Solar Panels Explained

Panels are the cheapest and most durable part of an off-grid system. They have no moving parts, routinely outlast their 25-year warranties, and cost less per watt than at any point in their history.

They are also the part people spend the most time agonising over and the least time getting wrong. The expensive mistakes in an off-grid system are almost always in the battery bank or the wiring, not the panel choice.

What does matter is understanding what the numbers on the datasheet mean, because several of them constrain everything downstream.

Reading a panel datasheet

Five figures do most of the work.

Pmax (rated power) is the wattage the panel produces at its maximum power point under test conditions. This is the number in the product name, and it is the one that describes the panel least usefully.

Voc (open-circuit voltage) is the voltage across the terminals with nothing connected. It is the highest voltage the panel will ever produce, and it is the figure that decides whether a charge controller survives.

Vmp (voltage at maximum power) is the operating voltage when the panel is delivering full power. Always lower than Voc.

Isc (short-circuit current) is the current with the terminals shorted, which is the maximum current the panel can deliver. Unlike most electrical equipment, shorting a solar panel does not damage it.

Imp (current at maximum power) is the operating current at full output. Slightly below Isc.

Temperature coefficients are the ones people skip and should not. They state how much output and voltage change per degree away from 25°C. Power typically falls around 0.3 to 0.4% per degree of warming. Voltage typically changes around 0.25 to 0.35% per degree, and it moves in the opposite direction from what you might expect.

Check these on the datasheet for the panel you are actually buying. The ranges above are typical, not universal.

Rated output is a laboratory number

Panel ratings are measured at Standard Test Conditions: 1,000 watts per square meter of light, a cell temperature of 25°C, and a defined atmospheric spectrum.

Your installation will not meet these. In particular, a panel in full sun runs far hotter than 25°C, often 20 to 30 degrees above ambient air temperature, and output falls as it heats.

Add the other losses and the picture is:

Loss	Typical effect
Cell temperature above 25°C	5 to 15%
Angle and orientation	0 to 20%
Dust, pollen, soiling	2 to 5%
Wiring and controller	3 to 8%

Expect 75 to 85% of rated output on a good day. That is normal and not a fault. Every sizing calculation on this site applies a factor in that range rather than using nameplate figures.

The counterintuitive consequence: panels often produce their best output on a cold, bright day rather than the hottest day of summer.

The specification that destroys equipment

Panel voltage rises as temperature falls.

This is the most important practical fact about panels, and it is the one most often missed by people building their first system. A charge controller has a hard maximum PV input voltage. Exceeding it damages the controller immediately and usually beyond repair.

The trap is that the rated Voc on the datasheet is quoted at 25°C. On a cold, clear winter morning, an array can produce meaningfully more voltage than its rating. That is also exactly when the array is likely to be fully illuminated by snow glare.

Size your string using the panel’s temperature coefficient and the coldest temperature your site actually reaches, not the rated figure. The charge controllers page covers how this interacts with controller selection.

Monocrystalline, polycrystalline, and the rest

Monocrystalline cells are cut from a single silicon crystal. Higher efficiency, typically around 19 to 22% for current commercial panels, and uniformly dark in appearance.

Polycrystalline cells are cast from multiple crystals. Lower efficiency, typically around 15 to 17%, with a visibly speckled blue surface.

The honest summary: this decision matters less than the internet suggests. Efficiency describes how many watts a panel produces per unit of area, not how many watts per dollar and not how well it performs. A 200 W mono panel and a 200 W poly panel both produce 200 W. The mono one is smaller.

That matters when area is constrained, on a tiny house roof or a small ground mount. It matters much less when you have a field.

In practice the price gap has largely closed and monocrystalline now dominates the market, so this is increasingly a decision that makes itself.

Other terms you will see on datasheets, briefly: PERC is a rear-surface treatment that recovers a little extra output and is now common. Half-cut cells reduce internal losses and improve shade tolerance somewhat. Bifacial panels also generate from the rear face, which is useful on a ground mount over a reflective surface such as snow, and close to useless flat against a roof.

Flexible panels deserve a warning. They are convenient for curved surfaces and light structures, but they generally have shorter lifespans than rigid glass panels, degrade faster under heat, and cost more per watt. For a fixed installation with room for rigid panels, rigid panels are the better buy.

Series and parallel

Panels wired in series add their voltages while current stays the same. Panels wired in parallel add their currents while voltage stays the same.

Series is usually the better choice for off-grid, for one specific reason: higher voltage means lower current for the same power, and lower current means thinner cable and less voltage drop over the run from array to controller. If the array is any distance from the batteries, that saving is substantial. MPPT controllers exist precisely to take a high-voltage string and convert it down.

Series has one significant drawback, covered next.

Parallel keeps voltage low, which suits PWM controllers and very small systems, at the cost of much heavier cable for the same power.

Shading

In a series string, the weakest panel limits the whole string. Current is the same at every point in a series circuit, so a panel in shade drags down the panels in sun alongside it.

Panels contain bypass diodes that route current around a shaded section, which limits the damage considerably. They do not eliminate it.

The practical guidance:

• Avoid shading rather than engineering around it. No wiring arrangement recovers energy from a panel that is in shadow.
• Watch the winter sun path, not the summer one. The sun is far lower in the sky in the months when you can least afford the loss, and a treeline that clears the array in June may cover it in December.
• If partial shading is unavoidable, parallel strings limit the damage to the affected string, and separate controllers or module-level electronics limit it further.

Tilt and orientation

In the northern hemisphere, panels face south. The further from south, the more you lose, though the penalty is gentler than people expect: a moderate deviation costs a few percent, not half.

Tilt is the more interesting decision, because the best angle depends on which season you are optimizing for. A rough starting point is to set tilt equal to your latitude for a good annual average, and steeper than latitude to favor winter, when the sun sits low and your system is under the most pressure.

For an off-grid system that has to work year-round, favor winter. The array is sized by the worst month, so extra summer output you cannot store is worth nothing, while extra December output is worth a great deal.

A steeper winter tilt also sheds snow better, which matters more than the angle itself in some climates.

Degradation and warranties

Panels lose output slowly, typically around 0.5% per year. Manufacturers commonly warrant something like 80 to 85% of rated output at 25 years.

Two things worth understanding about those warranties:

They are performance warranties, which is a different thing from a product warranty covering physical defects. Product warranties are usually much shorter, often 10 to 12 years.

They are only as good as the manufacturer. A 25-year warranty from a company that does not exist in 10 years is not a 25-year warranty. This is one of the few places where brand reputation genuinely earns its premium.

What actually matters when buying

• Voc and the temperature coefficient, because they determine whether your controller survives.
• Physical dimensions, because they determine whether your mount works.
• Total array watts, sized against your worst month.
• Rigid rather than flexible for anything permanently mounted.
• A manufacturer likely to outlive the warranty.

Efficiency and cell type sit well below all of these. Spend the attention on the battery bank and the wiring instead, which is where off-grid systems actually go wrong.

Before you install anything, read the solar panel safety page. An array cannot be switched off, and a series string reaches voltages that can kill.