarely see it, which is why NOCT or NMOT ratings show more realistic output.

Voc and Cold Weather

Panel voltage rises as temperature drops. A typical crystalline panel’s Voc coefficient is around -0.25% to -0.3% per °C. At -10°C (35°C below STC), a 0.28%/°C panel’s Voc rises about 10%.

Example: a 45V Voc panel at -10°C: 45 × (1 + 0.0028 × 35) ≈ 49.4V. Three in series ≈ 148V. That’s within a 150V controller, but with almost no margin. NEC 690.7 sets the method for calculating maximum system voltage.

Vmp and the MPPT Window

An MPPT controller needs the array’s Vmp to be above battery voltage by a margin (often about 5V) to start charging. In hot weather Vmp drops, so check the hot-day value too.

Isc and Fusing

Isc is the most current a panel can produce. NEC 690.8 generally sizes PV circuit conductors and fuses at Isc × 1.25 × 1.25 (1.56 × Isc) for continuous duty and irradiance above STC.

Temperature Coefficient of Power

A Pmax coefficient of -0.35%/°C means a panel at 65°C cell temperature (common on a hot roof) loses about 14% versus STC. Lower numbers are better in hot climates.

How to Read a Solar Panel Spec Sheet

Illustrative label for a 400W residential panel:

Label line	Example value	What it tells you
Maximum power (Pmax)	400W	Output at STC
Power tolerance	0/+5W	It tests at or above 400W
Open-circuit voltage (Voc)	37.0V	Highest voltage, no load
Maximum power voltage (Vmp)	31.0V	Operating voltage at Pmax
Short-circuit current (Isc)	13.6A	Highest current
Maximum power current (Imp)	12.9A	Operating current at Pmax
Module efficiency	20.5%	Watts per square meter of panel
Temp. coefficient of Pmax	−0.35%/°C	Power lost per degree above 25°C
Temp. coefficient of Voc	−0.27%/°C	Voltage gained per degree below 25°C
Maximum system voltage	1,000V or 1,500V DC	Highest string voltage the module is rated for
Maximum series fuse rating	25A	Largest fuse allowed in series with it
NMOT power	about 300W	Output at more realistic conditions

Check the math: Vmp × Imp = 31.0 × 12.9 ≈ 400W.

STC vs NMOT

	STC	NMOT (or NOCT)
Sunlight	1,000 W/m²	800 W/m²
Temperature	25°C cell	20°C air, with the cell running hotter
Wind	n/a	1 m/s
Use	Comparing panels	Estimating real output

NREL’s PVWatts calculator uses a default system loss of about 14% on top of temperature and sunlight effects, which is one reason real output sits below nameplate.

Cold-Weather Voc: Two Methods

NEC 690.7 lets you calculate maximum voltage using the manufacturer’s temperature coefficient, or a correction table in the code when that isn’t available.

Illustrative: the 37.0V Voc panel above, three in series, at a record low of −20°C (45°C below STC).

Method	Math	String Voc
Manufacturer coefficient (−0.27%/°C)	37.0 × (1 + 0.0027 × 45) × 3	about 124.5V
NEC Table 690.7(A) factor for −16 to −20°C (1.18)	37.0 × 1.18 × 3	about 131.0V

Both stay under a 150V controller, with more margin than the 45V example above. Use the lowest expected temperature for your site, often taken from ASHRAE design data, and the manufacturer’s method when it’s available.

Hot-Weather Vmp: Will the Controller Start?

Illustrative: the same panel at a 65°C cell temperature (40°C above STC), assuming a Vmp coefficient of about −0.35%/°C.

	Math	Result
Vmp per panel when hot	31.0 × (1 − 0.0035 × 40)	about 26.7V
Two in series	26.7 × 2	about 53V
Battery voltage, 48V-class LiFePO4	about 51–56V	Too close to charge reliably
Three in series	26.7 × 3	about 80V, a comfortable margin

Many MPPT controllers need array voltage several volts above battery voltage. Victron’s manuals, for example, say the PV voltage must exceed battery voltage by 5V to start. See MPPT vs PWM.

Isc and NEC 690.8, Worked Out

Illustrative: two strings of the panel above in parallel, Isc 13.6A each.

Step	Math	Result
Max circuit current per string (690.8(A))	13.6 × 1.25	17A
Conductor and OCPD sizing (690.8(B))	17 × 1.25	21.25A
Combined output circuit	2 × 17	34A max current

With only two strings in parallel, string fuses usually aren’t needed, because one string can’t backfeed more than the panel’s maximum series fuse rating. With three or more, the backfeed from the others can exceed it, which is when string fusing comes in. Follow the module and controller manuals. See what size fuse do I need.

Hot-Roof Power Loss

Illustrative, using the −0.35%/°C coefficient:

Cell temperature	Above STC	Power vs rated
25°C	0°C	100%
45°C	20°C	93%
55°C	30°C	89.5%
65°C	40°C	86%

A lower (closer to zero) coefficient helps most in hot climates. Ground mounts with airflow often run cooler than flush roof mounts. See roof mount vs ground mount solar.

Bifacial Panels

Bifacial panels collect light reflected onto the back as well. Spec sheets often list a bifaciality factor, such as 70%, plus extra output at several levels of rear-side gain. The front-side STC rating is still the baseline, and the extra depends on mounting height and the surface underneath, such as white gravel versus grass.

Mechanical Specs on the Datasheet

Spec	Typical example (illustrative)	Why it matters
Dimensions	about 1.7 × 1.1 m for a 108-half-cell module	Roof layout and racking
Weight	about 21 kg	Handling and roof load
Frame	Anodized aluminum, 30–35 mm	Clamp compatibility
Connectors	MC4 or MC4-compatible	Mate only with the same type
Mechanical load	Front and rear ratings in pascals (Pa)	Snow and wind areas
Cable length	about 1.1–1.4 m	Portrait vs landscape stringing

Mechanical load ratings depend on the clamping zones described in the installation manual, so follow its mounting diagram.

Warranty Specs

Most residential panels list two warranties:

Product warranty: covers materials and workmanship, often 12–25 years.
Performance warranty: guarantees a minimum output over time, often around 25–30 years, with a first-year limit and a yearly decline rate.

Illustrative: a performance warranty of 98% in year one and 0.5% a year after that guarantees at least 98 − (0.5 × 24) = 86% of rated power in year 25. See solar panel degradation per year.

FAQ

What solar panel specs matter most?

Voc (cold-weather), Vmp (MPPT window), Isc (fuses and wire) and the power temperature coefficient.

Why does my panel make less than its rated watts?

Heat, angle, dirt and less than STC sunlight. 70-85% of nameplate is common at midday.

Can I mix panels with different specs?

In series, match current (Imp). In parallel, match voltage (Vmp).

What do the specs on a solar panel mean?

Pmax is rated power, Voc and Vmp are voltages, Isc and Imp are currents, and the coefficients show temperature effects.

What is a good temperature coefficient?

Closer to zero is better. Many modern panels list about −0.30 to −0.40%/°C for power.

Where do I find a panel’s spec sheet?

On the label on the back, and as a PDF datasheet on the manufacturer’s website.

Sources: NREL PVWatts documentation; NREL Solar program; NFPA 70 (NEC) 2023 Articles 690.7 and 690.8; NREL PVWatts calculator; Victron Energy SmartSolar MPPT manual. Accessed Oct 2026.