Connecting Off Grid 130Watt System Safely - NEED HELP

[aaceve7]

Hello. I'm working with high school students to design and build five 130 Watt, 12V solar panel systems. Each system consists of two 65 Watt panels (DIY kits built by students), a Morningstar Sunsaver MPPT 15amp Charge Controller, Sun Xtender PVX-1040T battery, and Xantrex 806-1210 1000Watt True Sine Wave inverter. The panels have been built and the components are on their way. The next step is to install each system at a different community garden to power shed lights, charge tool batteries, and to power small projectors/speaker systems for outdoor movie nights. The systems will be installed on shed roofs with all components stored inside the shed.

This is where I need help. The systems won't be inspected by the city, but I need to make sure each system is installed properly and safe as I don't want any of these garden sheds to burn down. Can someone help me answer the following and let me know if I'm missing anything?

-The two panels will be connected in parallel to increase the amps. Should this be done outside close to the panels themselves? Or should I run both sets of wire into the shed and splice them inside just before the charge controller? The runs to the shed will be anywhere from 5'-10' and the panel wires are stranded 12 gauge.

-What should I use to splice the wires together? I read that wingnuts are not a great idea as they may loosen over time and may pose a fire hazard. We are on tight budget, but I also don't want to risk anything here.

-Once the panels are connected to each other, what gauge and type of wire should I be using to connect the charge controller to the battery?

-Similarly, what size/type of wire should I be using to connect the battery to the inverter? The largest draw would most likely be PA speakers (need to check amperage, but pretty sure they're around 5 amps) that will be approx. 50' away and connected to the inverter via an extension chord. If this is not a good idea please let me know.

-Do these systems need fuses or breakers anywhere? If so, what should I be using and where in the system should these components go?

Thanks in advance!

[Sunking]

There is so much wrong and unsafe not sure where to start.

-The two panels will be connected in parallel to increase the amps. Should this be done outside close to the panels themselves? Or should I run both sets of wire into the shed and splice them inside just before the charge controller? The runs to the shed will be anywhere from 5'-10' and the panel wires are stranded 12 gauge.

You are using a MPPT controller and the last thing you want to do is wire panels in parallel. To do so just waste money, defeats the whole purpose o fusin gMPPT, inefficient, and makes it more unsafe. Wire the panels in series keeps the current down, less and smaller wire than can be ran longer distance. Wire the panels in series and you can use 14 AWG up to 40 feet. Wire the panels in parallel and you can use 12 AWG up to 10 feet.

MPPT Output Current = Panel Wattage / Nominal Battery Voltage

Figure it out you are the teacher.

-What should I use to splice the wires together? I read that wingnuts are not a great idea as they may loosen over time and may pose a fire hazard. We are on tight budget, but I also don't want to risk anything here.

Technically to code you should use MC4 connectors. You can use Wire Nuts made for Low Voltage Landscape Lighting that are filled with a silicon grease made by Ideal Industries you can find at a Orange or Blue Box Store. They are made for underground Landscape 12 volt lighting. Being low voltage and low power is not dangerous, but you cannot cal it safe nor code compliant. One thing you do not want to teach your students because it will make them ask embarrassing questions is Budget and RE cannot be used together in a story. You want RE, pay the big bucks and forget budgets.

-Once the panels are connected to each other, what gauge and type of wire should I be using to connect the charge controller to the battery?

It is a 15 amp controller which minimum wire size is 14 AWG and can be used up to 5 feet away without a lot of loss. 12 AWG up to 10 feet. Smart money is 14 AWG same as used between panels and controller. But there is a catch. The fuse to the controller is connected directly to the battery Term Post, not the controller output like you are thinking and you will need two of them on a Floating System. One on each battery polarity +/-. Fuses are to protect wiring from too much current from the source and the panels are not the source source, the battery is.

