The Truth About Solar Panels

How much do you know about solar panels? How do you hook them up? To what?

First you must know all about solar panels and what they do and what they don't do.

1) A solar panel is a group of solar cells wired in series or parallel or both series and parallel for higher voltage and current. See definition of series and parallel in the glossary below.

2) A panel is usually waterproof for outdoor mounting. Keep wiring and connectors protected from weather, moisture, and exposure to persons.

3) A panel can't be just connected to a battery. This will overcharge the battery once it is sufficiently charged, or discharge the battery without a blocking diode, because no sunlight will make the panel look like a short and will back feed power from the battery back to the panel and discharge the battery. It is possible that some panels will have a blocking diode or shadow diode (for series hookups) installed inside the panel connector to prevent back feeding. Some panels may have a small charge controller attached to back of the panel.

4) A panel will generate only about 15% power with an overcast sky depending on the density of the clouds.

5) A single panel doesn't get damaged if shorted with an exception. A group of panels connected together with its total high current or voltage could burn up the wiring or cause injury if you are not careful. To confirm the amperage output of a single panel you can momentarily place an ammeter of sufficient current across the output in direct sunlight to measure the maximum current output capacity (in amps) of the panel. It is not recommended to place an ammeter across the whole array because it will create a fire and arc explosion, and you could be injured. To confirm the useable voltage of the panel, place a voltmeter across the output and measure the voltage. This voltage without a load will show a voltage much higher than its actual usable voltage. A 12 volt panel voltage could rise to almost 21 volts with no load. Once a load is placed, the voltage will drop to a more realistic level. See MPPT in glossary.

6) A solar panel converts sunlight to DC electricity. A solar panel is made up of many cells connected up in series on each panel to get a usable voltage typically 6, 12, or 24 volts.

7) A panel is rated in watts output and priced by the watt. Example, a 100 watt panel that sells for US$170 is considered US$1.70/watt which is a typical unit of measure with today's 2021 prices.

8) A panel requires a charge controller to regulate the voltage and current so as not to damage the battery or load.

9) A panel usually has specifications like Voc=Open voltage, V=usable Volts, A=Amps, W=Watts. Breakdown Volts=how many volts in series before arc-over.

10) There are three types of panels. Monocrystalline, polycrystalline, and amorphous. The better ones are the first two. They usually have a 25 year life span and are much more durable. Amorphous panels usually have a fragile glass surface and are seen in many cheap driveway lights. Amorphous panels will only last a few years before getting weak.

11) Panels should always be matched up with other identical panels when hooking them up in series or parallel.

12) You can hook up as many panels in series or parallel to a charge controller as long as you don't exceed the voltage or current specified on the controller. If your panel(s) don't provide sufficient voltage or current this won't necessarily be bad as long as the voltage doesn't drop below the charge controller's operating voltage. It will just mean the batteries you're charging will take longer to charge.

13) The voltage and current produced from the panel varies and is dependent upon the amount of sunlight hitting the surface of the panel. For maximum output the panel should be pointed directly into the sun. Perpendicular to the panel surface.

4) Large arrays are wired to dangerous voltages and must be wired and protected following NEC (National Electrical Code) regulations.


A cell is the smallest unit of a solar panel. A group of cells make up a panel. A single polycrystalline cell is shown below.

Each individual cell produces from .45 to 0.58 volts at an amperage dependent on the physical size (square inches) of the cell in full sunlight. The cells are wired in series and/or parallel or both to create a higher voltage or amperage. The larger the surface area, the more power out, and more costly. So 22 cells at 2 amps wired in series it will produce about 12.7 volts at 2 amps in full sunlight. If two banks of these series wired cells are wired in parallel, they will produce 12.7 volts at 4 amps. See "parallel" in glossary below.

A typical portable panel is shown below. This is a polycrystalline, 7.5 volt 250ma (1/4 amp) output in direct sunlight. This panel produces almost 2 watts. On a good sunlit day, this will amount to 20 accumulated watts!

The one shown below is a 14 volt 250 ma panel was made by Solarex. This one could be used for most charging applications specifically in the field. I use it to charge up my 4 watt VHF walkie-talkie when I'm in the woods back packing or out camping. It has an aluminum backing which makes it very rugged. This was purchased used for US$18.00 back in 2001.

