Solar panels

Solar panels, also called photovoltaic (PV) panels, produce DC power from sunlight to provide power to an electrical system. This power is used to charge batteries or to power appliances or devices. However, due to the nature of solar power, special equipment like charge controllers is needed to turn the generated power into something that can be used by batteries and appliances. There are numerous types of solar panels; this page will help you choose the type that meets your needs and budget.

Check the Solar power page for a primer on how power is generated.

Solar panel output

Solar panel output is the actual power generated by a solar panel. Due to factors like Sun angle, shade, and panel temperature, your panels will rarely (or more likely, never!) generate as much power as their nominal rating ("100 watt panel"); the rated wattage would be difficult to achieve even if the panel was located on the Equator at solar noon on a completely clear day. Continue reading about solar panel output …

Solar panel options

Solar panels are often mounted permanently on a vehicle roof, but portable panels are also available. There are several types of solar panels available, and many more in research stages. Panels are described by how the panel's cells are constructed.

Rigid
- Rigid Monocrystalline
- Rigid Polycrystalline
Flexible
- Flexible Crystalline
- Flexible Thin Film
Half-Cut

Rigid solar panels

Monocrystalline rigid solar 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.

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 poly, 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)

Polycrystalline rigid solar panels

Poly cells are made up of smaller pieces of slices. Poly cells are rectangular. Polycrystalline panels are somewhat analogous to OSB panels, 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.
slightly worse performance in high temps than mono

Flexible solar panels

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.

Crystalline flexible solar panels

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 flexible solar panels

[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.

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

Portable solar panels

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.

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 panels
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

Half-cut solar panels

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.

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.

Nominal 12v panels have 36 cells. They will generally have max power (Vmp) around 18v and open circuit (Voc) around 21v.1 These are commonly found on mobile and portable installations. Note: there are some “12v” panels that actually have 40 cells.12) 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.13) 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.14) 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 voltage) is an important factor when selecting a solar charge controller.

Efficiency

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%.

Specifications

PV are generally rated by several criteria

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 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)

This article is incomplete or needs additional work. You can help edit this wiki to improve it!

STC and NOCT 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)

STC NOCT Irradience 1000W / square meter 800W / square meter temperature cell temp 25C19) ambient temp 20C wind speed n/a 1m/s Edit 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.

Diodes 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

Is MPPT required when wiring panels in series?

MPPT controllers are not required for series panel wiring. Counterexamples:

Imagine a 24v battery bank being PWM charged by two 12v panels in series. The series configuration itself is not important, but the relationship between panel voltage and charging voltage is.
Imagine a the 24v panel string above MPPT charging a 12v battery bank. The MPPT controller dies and a backup PWM controller is installed to keep power flowing. PWM will run the panel string at Vbatt, making roughly 1/2 the peak power the MPPT could make. If it were three 12v panels in series then PWM would make 1/3rd the peak power. Note that during float with no additional loads there would be no practical difference.

So why do we often say that it is required? "MPPT is required for series" is shorthand for "when Vmp greatly exceeds battery bank voltages PWM output is hamstrung and MPPT advantage is overwhelming. (In our PWM backup scenario we are not concerned with maximum power, only getting some power from the panels until we can replace the dead MPPT.) For example, running 2 12v panels in series to charge a 12v battery bank. The flipside to this is "when Vmp is close to battery voltage then MPPT advantage is thinnest and PWM may actually make more power." For more on this edge scenario, see temperature derating.

Solar panels gallery

See more in the Solar panels category. For image credits, open image and click More Details.

Resources

	Resource	Description
	Adding solar panels	Learn how to add a solar panel to an electrical system
	Planning your solar system	How to plan your solar panel installation
	Solar panel heat transfer	Analyzes the effect that solar panels have on the transfer of external heat into the van.
	Electrical Solar Forum	Subforum of the Class B Forums
	DIY Solar Power Forum	A very active solar forum with 100k members, founded by Will Prowse. Lots of residential and commercial solar discussion, but the "Vehicle Mounted Systems" sub-forum has plenty of info specific for nomads. Other sub-forums for batteries and solar equipment.
	Vehicle mounted solar forum	A subforum of the DIY Solar Power Forum.
	NV200 adjustable solar panels	An example build of adjustable tilted solar panels on a Nissan NV200.
	Solar DIY	Subreddit with 67k subscribers as of August 2024. "SolarDIY is a community to share your ideas, projects, inspiration, and look for advice, ideas, and networking."
	Electrical matters forum	A sub-forum of VanLivingForum.com
	Search Amazon	Search Amazon for related products.
	Wikipedia	Wikipedia article on this topic.
	Search forums and groups	Search van life discussion groups for "solar panels"
	Search related sites	Search van life sites for "solar panels"
	Search NomadLife.wiki	Search other pages on this wiki for "solar panels"

Some or all of the content on this page was originally sourced from RVWiki