-Similarly, what size/type of wire should I be using to connect the battery to the inverter? The largest draw would most likely be PA speakers (need to check amperage, but pretty sure they're around 5 amps) that will be approx. 50' away and connected to the inverter via an extension chord. If this is not a good idea please let me know.

You are going to love this because it is so much fun and educational. This is where your whole idea falls apart and you are teaching kids how not to put a solar system together. Everything has to be built to work with each other. Lets works this backwards and see if you spot any issues.

A 12 volt 1000 watt Inverter requires a minimum 600 AH battery. Batteries have something called Internal resistance which cause the voltage of a battery to collapse as current increases. You Inverter will shut off when the input voltage to the Inverter drops to 10.5 volts. Will that happens to be around C/6 to C/8 generically on a battery where C = the AH capacity, and the digit = the Hour rate like 8 hours. A 1000 watt 12 volt Inverter pulls about 100 Amps at full power. Now for some Phd math.

Amp Hours = Amps x Hours, AH = A x H
Amp = Amp Hours / Hours. A = AH / H
Hour = Amp Hours / Amp. H = AH / A

So if you have a 100 amps load and the battery can only supply C/6 at best then you need 100 amps x 6 Hours = 600 AH. A 100 amp load on the wire from the battery minimum size required is 4 AWG copper about the size of of your finger and cost roughly $1ft. Also requires 2 x 100 amp fuses directly mounted onto the battery term post to protect the wire, and I will let you find out how much those cost. You have a Concord PVX 1040 12 volt an excellent very expensive 100 AH battery way to small for the Inverter and way to large for your panel wattage.

OK a battery must meet minimum charge requirements and has maximum charge current limits. Not going to dwell on max because you are so far meeting minimum requirements it is a moot point, but C6 to C/8 is about right for max. Minimum is roughly C/12 and perfect is C/10. So with a 12 volt 600 AH battery to drive a 1000 watt inverter is 600 AH / 12 H = 50 Amps and 60 amps is a perfect C/10. More Phd math.

Power = Volts x Amps.
Volts = Power / Amps
Amps = Power / Amps

One can assume if it takes at least 50 amps to charge a 600 AH battery might wonder how much wattage that is. Pretty easy we have a 12 volt battery x 50 amps = 600 watts. You can also assume if it takes 50 amps you need a 50 amp Charge Controller.

See anything wrong with your design? Told you it would be fun and educational.

-Do these systems need fuses or breakers anywhere? If so, what should I be using and where in the system should these components go?

Kind of already answered that but a picture is worth a thousand words. What is below does not show the solar panels connected to the CC.

[bcroe]

For openers, do I understand your students are building the solar panels from scratch? This is a good
learning experience, but it is quite difficult to build durable panels at home. Took me quite a few tries
before I had one I considered actually serviceable. After that I went to commercial panels.

Your whole project would be more practical using 12V LEDS and tool chargers, PA systems are readily
available for 12V operation. An AC inverter immediately adds a huge amount of inefficiency and extra
load to whatever is operated. You don't mention batteries, but keeping them alive takes a complete
system design with numbers for everything. Even then, those who have clouds and snow can easily
kill the batteries, not to mention periodic maintenance. good luck, Bruce Roe

[aaceve7]

Thanks for the feedback Sunking. So in summary my inverter is way too large for the system and my battery is too large for the panel. However, using your calcs this is what I'm getting:

-130W panel X 0.80 efficiency / 12V = 8.666 amps. Using minimum charge of C/12 that would give me a C=104AH, which would mean I can keep my battery size as is? I know it's at the minimum, but we will most likely be adding additional panels to these systems so I wanted to make sure we had a battery sized for additional capacity. We can keep the loads low for the time being to make sure we don't discharge the batteries to 50% every day.

-If we keep the battery at 100AH, and use C/6 as you mentioned then the inverter size should be: 100 AH/6 H * 12V = 200W. I was thinking of getting a 150W pure sine wave inverter, which would put me at C/8. Would this be sufficient? If so, any recommendations on a good quality inverter of that size?

