Solar Panels, Lesson One

What are Solar Panels?

Solar panels are panels comprised of smaller solar cells, and these cells when hit by photons(light) they create a voltage that can be used to power things electrically.

Solar Panels are Made Up of Solar Cells

Because solar panels are made up of smaller solar cells they can be any size, shape and power output that one would desire. But most consumer panels are designed to work with 12 or 24 volt systems and their shape is usually square or rectangle to be used in an even larger array of panels on a roof top or similar structure to obtain even greater voltages and currents to power house holds or businesses. Solar panels are rated in watts, but don't be fooled, their rating is based on a 5 hour day in full sun light.

Two important readings when measuring the output of a panel is the voltage open circuit or VOC and short circuit current or ISC. When these two readings are multiplied they should read higher than the rated wattage of the panel. Because their rating is based on a 5 hour day in full sun light and most of the day your panel will not be pointing directly at the sun so as to give you an average of your whole days output. These tests should always be taken with the panel totally disconnected from anything as the current test short circuits the panel. The nice thing about solar panels is that they love to be short circuited, solar panels will not be damaged by short circuiting them. But if your battery or charge controller is connected to the panel when doing this test severe damage will occur.

Solar Cells are Inexpensive, Can I Make My Own Solar Panels

No! Solar cells are like crackers, they are very fragile and very susceptible to impurities. Solar panels are created in clean rooms by robotics, and the major cost of a solar panel is the encasement of the solar cells. They need to be protected from the environment if they are to last 25 years or better, don't believe the junk you read on the Internet. A DIY solar panel is far beyond the capabilities of most.

Buy a box of saltine crackers and try and encase them in something like a solar panel design and see how long they last. This does not take into account the many solder connections that must be made to solar cells and each solder joint must be perfect if it is to last 25 or more years.

Solar Panels Cannot be Used Alone

In just about all situations solar panels cannot be used alone, you cannot just plug your TV into a solar panel and run it. Solar panels produce, what I like to call, raw power. It needs to be converted into usable energy. Energy that your electrical devices were designed to use.

Consumer solar panels are designed to work with 12 or 24 volt systems. A typical 12 volt solar panel will produce between 20 to 24 volts facing directly into the sun on a clear day. A 24 volt panel will produce around 40 volts. But the sun is constantly moving, with clouds, hazy, smog, and other obstructions partially blocking the sun. The output of a solar panel will vary greatly on a typical day.

This is one of the reasons solar panels cannot be used alone. They need to be set up in a system that will stabilize the output of the panel. This is done with a charge controller and an array of batteries. The charge controller keeps the voltage and current of the panel in the range that the batteries can use and the batteries keep the voltage of the charge controller in the range that your electrical devices can use. – charge controllers and batteries will be discussed later in the lessons.-- Lesson Two -- Lead Acid Batteries for Solar Energy Storage

Batteries are an essential part of an off grid solar power plant day or night.

How to Wire Solar Panels

Years ago you didn't have much choice, a 12 volt system would use 12 volt rated panels in parallel. This set up kept the voltage in the 12 volt range and with each panel you increased your current. Think of voltage as pressure on the system and current as the amount of charge.

Panels in series your voltage adds and the current stays the same, panels in parallel the current adds and the voltage stays the same. There is more to it than that but suffices for this lesson.

These days though, there are two main types of charge controllers for off grid solar systems and these are “pulse width modulation” or PWM and “Maximum power point tracking” or MPPT. Both of these will be discussed in greater detail in later lessons, but for now we will discuss how they relate to the wiring of the solar panels.

“Pulse Width Modulation” or PWM

PWM needs to only work with the voltage your system is rated for. For example, a 12 volt system would have a battery array of 12 volt batteries wired in parallel, with 12 volt panels wired in parallel, even though the panels in direct sunlight would have an output of around 20 volts. To charge a battery the voltage needs to be higher than the rated voltage of the battery. This is all controlled by the charge controller.

But there are a few drawback to this, one, you will lose power due to mismatch between panel voltage and battery voltage, this will be discussed in greater detail when we discuss charge controllers, and two, 12 volts needs very thick cables to travel any distance without suffering losses which brings us to.

“Maximum Power Point Tracking” or MPPT

MPPT charge controllers work very differently from PWM charge controllers. They play with the voltage and current produced by your panels to give you exactly what your system needs to obtain maximum power transfer from the panels to your batteries and electrical needs.

Power transfer can be hard to understand, so let me give you an example that will help you understand. Take a piece of wood and put it on a sturdy table and try to pound a nail onto it, no problem, the power from the hammer when it hits the head of the nail flows to the tip of the nail and drives the nail into the wood. Now take a piece of wood and hang it with rope so it swings freely in the wind. Now try and pound a nail into it, with out holding the wood still in any way, the wood won't stay still. No matter how hard you hit the nail the wood finds a way not to except the nail's penetration. No power is being transferred from the tip of the nail to the wood, well not enough. The same thing happens when ever you are trying to transfer power from one item to another.