 

A full size 110 watt commercial panel is shown below. It will produce 21 volts at 6.5 amps.

 

Most full size panels are rated at 12 volts at 8 amps or so. Using Ohm's Law, P=E X I, P = 12 X 8 = 96 watts. Watts is a unit of measure and is used to determine how much power you will need. If you put a volt meter across the output of the panel in full sunlight you will read a voltage much higher than the 12 volts stated on the panel label sometimes 17 to 21 volts. This is considered the "Open voltage" or no load reading. This is normal. Once the panel has a load on it, the voltage drops back to almost 12 volts depending upon load.

Below is my house with a portion of the array installed. Right now it produces about 3.3kw or 300 amp/hours per day @ 53.2 volts, roughly 15.6 kwh per day which is pushed back to the power company with a net metering system.

This system is configured for a grid tied 48 volt system. It uses eight 6 volt flooded lead acid batteries. The panels are wired, four in series for 48 volts and eight sets of 48 volts in parallel to provide the 60 amps to the two 40 amp charge controllers. A Trace SW4048 inverter system pushes any excess power back to the power company when the batteries are fully charged and I'm not using much current in the house.


Wiring up panels correctly is important. You can wire them in series or parallel or a combination of series and parallel to get the voltage and current desired. Wiring them wrong can be a fire hazard and can do serious damage.

You must also wire them with diodes to prevent "shadowing". This can damage the panels and significantly reduce output. Shadowing occurs when a group of panels are wired up and an object like a tree or building shadows one or more panels. This will cause problems. You have to understand that a solar panel if not exposed to sunlight is actually a short circuit and power will feed back from the battery or other panel into the shadowed panel. This reduces the efficiency and output voltage.


If you want to know more about battery types view this link about the theory of batteries here.


This is how you hook up a panel to charge batteries. First let's focus on charging AA batteries with NiMH or NiCad chemistry. The charger has a smart chip in it to carefully regulate the charging current and voltage so as not to damage the batteries with too much current. They can be purchased on Amazon. Search "Powerex MH-C204F" for approx US$29.95. Various chargers and batteries can also be purchased from Thomas Distributing.

Note the wall wart (power adapter) that comes with your charger. It should be marked on the back of the adapter and on the charger. Most adapters are outputting 12 volts at up to 1 amp. This means you need at least a solar panel with an output of about 15 to 18 volts at an amp or so. You will need a buck/boost regulator to take the voltage to 12 volts. These buck/boost converters can be purchased from Amazon or Ebay for about US$2.50. 


Actual size of buck/boost regulator

Below is a simple circuit diagram showing how to connect to a smart battery charger. 

A word of caution: These smart chargers can require hours of operation to properly condition or charge the batteries. If you have scattered clouds or only a short time of good sunlight or intermittent power, the charger may not fully cycle charge the batteries. This usually holds true with any type of battery charger. Smart chargers may restart its charge cycle every time there is a temporary loss of power (momentary lack of sun). Some smart chargers take as long as six hours to shape the battery and bring it to full charge. 

An ideal circuit is below. Charge up a gel cell or other deep cycle battery and send the 12 volts to the battery charger/conditioner. This way if the sun drops behind a cloud the 12 volt battery will maintain the charger cycle until the sun reappears. The solar charge controller keeps the 12 volt battery from overcharging.


Why 12 volts? It seems like everything is designed around 12 volts. The main reason is, it is a low voltage, a safe voltage, and has become a standard because all modern automobiles generate and use 12 to 14 volts. Because of this, most household electronics such as battery chargers, radios, computers, etc. run on 12 volts also. All these household appliances can then run on power from an automobile without any conversion. Look at the label on a few of your transformer "wall warts" and note the voltage and current rating. Also note the polarity, usually center positive. When making up your own cables, observe the polarity so you don't damage your equipment.

 

This transformer outputs around 12 volts DC and can provide up to 500 ma or 1/2 ampere of current. See glossary.