A few other things I got from your post:

-Panels should be wired in series to keep amps low if I'm using MPPT. If I use a PWM, should I wire in parallel?

-MC4 connectors are the way to go

-The 12 AWG panel wire is sufficient to connect the panels to the CC with a 10' run

-14AWG wire should be used to connect the CC to the battery with fuses (15amp?) for each polarity connected at the battery terminals

-Fuses will be needed between inverter and battery and connected at the battery terminals Based on a 150W inverter would 15amp fuses be sufficient?

[aaceve7]

For openers, do I understand your students are building the solar panels from scratch? This is a good
learning experience, but it is quite difficult to build durable panels at home. Took me quite a few tries
before I had one I considered actually serviceable. After that I went to commercial panels.

Your whole project would be more practical using 12V LEDS and tool chargers, PA systems are readily
available for 12V operation. An AC inverter immediately adds a huge amount of inefficiency and extra
load to whatever is operated. You don't mention batteries, but keeping them alive takes a complete
system design with numbers for everything. Even then, those who have clouds and snow can easily
kill the batteries, not to mention periodic maintenance. good luck, Bruce Roe

Thanks Bruce. Yes the students built these from scratch. They are painted wood frames with plexiglass sealed on top. The idea was that they would learn how to solder, use power tools, and learn about solar energy at the same time. This isn't my class, I was just asked to come in and assist with the construction. I've built a panel like this in the past and it's still going strong 10 years later. I know it's not the most efficient, but it accomplishes what it was designed to do (run a 25W light bulb and charge drill batteries occasionally). If these systems were for my home I would certainly just buy commercial panels, but it was more about letting the students get some hands on experience building something, even if it's not the best option.

As for the DC, most of these systems are being installed in gardens that need AC to recharge their tool batteries so I need to have at least a small inverter. One of the gardens, however might end up just being DC as they really have no need for AC power.

[J.P.M.]

Thanks Bruce. Yes the students built these from scratch. They are painted wood frames with plexiglass sealed on top. The idea was that they would learn how to solder, use power tools, and learn about solar energy at the same time. This isn't my class, I was just asked to come in and assist with the construction. I've built a panel like this in the past and it's still going strong 10 years later. I know it's not the most efficient, but it accomplishes what it was designed to do (run a 25W light bulb and charge drill batteries occasionally). If these systems were for my home I would certainly just buy commercial panels, but it was more about letting the students get some hands on experience building something, even if it's not the best option.

As for the DC, most of these systems are being installed in gardens that need AC to recharge their tool batteries so I need to have at least a small inverter. One of the gardens, however might end up just being DC as they really have no need for AC power.

Not a knock but just a question: Has anyone taken a few minutes or made an attempt to explain, in a fundamental and simple way, how a PV cell converts sunlight to electricity ? It ain't rocket science.

Kind of like building a rocket without a very basic mention and explanation of gravity or how/why F = m*a is important to rocketry. I'd respectfully suggest it may be worth considering keeping the cart before the horse just a bit. Otherwise, seems to my ignorant way of seeing it, students would be as well or better off building window frames.

[Sunking]

Thanks for the feedback Sunking. So in summary my inverter is way too large for the system and my battery is too large for the panel. However, using your calcs this is what I'm getting:

-130W panel X 0.80 efficiency / 12V = 8.666 amps. Using minimum charge of C/12 that would give me a C=104AH, which would mean I can keep my battery size as is? I know it's at the minimum, but we will most likely be adding additional panels to these systems so I wanted to make sure we had a battery sized for additional capacity. We can keep the loads low for the time being to make sure we don't discharge the batteries to 50% every day.

-If we keep the battery at 100AH, and use C/6 as you mentioned then the inverter size should be: 100 AH/6 H * 12V = 200W. I was thinking of getting a 150W pure sine wave inverter, which would put me at C/8. Would this be sufficient? If so, any recommendations on a good quality inverter of that size?

