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Solar panels

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 much less than their rated wattage under normal conditions.

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.

Types of panels


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.

Thin film

[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 solar panels10) may 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 panels11)
  • 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
  • not all portables are weatherproof
  • can be blown over by wind

Note that regular framed panels can also be carried as portables. To make storage/placement easier they are usually 100x times however many you need.


Some panels are “half-cut” which means the cells are cut in half and wired to make 2x as many cells. This can result in better harvest in some partial shade conditions with some increase in complexity and expense.

{Secessus reminds us we should avoid shade in the first place}

Panel voltage

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.12) These are commonly found on mobile and portable installations.
    Note: there are some “12v” panels that actually have 40 cells.13) Example: Renogy 200w "12v" panels, with Vmp of 22.6v and Voc 27v. The extra voltage cannot be harvested by PWM so MPPT is effectively required with these.
  • Nominal 20v panels have 60 cells. They will generally have max power (Vmp) around 30v and open circuit (Voc) around 36v.14) 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.15) These are commonly found in commercial or residential ground level installations.
  • even higher cell counts (and voltages) can be present in very large panels, like >400w.

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 they 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 are more likely to stay above charging setpoints in poor insolation or high heat.

Panel voltage16) is an important factor when selecting a solar charge controller.


The formula for panel efficiency is simply [ rated kW / square meter ]. A 20% efficient panel will make 200w per square meter under lab conditions, ie, 20% of the lab's 1000w standard. (0.20kW per square meter)

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 higher efficiency panels might be worth the premium.


Framed panels are often warranted to make 80% of their rated output for 25+ years.

There is some evidence that panels with a load connected degrade slower than those stored unconnected (open circuit)


PV are generally rated by several criteria17)

  • Power (W, or Pmax): 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
  • Temperature coefficient; see below.

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.

temperature coefficient

Panel voltage decreases as cell temperature increases. Ramifications:

  1. since panels are rated by Vmp x Imp, anything that reduces voltage will reduce power.18)
  2. the Vmp your MPPT controller finds in human-comfortable temps will likely be lower than rated Vmp

Example: 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%.19)

The actual amount of drop is dictated by the temperature coefficient, expressed as -0.X%/ºC. In other words, the Vmp will go down by X% per degree celsius of cell temp above 25C. Since

This coeffcient ranges from 0.3% to -0.5%. Crystalline panels average around -0.44% and thin-film lower (in the thirties). The general pattern seems to be that cell chemistries with higher Vmp tend to experience greater heat-related losses. “Mind the [air]gap”.

Let's consider three different panels in 80F ambients:

  • 100w mono, rated 19Vmp, -0.445%/ºC20) == 14.65w lost @ 16.22Vmp
  • 100w poly, rated 18Vmp, -0.440%/ºC == 14.48w lost @ 15.39Vmp
  • 100w CIGS21), rated 18.66Vmp, -0.36%/ºC == 11.85w lost @ 16.45Vmp


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:22)

Irradience 1000W / square meter 800W / square meter
temperature cell temp 25C23) ambient temp 20C
wind speed n/a 1m/s

We can see that with NOCT the sunlight is not as strong, and the panels are assumed to be much warmer24) though some cooling from ambient breezes is present.25)

While NOCT may be useful for gauging normal harvests, STC is used for system component26) sizing because the panels really can make STC power27) in some real world conditions. The system needs to be sized to deal with high-output situations, particularly overly high panel voltages.

NOCT is useful for comparing two panels [that have] the same STC rating. A panel with a higher rated power at NOCT for example, will generally result in a higher performing panel.28)

PTC (PVUSA Test Conditions)29) 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@AltE30)
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). – Pappion31)

Further reading:

a bit of an oversimplification, admittedly
wrapping, basically
which do not have the benefits described below
overcast light is blue-shifted
folding, briefcase, framed or otherwise
nominal 13v?
more precisely, the relationship between panel voltage and battery voltage
PWM are generally not affected since they are not using that higher voltage anyhow
not really, trying for apples-to-apples here
which would be about 0C ambient
therefore Vmp and Pmax reduced
panels are installed with an airgap to let breezes cool the underside
like controllers
or even more
electrical/solar/panels.txt · Last modified: 2024/05/28 22:01 by frater_secessus