Solar panels, also called photovoltaic (PV) panels, produce DC power from sunlight to charge batteries and provide electrical power. The panels can be used single or in series or parallel arrays.

To avoid overcharging,¹⁾ a charge controller is placed between the PV and the battery bank.

Many factors will affect output; and panels will rarely generate power equal to their laboratory rating.

As a general rule, expect panels to put out 60% of their rating under absolutely best-case scenarios.

There are several types of PV widely available on the market, and many more in research stages. PV are described by how each panel's cells are constructed.

Mono or single crystal cells are made from complete slices of silicon crystal. Mono cells have rounded edges because they were cut from a single cylindrical crystal.²⁾
Strained metaphor: if poly cells were it were paneling they would be veneer because the visible surface is made from one piece of material.

• PRO
  • most efficient (15%-20%) overall due to highest quality silicon
  • most efficient for the area, so best where space is extremely limited
  • slightly better than poly in uniform low light
  • theoretically greater longevity than poly, much better longevity than thin film
  • slightly higher Vmp than poly³⁾, for situations in which that could be a benefit
  • slightly better performance in higher temps than poly, possibly due to higher Vmp
• CON
  • most expensive to produce
  • slightly worse than poly in partial shading
  • ill-suited to PWM/shunt controllers because of their higher voltage (and lower current)

Poly cells are made up of smaller pieces of slices. Poly cells are rectangular.⁴⁾

If poly cells were paneling they would be OSB, because they are made up of many flat pieces of silicon.

• PRO
  • least expensive PV per watt
  • better match for PWM due to slightly lower Vmp.
• CON
  • efficiency (13%-16%) is less than mono. This works out to 3%-5% less power for a given area.⁵⁾
  • slightly worse performance in high temps than mono

Flexible panel configurations place unusual demands on the materials. Framed panels use extremely durable glass encapsulation⁶⁾ but this would not work for flex panels. One challenge is to come up with encapsulation that is transparent, strong, and durable.

Originally a clear film called PET was used for the exterior but over time durability issues related to yellowing, increasing opacity, and delamination (“peeling”) were revealed, especially in high temperatures or harsh environments. ETFE is now commonly preferred for exterior encapsulation layers due to greater UV transparency and durability. Flex panels that do not mention ETFE are likely encapsulated in PET.

The other challenge is to make solar cells that can slightly flex.

While the flex panel market has largely gone to poly/mono crystalline, there are obvious issues related to making flexible objects out of brittle silicon crystals. It is not clear why the market went that direction.

Handle with care, and bend as little as possible.

[Note: actual thin-film panels are rarer as of 2018. The market has gone to flexible mono or poly⁷⁾]

Thin film or amorphous PV uses photovoltaic material deposited on a substrate rather than silicon crystals.

If thin film cells were were paneling they would be colored plastic veneer because it is produced inexpensively and is least affected by ambient conditions.

• PRO
  • semi-flexible so it can be applied to curved shapes
  • least affected by high temps
  • least affected by shading due to cell construction
  • least affected by cloudy/hazy conditions, due to increased sensitivity at the blue end of the light spectrum⁸⁾,⁹⁾
  • very light
• CON
  • most expensive per watt
  • least efficient (about 9%), requiring large physical area.
  • shorter longevity compared to crystalline panels
  • lower Vmp

While most panels are hard mounted horizontally on the roof of the van, portable folding portable solar panels have dropped in price and have some advantages.

Amazon search

• PRO
• no mounting costs
• can be stored securely in the camper
• can collect sunlight while the camper stays in the shade
• Can be angled and moved follow the sun and collect maximum power
• Sometimes have USB charging ports directly on the back of them, allowing charging of small devices directly without the use of a charge controller and battery setup.

• CON
• animals may damage or pee on the panels¹⁰⁾
• long wiring runs can cause voltage drop
• deployed panels can make quick getaways impractical
• it is easy to get lazy and not put out the panels every time
• Questions exist about long-term durability
• Possibility of panels getting stolen

12V panels are not really 12V; they are called that because they charge 12V battery banks (which aren't really 12v either!). Since they are called 12V we say nominal (ie, “named”) 12v. [yes, it's confusing. – frater secessus]