Very good you catch on quick. I wished you had stated your goals in your original post.

Yes your Inverter is way to large and it is really easy for use to catch because panel and inverter wattage should be roughly equal. The biggest mistake we see here everyday id folks come here with say a 100 watt panel, some arbitrary controller they through darts at on a web site. a car battery, and huge honking 1000 to 5000 watt Inverter.

If you work the battery math and science out, you discover the secret of why panel and inverter wattage are roughly equal. The range generically is C/8 to C/12 on the battery charge and discharge current with C/10 being a perfect match. At C/10 the panel wattage is equal to inverter wattage. Give the numbers a spin for yourself.

A few other things I got from your post:

-Panels should be wired in series to keep amps low if I'm using MPPT. If I use a PWM, should I wire in parallel?

Correct and the reason again is the math and science.

PWM Output Current = Input Curent
MPPT Output Current = Panel Wattage / Battery Voltage

Example if you have a 100 watt panel the Vmp is roughly 18 volts and Imp = 5.55 amps. With PWM on the output would be 5.55 amps into a 12 volt battery gets you 12 volts x 5.55 amps = 66.6 watts. Now I will let you calculate what current and power a MPPT controller generates. Don't worry about efficiency derating for now on panel wattage as we can ignore it because it is equal in both models.

There is a limit to how many panels you can put in series and it is limited to the charge controller input Voc which can range from 40 to 600 volts. Think of a MPPT Controller as a AC transformer where you can Buck (divide or lower), or Boost (multiply or raise) the voltage and where voltage and current are inversely proportional. So in other words power is equal on both sides, we are just changing the ratio relationship of voltage and current. Example

100 watts = 1 volt x 100 amps. Would take one huge expensive wire.
100 watts = 100 volts x 1 amp. Small skinny cheap wire. Now think electric utility and why they use high voltages.

-MC4 connectors are the way to go

I would not put it that way. That just seems to be an industry default choice which meets code requirements. You wanted budget friendly so I gave a connector that is budget friendly, safe, works, and compliant. More then one way to skin a cat.

-The 12 AWG panel wire is sufficient to connect the panels to the CC with a 10' run

-14AWG wire should be used to connect the CC to the battery with fuses (15amp?) for each polarity connected at the battery terminals

-Fuses will be needed between inverter and battery and connected at the battery terminals Based on a 150W inverter would 15amp fuses be sufficient?

I am going to lump all these questions together because they are related. Wire size from a minimum safety point of view is determine by the amount of current imposed on it, and the temperature rating of the wire insulation. A little more math.

Power = Current x Current x Resistance.
Voltage = Current x Resistance
Power = Voltage x Current

Power is heat, and more power is more heat. Wire has resistance and is directly related to the cross-sectional area of the diameter of a conductor aka wire. Smaller the wire, the more resistance it has. So if you do the math only stands to reason the more current you push, the power and heat it generates and also is a LOSS of both Power and Voltage. In fact if you double the current, you double the voltage loss, and generates 4 times the thermal heat.

Low voltage requires an extra step to make sure we are not loosing too much power as waste heat in the wiring, so minimum safety requirements may or may not work because as the conductor is lengthened we are adding more resistance which results in higher voltage and power losses. OK minimum safety requirements are determined by the Fuse/Breaker size, or in the event there is no breakers or fuses we have to know the maximum fault current the system can impose of a wire. Fuses are installed to protect the wiring and nothing else They are not used nor are they capable of preventing you from being electrocuted.

OK lets say we either have a circuit source whose maximum fault current is 15 amps or we have a 15 amp fuse. Code requires a minimum 14 AWG conductor, and if it were 20 amps is 12 AWG. So 14 AWG is going to work between the panels and controller up to 20 feet. (I used 36 volts and 9 amps to calculate) On the output using 12 volts and 10 amps, use 14 AWG up to 5 feet, or 12 AWG up to 10 feet.

