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Newbie needs advise on stand alone system please


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Hey all! My 1st post so please forgive the newbieness.....

I have an out building I need to power. really just lights and a water booster pump. 0.37kw pump running prob 15-20 min a day only. Lights estimated 250 (all LED, some tube, some spotlights) watts at any one time. all for about 1 hour a day, and 30 watts all night. Kay thats all pretty easy, but then I got offered a really good deal on an older (but new) cyberpower inverter (CPS2200EI simulated sine wave, 1230 watt unit on a 24 volt system.

Its designed for a grid tie in system. I want to hook it up to a solar panel with a PWM controller and 1 or 2 panels 250 odd watt in total. i'll take off the AC input into the inverter and hook up the controller direct to the 12 volt batteries in series. 

Am I being an idiot? should i rather get the right type of inverter? Is it gonna damage the water booster pump? is the inverter efficient enough?

Comments would be massively appreciated!

Regards,

Peter

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16 hours ago, Bonova said:

Is it gonna damage the water booster pump?

It's mostly LED loads, plus that pump that runs half an hour or so. The LED lamps usually have switch mode power supplies and won't care about the MSW inverter. The motor similarly would probably survive just fine, though it will run a little hotter than it does under a pure sine-wave unit.

Just keep in mind that this older kind of inverter isn't particularly efficient (mid 80% on conversion, then some more on the poor THD), and with a PWM controller you're throwing away another 33%. So definitely go with a larger PV array, but stay within battery charge limits.

Consider running the LEDs directly from 12V, using a buck converter (much more efficient and cheaper than an inverter)?

How good a deal is good though?

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Thank you for your reply! Good deal is basically free. Paying same as I would for just the 2 batteries so the inverter is free. The reason I didn’t go 12v on the lights is that the distances are quite big- 50 m long building so the higher amp cable comes into the equation. And I guess I need the 220v for the pump anyway

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  • 2 weeks later...

OK so my inverter and batteries are working well! No lights up yet but it runs the booster water pump perfectly. I'm keeping the batteries charged every few days with Eskom and a long extension cord haha.

Now I need to buy a solar panel and charge controller and I'm not certain of the specs. My batteries are set up on series so its a 24volt system. Do I need a 24-35 V dc module voltage panel? Or a 12-23V dc one? Does it need to be above or equal to 24 V to be able to charge my batteries in series?

There seem to be a lot fewer 24-35V panels on the market.

OR - do I get 2 smaller panels and connect them in series? Is that even possible? I'm realizing how little I know now :(

 

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58 minutes ago, Bonova said:

There seem to be a lot fewer 24-35V panels on the market.

It depends on the number of cells in the panel. The common options are 36 cell (called a 12V panel by laypeople, though it makes between 18V and 22V), a 72 cell module (called a 24V panel, though it makes between 36V and 45V), and more recently there's also 60-cell modules.

You need a voltage that's about 5 volt higher than the absorption voltage of your batteries, so starting around 34V. You can also put modules in series, eg, you can put two 36-cell modules in series to make a 72-cell string.

Usually, modules below 150W are 36-cell modules, modules above that are 72-cell or 60-cell modules. Modules above 300W are almost always 72-cell. You can count the blocks in the picture if you are unsure.

Now... if you're using a PWM charge controller, aim for a lower voltage. One 72-cell module, or two 36-cell modules in series ought to do.

If you're using an MPPT charge controller instead, aim for a voltage about twice your battery absorption voltage, or around 60V.

Consult the specs of the charge controller. MPPT controllers have a maximum input voltage. The open circuit voltage of your total string must be less than this specified voltage, or smoke will come out.

Some cheap MPPTs are limited to 50V. Decent ones usually allow up to 150V.

Should you get MPPT or PWM? Rule of thumb, for anything under 300W... go PWM, and remember that you'll only get two thirds of the power out of the PV modules. Over 300W, go MPPT. It's usually worth the extra cost vs more PV modules.

Personally I can always recommend the Victron BlueSolar series of MPPTs. They are not significantly more expensive than other brands, even locally made ones, and even the smaller units have a Maximum voltage of 100V.

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Thank you kindly for that reply plonkster! Very informative!

I will go with your advise.

Last things for now - What size cabling would you suggest from the panel to the batteries via the controller? Obviously the larger the more efficient. I need about 8 m in length. The max out put is around 8amps so will a 15amp wire be adequate? Does it need a fuse? I read a bunch about fuses etc but seems more related to grid tied in units? I have a DB board after the inverter but I want to ensure no burny wires between the panel and the PMW controller ;)

If one puts 2 panels in series (or parallel) for that matter - do they have to be the same wattage?

Thanks again!!!!

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Hi @Bonova,

Quick answer is that solar type cables come in two sizes. 4 mm2 and 6 mm2. The general rule is that the longer the distance between panel and inverter, as well as the higher the current pushed over said cables, the more wires you need to mitigate the voltage drop.

I would think that in your case 4 mm2 cable is just fine. I used 6mm2 on 4x900w panels to the combiner box, +- 15 Amps per string. If budget is very tight, 4 mm2 is sufficient.

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37 minutes ago, Sidewinder said:

I would think that in your case 4 mm2 cable is just fine

Agreed. In your case 4mm^2 will be fine. That cable is good for 30 amps in open air (though I won't do it due to the serious voltage drop you'd experience :-) ).

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9 hours ago, Bonova said:

If one puts 2 panels in series (or parallel) for that matter - do they have to be the same wattage?

Yes they should, not only same wattage but preferably same spec exactly. In parallel you would force them to operate at the same voltage, in series you would force them to operate at the same current - even though this may not be ideal for the the panel in question.

If you really feel that you need to do it then make sure that they have the same operating voltage and then put them in parallel (which would be easier to match in dissimilar panels).

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  • 1 month later...
On 1/4/2018 at 12:10 PM, Sidewinder said:

Hi @Bonova,

Quick answer is that solar type cables come in two sizes. 4 mm2 and 6 mm2. The general rule is that the longer the distance between panel and inverter, as well as the higher the current pushed over said cables, the more wires you need to mitigate the voltage drop.

I would think that in your case 4 mm2 cable is just fine. I used 6mm2 on 4x900w panels to the combiner box, +- 15 Amps per string. If budget is very tight, 4 mm2 is sufficient.

You get 10mm2 as well. 

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On 1/4/2018 at 9:33 PM, pilotfish said:

Yes they should, not only same wattage but preferably same spec exactly. In parallel you would force them to operate at the same voltage, in series you would force them to operate at the same current - even though this may not be ideal for the the panel in question.

If you really feel that you need to do it then make sure that they have the same operating voltage and then put them in parallel (which would be easier to match in dissimilar panels).

I have run 235W and 255W panels together, purely cause "that's all there was" for a long time. In parallel, it will default to the lower amperage. In serial, it won't matter much, but the total voltage will be the sum of the panel's voltages. It is not ideal but will work if that is what you have to work with

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