Solar panels (aka photovoltaic modules) consist of thin layers of polycristalline or monocrystalline silicon.
How They Work
Silicon is a semiconductor. When photons hit its surface, this dislodges electrons which in turn create a flow in electric charge.
Normally, these dislodged electrons would move in random directions and not produce a usable current. By special treatment, solar panels make sure the dislodged electrons move in the same direction, producing an electrical current.
Small Cells Form Large Panels
Solar panels are made of individual small cells: Monocrystalline silicon cells typically produce 0.5-0.6V each. Polycrystalline cells can produce 0.5-0.55V.
By connecting these cells in series, the overall panel voltage can be set. By connecting cells in parallel, the maximum panel current can be set.
On the market, a rich variety of solar panels exist with different maximum voltages, catering your needs. For DIY projects, solar panels with 3V, 5-6V, 9V, 12V, and 24V are common.
Evaluating Panels
Do not naively trust vendors or technical specifications. While most vendors provide accurate information, you can commonly find astonishing offers for solar panels supposingly capable of producing hundreds of watts for little money.
Rogue vendors cannot bend physics though, so before you purchase a particular panel, do the physics plausibility check:
0.015-0.023W/cm2 Yield
Physically, the area of a solar panel matters most: sunlight delivers roughly 1kW of energy per square meter (1000W/m2) on a clear day at solar noon in the summer. That is 0.1W/cm2.
Solar panels have a conversion efficiency anywhere between 15-23% and produce 0.015W-0.023W/cm2. This is the formula to roughly check solar panel specs.
Let’s take a look at these polycrystalline mini solar panels. They have a size of 53x30mm:
Test 1: Polycrystalline Panels
The vendor claims these cells produce a current of 30mA at 5V (0.03A x 5V = 0.15W). Their surface is 5.3cm x 3cm = 16cm2. Being polycrystalline, their efficiency is at the low end and can be estimated at 15%.
Here is the plausibility check: physically, these panels should be able to produce this:
Area x Solar Input x Efficiency = Watts 16cm2 x 0.1W/cm2 x 15% = 0.24W
The irritating result: that is too much, actually much more than the vendor claimed.
Identify The Active Area
The unexpected result illustrates how important it is to look at the relevant dimensions: while the total physical size of the panel is indeed 5.3cm x 3cm, the effective light-converting area is much smaller: 4.5cm x 2.3cm = 10cm2.
Let’s repeat the math with the corrected surface area:
10cm2 x 0.1W/cm2 x 15% = 0.15W
A 0.15W yield is exactly what the vendor claimed.
Test 2: Monocrystalline Panels
Monocrystalline panels have a higher efficiency (up to 23% compared to 15%). They cost more, too. Let’s test plausibility for this larger monocrystalline panel:
The vendor claims this panel delivers 3W at 5V. The panel size is 12.1cm x 12.5cm, and on closer inspection, there is a 6.4mm inactive rim on both sides:
The 12.5cm physical width of the panel is thus reduced by 2 x 0.64cm to 11.2cm, providing an active area of 12.1cm x 11.2cm: 135.5cm2.
Performing Plausibility Check
Let’s do the math again. This time, a 23% efficiency is used as it is claimed to be a monocrystalline panel (plus the vendor explictly claimed a 23% efficiency):
135.5cm2 x 0.1W/cm2 x 23% = 3.11W
This panel (and this vendor) passed the plausibility check.
Whether or not the panel in fact yields the full energy calculated is a separate question. The plausibility check uncovers vendor specs that are physically impossible, i.e. a “200W panel” for less than EUR 10.00 and sized 20x20cm. Only practical testing can verify the claimed panel efficiency.
Choosing A Good Panel
There is a huge variety of solar panels to choose from. With the following quick guide you can effectively compare different panels and identify the one that works best for you at the lowest price.
It’s a two-step process:
- What Is Needed? First, make up your mind what you actually need: what should be the solar panel voltage, and what are your power requirements and space constraints.
- Choose Model: Once you know the solar panel voltage and the maximum area you have available, compare different models in an educated manner to find the best and most cost effective solution.
Panel Voltage
Solar panels can come with a variety of maximum output voltages. This solely depends on how the vendor has connected the internal basic solar cells.
The first decision for you should always be: what is the solar panel voltage that you need. As a rule of thumb, the voltage should be as close as possible to the voltage you actually require. DC-DC voltage conversion is most efficient when the raw input voltage and the fixed output voltage are close to each other.
So if you plan to power a microcontroller, a 5-6V panel is best. If you’d rather like to charge your car battery or a power bank, then a 24V panel might be best (as it can easily power the high performance 20V USB charging standard).
In any case, never use the solar panel output voltage directly to power devices. Solar panels deliver an inconsistent, raw and strongly fluctuating voltage that can be dangerous and destroy electronics if applied directly. Always use a charger module to convert the raw solar voltage to the desired fixed output voltage!