Use fuses between the Controller and Battery, and between the Battery and Inverter. Install them on the battery Term Post to protect both wire directions from the battery. Solar panels are limited current sources and in your case less than 15 amps so the wiring between the panels and controller do not require fuses, but you can use them if you want.

I get paid $1 for every word typed.

[aaceve7]

Not a knock but just a question: Has anyone taken a few minutes or made an attempt to explain, in a fundamental and simple way, how a PV cell converts sunlight to electricity ? It ain't rocket science.

Kind of like building a rocket without a very basic mention and explanation of gravity or how/why F = m*a is important to rocketry. I'd respectfully suggest it may be worth considering keeping the cart before the horse just a bit. Otherwise, seems to my ignorant way of seeing it, students would be as well or better off building window frames.

These students are juniors and seniors in an honors physics class. The physics teacher had a long course block focused on solar energy and circuits before I came into the classroom. The goal was to take what they learned over the past month or so and apply it to a "real world" project. I'm friends with the physics teacher so offered my assistance in building these simple panels. We have to work within the confines of a grant the school received so we tried to balance everything to make sure the students had a good experience, but also make sure the gardens that are receiving these systems are getting something useful. Still, these are essentially window frames

[aaceve7]

Got it. Thanks Sunking. I believe you answered all of my questions so now I just need get a different inverter, purchase the MC4 connectors, fuses, and some more wire.

[Sunking]

You are welcome.

[Heybob]

Hi Sunking,
just bought a solar kit I am happy with. One question is they do not mention a fuse between the controller and the battery which I would like to use in my setup. Can you please advise me on fuse size and distance from battery. I have a 12v Epever pwm 20amp controller, Max PV voltage 50V, going to a 130ah AGM battery. Hope you can help. Thank you Heybob.

[J.P.M.]

These students are juniors and seniors in an honors physics class. The physics teacher had a long course block focused on solar energy and circuits before I came into the classroom. The goal was to take what they learned over the past month or so and apply it to a "real world" project. I'm friends with the physics teacher so offered my assistance in building these simple panels. We have to work within the confines of a grant the school received so we tried to balance everything to make sure the students had a good experience, but also make sure the gardens that are receiving these systems are getting something useful. Still, these are essentially window frames

Understood. Thank you.

One more query: So the honors Physics students leaned of things like the photoelectric effect, how photons can lead to formation of electron-hole pairs in silicon, and why we have Einstein to thank for our PV panels along with some principles of electricity ?

Regards,

J.P.M.

[Sunking]

Hi Sunking,
just bought a solar kit I am happy with. One question is they do not mention a fuse between the controller and the battery which I would like to use in my setup. Can you please advise me on fuse size and distance from battery. I have a 12v Epever pwm 20amp controller, Max PV voltage 50V, going to a 130ah AGM battery. Hope you can help. Thank you Heybob.

Scroll back up to post #2. The fuse between the battery and controller is installed directly to the battery Term Post. You have a 20 amp controller requires at minimum a 20 amp fuse with a minimum 12 AWG copper wire and is good up to about 5-feet 1-way distance.

You will need another fuse between the battery and load device like an Inverter also installed directly on the battery term post. Size depends on the load connected which you have not specified.

[Heybob]

Hi Sunking,
thanks for answering my post so quickly. The only accessory that I will be using about 3 days a Month is a Waeco CFX75 fridge, to keep drinks and food cool for that period. With the kit that I bought was a power station. The power station :- Projecta, with a 20amp fused accessory socket and also a fused 10amp cigarette socket. I do not intend to use an inverter. What I want to do is use the battery posts + & - to connect the controller to the 130ah battery. I was thinking of a distance for the fuse @ 150mm or 6" from the battery. I want to use a copper wire which will just fit the controller. Thank you for your input. Heybob.