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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.

Since PV necessarily run at higher voltages than the batteries they charge, 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.

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.1)
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 poly2)
    • 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


Poly cells are made up of smaller pieces of slices. Poly cells are rectangular.3)

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
  • CON
    • efficiency (13%-16%) is less than mono. This works out to 3%-5% less power for a given area.4)
    • slightly worse performance in high temps than mono

thin film

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
    • 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, some are mounted on tilt panels to increase yield, and some are portable.


  • no mounting costs
  • can be stored securely in the camper
  • can collect sunlight while the camper stays in the shade


  • animals may damage or pee on the panels5)
  • 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

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 17v and open circuit (Voc) around 21v.6) These are commonly found on mobile and portable installations.
  • Nominal 20v panels have 60 cells. They will generally have max power (Vmp) around 29v and open circuit (Voc) around 36v.7) These are commonly found in residential rooftop installations.
  • Nominal 24v panels have 72 cells. They will generally have max power (Vmp) around 35v and open circuit (Voc) around 42v.8) These are commonly found in commercial or residential ground level installations. Due to internal construction (actually 2 36-cell segments in parallel)9) 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.


PV are generally rated by several criteria10)

  • Power (W): 190 Watts. ← rated power in Watts under lab conditions. You can derive price-per-watt by dividing $/watts.
  • Open Circuit Voltage (V): 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 (A): 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 (V): 28.60 Vmp ← voltage at which max power is generated in lab conditions
  • Maximum Power Current (A): 6.64 Imp ← Amperage 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%.11)

electrical/solar/panels.txt · Last modified: 2018/05/19 05:18 by frater_secessus