• Nominal 12v panels have 36 cells. They will generally have max power (Vmp) around 18v and open circuit (Voc) around 21v.¹¹⁾ These are commonly found on mobile and portable installations.
• Nominal 20v panels have 60 cells. They will generally have max power (Vmp) around 30v and open circuit (Voc) around 36v.¹²⁾ These are commonly found in residential rooftop installations.
• Nominal 24v panels have 72 cells. They will generally have max power (Vmp) around 36v and open circuit (Voc) around 42v.¹³⁾ These are commonly found in commercial or residential ground level installations. Due to internal construction (actually 2 36-cell segments in parallel)¹⁴⁾ they can be more resistant to partial shading.

higher voltage panels (24v = 72 cell, 20V = 60 cell) are usually cheaper by the watt than 12V (36 cell)

higher voltage panels are more common on the used market, as people upgrade residential/commercial installations

MPPT controllers do a DC-DC downconversion that is quite efficient. If Vmp isn't required most will move PV voltage away from Vmp to prevent power from ever getting to the CC.

Higher voltage panels can reduce wire costs; amps are cut in half for the same amount of wattage. They more likely to stay above charging setpoints in poor insolation or high heat.

Panel voltage¹⁵⁾ is an important factor when selecting a solar charge controller.

Higher efficiency doesn't mean the panel makes more power; it means it makes more power from a given area. 100w is 100w, but you might be able to fit a 110w panel high-efficiency panel in the same space as a regular-efficiency panel.

In general, mono has higher efficiency than poly, and poly has higher efficiency than thin film.

The cost of higher efficiency panels typically outstrips the increase power, so unless you are tight for space they typically aren't a good value for money. If you are tight for space and need the power then pony up the cash and enjoy the premium product.

At zenith, sunlight provides an irradiance of just over 1 kilowatt per square meter at sea level. Of this energy, 527 watts is infrared radiation, 445 watts is visible light, and 32 watts is ultraviolet radiation. – wikipedia

Mono panels can capture about 15-20% of this visible light energy. Poly panels capture 13-16%, and film panels capture 9%.

PV are generally rated by several criteria¹⁶⁾

• Power (W): 190 Watts. ← rated power in Watts under lab conditions. You can derive price-per-watt by dividing $/watts.
• Open Circuit Voltage (Voc): 36.00 Voc ← Volts in full sun with no load. In practice you will likely not see Voc when hooked to the controller, but all parts of your solar installation need to be able to cope with the theoretical Voc. In serial arrays the Voc ratings are added.
• Short Circuit Current (Isc): 7.42 Isc ← Amps in full sun when shorted. Also theoretical, but connectors and cables need to be sized to handle Isc.
• Maximum Power Voltage (Vmp): 28.60 Vmp ← voltage at which max power is generated in lab conditions
• Maximum Power Current (Imp): 6.64 Imp ← Amps at Vmp in lab conditions

Note that in this example the Power (W) rating is 190, which is the Vmp x Imp (28.60 x 6.64 = 189.904W).

In real world conditions power output will likely be less than under optimal lab conditions and the Vmp may not be at the voltage given on the label. Vmp will vary due to local conditions like temperature, shade, and sunshine. An MPPT charge controller, if present, will sweep the range of voltages regularly to find Vmp under the existing conditions.

Panels will get closest to their rated output at ambient temperatures around 32f/0f when the panels will be running about 85F. By the time ambient temperatures are 90F panel temps will rise to 145F and power output will drop about 18.45%.¹⁷⁾

To help consumers understand output outside the lab (Standard Test Conditions - STC) some producers also publish specs for conditions that might be more applicable to actual use (Normal Operating Cell Temperature - NOCT). Here is how they differ:¹⁸⁾

We can see that with NOCT the sunlight is not as strong, and the panels are assumed to be much warmer²⁰⁾ though some cooling from ambient breezes is present.²¹⁾

While NOCT may be useful for gauging normal harvests, STC is used for system component²²⁾ sizing because the panels really can make STC power²³⁾ in some real world conditions. The system needs to be sized to deal with high-output situations, particularly overly high panel voltages.

PTC (PVUSA Test Conditions)²⁴⁾ is a rarer standard.

A diode is designed to let current flow in one direction…. [it] is the electrical equivalent of a [plumbing] check valve. – Amy@AltE²⁵⁾

Bypass Diodes are inside the panel junction box, wired parallel to each cell group. It conducts when the cell is shaded and has reverse polarity due to other cells producing. Blocking Diodes are external to the panels. It blocks reverse current from other panels. It must handle the full voltage of system (series panels). – Pappion²⁶⁾

Further reading:

• Alt-E: Bypass Diodes & Blocking Diodes in Solar Panels
• solar panel diodes