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Noob- lots of questions

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Hi. I'm new to the solar world.

I'm building a basic system just to get started

- 4x 100watt panels

- A cheap pwm Chinese charge controller

- A cheap Chinese 3000watt inverter

- I'm buying batteries next month...I'll probably start with two 105 ah gel batteries.

Lots of questions. About which gauge wire to use and where to place fuses to protect the inverter and charge controller.

Would anybody be able to give me a hand?



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Hi @Donwil

There is a lot of info on this forum and on the blogs. But I can send some basic info your way here:

  • A PWM charge controller is designed for basic use and will not optimise your 400W of Panels, so only use this if you intend on drawing much smaller amounts from the batteries (eg: I use a PWM controller for my Camper Van, where the only draw is usually a fridge and a few LED lights).
  • Secondly, most chinese controllers are very poor at converting your available 400W to something decent and many cant cope at the rated capacity. Rough calc 400W/12V = 33.33A so nearly already bigger than a standard off the shelf 30A controller can handle. As chinese controllers go, look at the ECCO brand with a LCD interface, but still only expect to get about 60-70 percent of your PV back in real terms. In full sunlight my 300W of PV through a PWM controller is closer to about 12A of charging current for your batteries.
  • FORGET about chinese invertors! They Lie, Lie, Lie about the ratings, and they can get away with it for the most part. Just think about the price and it should give you a hint. Many of the chinese invertors out there that are rated even at 1500W can barely push out 500W before letting the smoke out. Just try find a decent invertor, possibly on this forum or web store, that at least has been proven (the small axperts, although chinese, are at least proven as basically capable by a few members here)
  • Next is your load - You haven't specified what you are thinking of running off this small power plant? Is it your whole house, a few PC's?
  • 2 x 12V 100Ah batteries will give you a maximum (if you don't plan on keeping the batteries too long and working at 50% which is reasonable assumption for Gel batteries) of 100Ah usable power (extremely rough calculations - I don't want to be shot down by the experts with the mathematical calculations and discussions about specific batteries). If you drain them at a decent rate then roughly 4 hours at 25A (300W - 3 incandessant light bulb or 1 or 2 basic Desktop PCs). Drain them slower at 10A, then about 10 hours (1 incandessant globe)

I have a blog on this forum called "KLEVA's Lessons Learned", and my lessons have been expensive for someone with limited funds.

How to start? Well you already have some panels... For a 12V setup you will need to connect these in parallel to a decent PWM (or better if you can) Controller. This will be able to charge a decent 2 battery setup (also in parallel for 12V) to start with. But you are definately overestimating the amount of Load power you can feasibly get out of that arrangement, if you try draw 3kW (or kVA) out of 100Ah of available batteries you will only get about 20 minutes of power (and you are going to horribly stress your batteries)

Now that I have scared you, I apologise. You have some decent starting ideas, but you need to think a bit more carefully about your load and batteries to store the power. Your 4 solar panels have the ability to generate roughly 3kWh on a perfect day (your batteries will only keep roughly 1,2kWh and a decent PWM controllermight only just be able to supply that), so you need a better way of trying to collect that energy, store it somewhere, and to use it carefully.

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There's a perception that 'solar' is the answer to all electricity supply issues and that it replaces grid power painlessly.

This is not the case, hence the variations: e.g. backup power, off grid and grid tie systems.
Independence from intermittent grid supply is one thing but installing an off grid system is another thing altogether.
So when someone posts a thread like this one you need to understand where they're coming from...

I am not trying to be difficult or obstructive but this is a complex subject.

If you live in Australia all you do is install a grid tie system and check out for your electricity bill to see what your credits amount to. Here in SA we have more complex issues trying to mitigate the problems associated with a flaky grid.. 

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Thanks Kleva

That is an excellent response and the advice is greatly appreciated.

My thought process is build a cheap small system and gain some understanding before I spend a substantial amount of money on a 48 watt system. My knowledge and electrical skills are limited, but I'm leaning.

I plan to power one 300watt TV and one 3watt LED lamp off the system. I know to calculate amps you divide watts by volts. How would I calculate running time on two 110ah batteries then? What exactly is an amp hour?

I'm following this video as a rough guide

So far I've spent the following

3000w Chinese inverter; R1060 

50 amp Easton Chinese pwm controller; R1000 

4x 100 watt panels; R3600

Batteries yet to be purchased. I'm looking at buying two gel batteries. Does it really matter if they are gel or lead acid on a small system like this?