If you need to charge a (SLA) sealed lead acid battery or 12 volt deep cycle battery you will need a special charger (shown below) designed for lead acid batteries. You can find these special chargers on Amazon for less than US$15.00.


Size 3" x 5"

A feature of this controller is that it monitors the load and battery and disconnects the load if the battery reaches a low charge thus protecting the battery from a deep discharge and damaging it.


Glossary

Load - A light bulb, radio, or any equipment, or even a battery that requires charging.

Series - A group of cells or batteries wired together as shown below. Series increases the voltage at the same current. Each battery plus and minuses are connected back to back.

 

Parallel - A group of cells or batteries wired together as shown below. Parallel increases the current while maintaining the same voltage. All the plus terminals are tied together and all the minuses are tied together.

 

Series/Parallel - Wiring two series strings in parallel. Two 60V 1A strings in parallel will provide 60V at 2A

 

Buck/boost - A switching supply that will take an input voltage from 3 volts to 25 volts, and regulate, increase, or decrease the voltage to a set value using an adjustment on the board that is shown in blue component.


Actual size

Charge controller - An electronic device connected between solar panels and the battery bank to safely charge the batteries such as an MPPT charge controller shown below. This whole house unit will handle 40 amps at 12/24/48 volts.


4" x 8" x 2"

NiMH - Nickel Metal Hydride chemistry for batteries. This chemistry is currently more popular for most rechargeables and has replaced NiCad chemistry. Some batteries are LSD (low self discharge) which means they can sit on the shelf for 6 months or more without self discharging.

NiCad - Nickel Cadmium chemistry for batteries has been around since the mid 1960's. They are robust and durable and can be recharged over a thousand times. They will perform in lower temperatures and are easier to charge. The drawback of this chemistry is that they will self discharge sooner than other rechargeables and the Cadmium is a more dangerous element to the environment. NiMH has replaced this chemistry.

Arc-over - Panels can be configured in series as much as 300 volts. This is lethal and dangerous if not respected. Hooking up lower breakdown panels in a high voltage configuration can cause arcing and burning, and can be deadly. The reason for higher voltage is the use of thinner gauge wire in long distance runs from the panel array to the charge controller and storage batteries.

Grid tied/Net Metering - An AC inverter will convert the high current 48 volt DC into pure sine wave AC at 117 volts providing whole house power and also connects to the grid to sell power back the the power company. Special monitoring in the grid tied system controller prevents back feeding AC to the grid should the main power fail from the power pole. This will prevent power line workers from getting injured if power is fed back to the line. In most jurisdictions you must have permission to sell power back to the power company.

Voc - Volts, open circuit. An output voltage measured from solar panel with no load.

MPPT - Maximum Power Point Tracking. An algorithm designed to transfer power to a battery or other load at an input voltage higher than its load rating. The power curve (efficiency) of a panel usually provides more power transfer well above the battery load voltage so the charge controller pulse widths the power at a higher voltage.

Wall wart - A wall transformer that converts the house voltage, 117 volts AC down to a safe usable voltage such as 12 volts. Note that there are two basic types of wall "transformers".

One has a transformer and diode and capacitor to provide 12 volts. The caveat is that there usually is no regulation to 12 volts and if you actually measure the voltage of this transformer, you will find a voltage as high as 17 volts if it does not have a load connected to it. This can be dangerous to some equipment but if it comes with your equipment, it will not be a problem. These wall transformers are noticeably heavier and more expensive than more modern ones.

The second and more modern type is a switching supply which has no iron transformer and is noticeably lighter. Since they don't have iron plates and copper coils in it they are cheaper to build. The voltage output is usually regulated and closer to the voltage stated on the label. They can usually provide a higher current so they can actually be smaller. The only caveats are that they can be noisier and cause Radio interference to more sensitive receivers and they are more susceptible to damage from lightning or surges on your house current. 

Diode - A component that allows current to flow in only one direction.

Bypass diode - Diodes placed across each panel to allow other panels to continue providing power while a panel is shadowed.

Blocking diode - A diode that prevents power back feeding from the battery back into the panel.


Diodes placed between terminals on back of panel.

 

Copyright 2021 Rick C. Ver 0.94 Nov 8, 2021