Getting my head around Solar

[Askari]

Hi 

Trying to learn and understand the basics with a view to self-installing a completely off-grid system. The plan is to get rid off Eskom completely. Just received a R97 000 bill , cant be correct but Im tired of dealing with these idiots. And the power has been on and off all week so really want to show them the middle finger and be self sufficient.

Have done my initial sums and figure a 2,5 to 3kw inverter should suffice for now. I can move this to another building if i upgrade later so not too worried about future proofing. Would rather ensure I can add panels and batteries.

I have a question regarding 2 Axpert inverters. The first is the Axpert KS 3K-24 and the second the Axpert MKS 3KP-24. On the second one I see in the specs the MPPT Range @ Operating Voltage is 30 VDC~80 VDC and if I understand correctly it will not work unless the panels are supplying in that range? If so I would need to connect panels to ensure at least 30VDC. Now the first example I linked to there is no mention of voltage range. Is this because it does not have a MPPT charge controller? Would I have to buy a separate charge controller if I went with the KS but not if I went with the MKS+?

The KS3K 24V is a good price on Takealot at R5999 

One last question for now: Was looking at the SAMIL Solar Solar River range ans I see the voltage range is 165-500V. That is a bit difference to the Axperts and I would need a lot more panels in series to reach the minimum. 

 

Thanks in advance.

Mike

 

 

 

 

[pilotfish]

Hi Askari,

I think the guys on the forum will need a hellova lot more info regarding your current consumption and habits, and how much you are prepared to sacrifice, before being able to assist in any way - your last 12 months meter readings would be a good place to start.

As a conversation starter... the inverters that you are looking at are unlikely to be suitable for off-grid applications unless your current (or future) power consumption is extremely low - they are limited to a PV array size of (KS 3K = 1.2kW) (MKS 3KP = 1.0kW) which will provide you about 5 to 6kWh /day, on a very good day if all is well installed with panels well aligned and zero shading problems - this is about how much an average pool pump would consume on a daily basis, so do you have a pool? 

[pilotfish]

... the Samil inverters in your link are for Grid Tie applications (where you use Eskom as your battery), not Off Grid applications - so not useful if the plan is to get rid of Eskom completely. 

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14 hours ago, Askari said:

The KS3K 24V is a good price on Takealot at R5999 

The guys around here know me. I am not a particularly big fan of the Voltronic stuff. Giving credit where it's due, yes these inverters come in at an incredible price point and represent really good value for money, that is: you get a lot of inverter for really little money. But that is precisely what's been bothering me for years. My advice would be to use the low price to your advantage and buy the bigger one instead. Get the 4kva 48V model. You'll thank me later. Conversely, if you want to start small, again use this to your advantage (smaller inverters cost less) and buy a better inverter, for example a small 24V Victron Multiplus (The usual disclaimer applies: Vested interests at play). Either way, I'm saying look into spending at least three times more than this... if you can.

The main reason I always push the smaller Multis is this: It's a hybrid unit. It can work with the grid. It has 200% surge capability and a big heavy transformer (not low-frequency big, but bigger than the cheapies). It has a 5 year warranty. It supports Lithium Ion batteries out of the box (which you will get into in the next ten years if you stick with this, almost guaranteed). Support is exemplary. The protocols are open and the software is free. They integrate well with other equipment. Okay I'll stop singing the praises there :-)

14 hours ago, Askari said:

Was looking at the SAMIL Solar Solar River range ans I see the voltage range is 165-500V.

That's a grid-tied inverter. Interesting input voltage range. The fact that it starts below 350V means it has a boost stage, so it is in the same category as the Fronius Galvo. I don't know these inverters, price seems very attractive, but again, it's about a third of other decent GTIs like the mentioned Fronius or even a Goodwe.

If you want to go grid-tied, look into SMA's SunnyBoy range. It's a German brand, complies with our local regulations, and you can have a 1.5kw for about the same price (around 9k).

A bit of side info about input voltages. Many people don't know that your mains voltage isn't actually 230VAC, the peak is closer to 325VAC. The lower value is an average, an RMS value. It's the equivalent DC voltage that would provide the same motive force. In order to make a 230VRMS output from DC, you need to start with something above 325VDC. Most off-grid inverters (that use batteries) start with something below 100V, 24V or 48V. These inverters (the modern ones) first have to boost the voltage to something above 325VDC before it can chop it up into AC. This is called the "boost stage" of the inverter.