I'm aware that you can only discharge your batteries to 50%. On a pwm controller there is setting that disconnects the load at a specific voltage, is this a reliable manner to manage the battery?

I'm really having fun learning about this stuff, a new hobby. Are there any magazines or international forums that are any good at looking at?

Thanks Again


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Hi @Donwil

Sorry about delayed response - Was away for the weekend and my current workload is nuts... I will try answer some stuff quickly before returning to work...

  • Blog can be found on this forum at https://powerforum.co.za/blogs/blog/7-klevas-lessons-learned/
  • An Amp/Hour or Ah sounds exactly what it is - It's how many Amps per Hour you need or need to use. Batteries are usually given in Ah to give a basic idea of how much power they have available (in absolute total) before they are "shot". Practically you should work on half of that, especially since any decent controller will not allow them to get that far depleted.
  • But there is also something that is rarely found (unless you hunt the specs) and that is a "C" number (a symbol for current) which indicates how the Ah value was calculated. The C value basically indicates how they carried out the test to prove that it is a certain Ah battery. For example: 200Ah at C5 means that if you draw 5A maximum then the battery promises to deliver 40 hours total (although that max would kill the battery - roughly work on half of that or less to not destroy your batteries), lets say practically 20 hours. If you draw 10A from the same battery then there are no promises, but I generally would expect it to be exponentially worse, say closer to 5 hours (4 times worse). The C value in most cases also applies to how fast the battery can be charged without damage. Charging a C5 at 20A would not be good for the battery and most likely will damage it (and even risk dangerous stuff like explosions and meltdowns). Again I have given the worst first! Most decent batteries are C20 or C10.
  • Gel Batteries are almost always better than standard Lead Acid for longevity. It really depends on how they are used. Gel Batteries are in a way also Lead Acid, the difference is that the liquid inside is a lot thicker (think grease vs water). Some of the Pro's of Gel are that they don't build up sulphates at the bottom of the battery as easily, they don't lose "liquid"/acid as quickly when charged heavily/quickly, they generally last longer, they need less maintenance than a good quality Lead Acid. That said, a well maintained Trojan Lead Acid battery is the best cost effective product - but you will have to have a proper maintenance schedule to keep the pH right and the batteries topped up properly.
  • Using your example: (Although you must have a very old CRT TV or a HUGE flatscreen if it uses 300W - he he)
    • 300W + 3W = 303W, which equates to a roughly 340VA (slight increase to take Power Factor into account with the products you intend to use, but don't worry about Power Factor yet - That's for later lessons - Power factor only applies when converting to AC)
    • 350VA (rounded up) gives approx 1.6A AC current, but more importantly, is over 25A from a single 12V DC source
    • You are looking at 2 batteries, so 30A (rounded up) is 15A draw each - So if you get C20 batteries rated at 100Ah, you will safely get 100Ah available, which is 100Ah/30A = 3.33 hours usable
  • The batteries in this scenario are holding you back. While it seems like you should be easily able to power what you have, it's the storage of the power that often confuses. You have 400W in panels, so during the day there probably wont be a problem, but your batteries are only storing about 2 hours of that power maximum. For simplicity I will refer to everything in Watts here -> Think 400W of Panels x 2 hours of full sunshine = 800W, and you will be draining it at about 350W, therefore my max 3.3 hours estimate. You just don't have enough space to store all that sunshine for later use.
  • To answer one of your last questions - a generic PWM controller is usually settable at what Voltage it cuts off (10.5VDC should be absolute minimum, but by default they are normally set at 11.5VDC), which means that they are generically fairly decent about that part. Especially since battery voltage under load is usually lower than what it actually is. So the cutoff on a even a generic PWM will be safe for your batteries, but might annoy you due to the limited power/time you expect.
  • You overpower your panels and controller to take into account the bad days for charging the batteries, but the batteries ultimately are what is supplying your load. You have to get that calc right.

I have given the worst case values here, so that you understand the problem and limitations. As an example again: I have 300W of panels on my Camper Van normally, with 4 x 100Ah batteries, which only gives a feasible draw of 200W over the whole night. If I have to run the Freezer overnight then that is only 3 hours runtime (70W).

tldr; You have to store all the power you generate - 400W of panels is 3.2kW of power per day (at max), where do you store it? Your storage is what is supplying your power to your load, not the panels and controller.

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