Solar Panel Size
There is an easy rule of thumb for solar panels: size (area) rules.
As you have seen, the active area of a panel is the most important factor determining the power you can get.
You can connect multiple smaller panels to one large active area yourself, too: just connect the panels in parallel to not change their total voltage. By connecting multiple smaller panels in parallel (using panels with identical nominal voltage each!), you may be more flexible in where to place the panels, and at which angle. What matters is the total active panel size, regardless of whether it’s one piece or a bunch of smaller ones.
Oversizing: Cost Effective
Oversizing a panel can compensate many other flaws, such as less efficient panel technology, less efficient charger technology, less that ideal location for the panel in respect to sun intensity.
Rather than spending extra money on high efficiency components such as super-efficient panels or super-efficient MPPT solar chargers, this money is often spent more effectively in purchasing a larger solar panel.
Space Constraints
If space constraints or design issues impose a hard limit on the maximum solar panel size you can use, then tweaking panel efficiency and/or charger efficiency can compensate this.
Compensating space constraints has its price: more efficient panels and more efficient DC-DC converters in total are more expensive than a larger cheap panel.
Compensating Via Panel Efficiency
10cm2 space can be reduced to 6.5cm2 by switching a polycrystalline panel with a monocrystalline panel:
10cm2 * 0.015W/cm2 / 0.023W/cm2 = 6.5cm2
A polycrystalline panel has a 15% efficiency (yielding 0.015W/cm2) whereas a monocrystalline panel has a 23% efficiency, yielding 0.023W/cm2.
Compensating Via Charger Efficiency
Post-processing the raw solar panel voltage consumes energy. The efficiency of DC-DC converters vary greatly and can eat up all your other efforts or - when done correctly - provide great optimization potential.
The raw solar panel voltage needs to go through a DC-DC converter to turn it into a stable fixed output voltage. This is often combined with charger technology and a buffer battery.
All of this can be performed with efficiencies in the range of 50-90%, depending on technology used:
- Solar panel voltage: Choosing a solar panel with a maximum output voltage close to the intended fixed voltage improves the efficiency of DC-DC converters. Converting a 24V solar panel to 5V USB power is far less efficient than using a 12V or 9V panel.
- MPPT: Specialized MPPT converters (Maximum Power Point Tracking) take into account that solar panels have a sweet spot (a current) at which they deliver the most energy. MPPT converters control the current they draw from the panel to optimize and improve solar panel efficiency.
- Linear vs. Switched Regulators: Linear converters (regulators) are much less efficient: they dissipate extra energy as heat. Linear converters exist just because they are cheaper, and in many scenarios efficiency is not important. Slightly more expensive switched converters utilize significantly more of the input energy and are much more efficient.
Cost Optimization
Once you know the solar panel voltage and roughly the size of the solar panels, you can now search for deals and compare models.
To identify the best buy, rate the panels by cost per watt.
Cost per Watt
Let’s calculate cost per watt fro the two quite different panels from above.
- Small, cheap, less efficient: these panels were purchased in a 10 pack for a total of EUR 2.38. Each panel cost EUR 0.24 and yielded 0.15W. That is EUR 0.24 / 0.15W = EUR 1.60/W.
- Larger, more expensive, more efficient: these panels were purchased as 5 pack for a total of EUR 14.99. Each panel cost EUR 3.00 and yielded 3W. That is EUR 3.00 / 3W = EUR 1.00/W.
If you had to choose between the two panel models above, the cost per watt calculation would clearly favor the more expensive and larger monocrystalline board which yields the same energy for 37.5% less cost:
100% - (EUR 1.00 x 100 / EUR 1.60) = 37.5%
Final Thoughts
A few practical facts:
No Covers Please
Often, small DIY solar panels are protected by a thin plastic cover. Make sure you remove this before use, and make sure you do not place anything else inbetween the solar panel surface and the sunlight (i.e. a plexiglas cover, etc).
Solar energy can be very powerful when it can reach the solar panel. Most materials - even when they appear transparent to the human eye - filter out a great portion of the energetic light spectrum, severely impacting the solar panel efficiency.
Solar Panels Inside The House
It makes no sense whatsoever to use solar panels inside the house. Only natural sunshine provides the raw energy needed to produce significant currents.
Even on a cloudy day, solar panels work surprisingly well outside. Inside, and with artificial light, they are useless.
That’s actually a good thing: it illustrates how efficient artificial light sources have become: light is emitted only in the visible spectrum. Infrared and ultraviolet ranges not visible by the human eye are filtered out. Coincidentally, these are the ranges that transport the most energy to solar panels which is why - for them - it is always pitch dark inside.
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(content created May 01, 2024)