An MPPT charge controller looks at what you are transferring power to and plays with the voltage and current so that maximum power is transfer in the form that is needed to be excepted by your system. For example, if it were trying to pound that nail in the swinging piece of wood it would shoot that nail at a high enough velocity that the wood would not have time to get out of the way and bang, power from the tip of the nail would be transferred to the wood in a manner that the nail would penetrate the wood.

Because an MPPT charge controller can play with the voltage and current you no longer need to only use the voltage your system is designed for, MPPT charge controllers have a solar panel maximum voltage rating that can be far greater than the voltage your system is designed for. This means you no longer have to deal with 12 volts if your system is a 12 volt system, but now you can wire your panels to deliver greater voltages which can translate into thinner cables from the panels to the charge controller with less cable losses and a greater transfer of panel power to battery power.

For example, you have an MPPT charge controller rated for 100 volts maximum panel voltage you can take four 12 volt rated panels, four because actual voltage of the panels is around 20 to 24 volts and you always want to use less voltage than what your unit is rated, connect them in series giving you around 80 to 96 volts. This also gives the controller a lot to play with when trying to match your power transfer from panel to battery.

MPPT can only play with voltages greater than what your system is rated for, for example if your panels are only outputting 20 volts for a 12 volt system then you won't be getting much of a benefit, power wise, from your much more expensive MPPT controller.

Mixing and Matching Solar Panels

It is always best to use the same panels in your solar panel array but with “Do It Yourself” systems many times great deals can be found with differing types of panels. So the question is can you mix and match different panels in your array. The answer is yes if the voltages and currents are close. An MPPT controller can make this easier.

With PWM you are limited to using panels that are rated for your system, you can use two 12 volt panels in series to create a 24 volt panel but you can not use a 24 volt panel on a 12 volt system. With MPPT you could for example, use a 24 volt rated panel which output around 40 volts with two 12 volt rated panels in series which will output around 40 volts on a 12 volt rated system.

Remember solar panels in series voltage adds, in parallel current adds. But the voltages and currents of the individual panels need to be close or you will incur losses. For example connecting a panel that outputs 20 volts in series with a panel that outputs 10 volts will not be wise. That's because even though the voltages will add the current will be limited to the current of the smaller panel and may cause excessive heating and will cause a loss of power.

But if you connect two panels in series, lets say, that both output 20 volts at 3 amps, this will give you 40 volts at 3 amps. This now can be connected in parallel to a panel that outputs 40 volts at around 3 amps to give you an array of 40 volts at 6 amps. This now can be connected to an MPPT controller to power your 12 or 24 volt system.

How Many Panels Do I Need?

This is a very difficult question to answer. You will never get the rated power from your panels. These ratings are always calculated under ideal situations which never happens in real life. Where you live, your climate, air quality, and many other factors will all degrade your expected power output. Most professional installations are grid connected and over done to protect against insufficiencies.

But if your situation is going to be a Do It Yourself off grid deal, how much is this going to cost, most likely, will be a factor. So you may not want to go hog wild with the “over doing it”.

But let me try and help here, first you must have enough power to recharge your batteries everyday rain or shine. This is a must or you will destroy your most costly part of the system, your batteries. Lead acid batteries must be kept at full charge at all times. Well as close to full charge as possible, lead acid batteries are very slow to fully charge --can take days-- so in a working solar system they will always be a bit under charged but this must be kept to a minimum. Charge controllers try to cut this charge time by employing different charging schema.

It is always better to have too many panels rather than too many batteries. Lead acid batteries go bad if they are left undercharged. And even if your charge controller is telling you the batteries are charged they most likely are not fully charged. Lead acid batteries are very sluggish to fully charge.

A rule of thumb for a cloudy day is your panels will only give you 10% of the power they give you on a sunny day. So if your off grid you must have enough panels so that 10% of normal sunny day power will give you the minimum power needed to recharge your batteries and power your needed electrical necessities like fridge and lights.

If your building your DIY solar system on a shoe string I will show you how to meet your power needs half way. I've been off grid for over a year now with only three deep cycle 60 AH flooded lead acid batteries from Autozone. Not top quality batteries. And I only discharge them to around 10% each night and we; three of us, live comfortable with just a bit of sacrifice.

For solar panels, I have 800 watts of rated power. Total cost of system was around 2 grand dollars, this includes everything. In another year I will have broken even compared to what I was spending on grid. Not bad if I say so myself.

Now lets move on to batteries in lesson two.

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