Some inverters don't have a boost stage, usually the grid-tied units (without batteries). You can spot them by the fact that their input voltages are above 350V. These inverters are more efficient than those with a boost stage, but they need more panels in series to get the voltage high enough.

Final note about PV voltage and your concerns that it has to be above 30V. In practice, you will have trouble with voltages that are too high rather than the other way round. Even a 36-cell module (a so-called 12V panel) makes a good 18V-22V, and 2 in series will be enough to get above 30V. As I said, in practice you're more likely to run into the opposite problem: two 300W panels (72-cell modules) will push a good 90V in series, and that will be too much for your inverter. Then you'll have to put them all in parallel and that will mean thicker cabling etc...

14 hours ago, Askari said:

first example I linked to there is no mention of voltage range

That's because it's a PWM controller. You want to match the panel voltage to the battery voltage because there is no power conversion (where volts are swapped for amps) going on. If you have a 24V battery, you'll use either two 36-cell modules in series or one 60-cell or 72-cell module on its own. When the charger turns on it pulls the panel voltage down to the battery voltage with a resultant loss of power. Eg, a 60-cell module will do its best work at 30V, but you will pull it down to between 24V and 28V and lose between 7% and 20% of the available power because of that. I think they don't publish the voltage range because PWM controllers are usually paired with PV voltages no more than 50% higher than the battery voltage.

[Askari]

Very helpful info there plonkster, cleared up alot of  other questions for me. I do not want nor need the system to be grid tied and so many of these inverters are designed to accommodate that function. So back to hunting for a suitable Inverter .

 

 

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52 minutes ago, Askari said:

I do not want nor need the system to be grid tied

If you need a pure off-grid inverter, I'd love to recommend a Victron Phoenix, but the Axpert is not a bad inverter. It's sort-of middle of the bunch. You can do much worse than an Axpert, that is for sure, but you can also do a lot better :-)

I would still advise looking into Hybrid inverters. That's inverters that can either run off-grid, or they can tie with the grid and mix grid power with solar/battery power. The best known ones are the Voltronic Infinisolar and the Victron Multiplus.

This is what I've done: I have a 1600VA inverter that is sufficient to power all my base loads and take care of essentials during an outage. It can't run the microwave, which is the only appliance I miss, but a gas stove is cheaper than a larger inverter. That small inverter is enough to offset 50% of my consumption. Presently I'm doing closer to 20%... but that's because I lack generation and storage capabilities. The few times a day that I need power... I simply take it from the grid. It's a useful feature to have.

If you plan on disconnecting from the grid completely -- that is ask the council/supplier to remove your connection -- well then you're in for a steep learning curve :-)

[Askari]

Im on a farm and have the whole standing charge issue so they will simply disconnect the links feeding my transformer. If I didnt have these standing charges I could stay on grid and use it for when I need extra power or when its cloudy for long periods. I wish I could just pay for consumption...... 

In general are purely off grid inverters cheaper than hybrids?

[Chris Hobson]

16 minutes ago, Askari said:

Im on a farm and have the whole standing charge issue so they will simply disconnect the links feeding my transformer. If I didnt have these standing charges I could stay on grid and use it for when I need extra power or when its cloudy for long periods. I wish I could just pay for consumption...... 

In general are purely off grid inverters cheaper than hybrids?

Have you considered prepaid? You get 16kVA single phase. No standing charges R4 per kWh.

 

[Askari]

3 minutes ago, Chris Hobson said:

Have you considered prepaid? You get 16kVA single phase. No standing charges R4 per kWh.

 

I havent but definitely something to consider I think. Its just this latest erroneous Eskom bill of R97 000 that has got me in a foul mood and Im expecting a big boxing match with them so I would rather cut ties completely. 

[Guest]

FWIW, a few random thoughts:
- Matter of principal and / or anger are two emotions that should not be used to justify drastic changes. Take a breath, step back and become a cunning consumer.
- "Wants" vs "needs" have huge price differences. Determine your "needs" first and reduce it to the lowest you can go.
- Batteries are more expensive than Eskom - you need to fully understand the costs and implications over 5-10 years.
- Married? Prepared for very interesting chats. 

Steps I would suggest one takes:
1) Reduce your base load, the one that is on 24/7/365, to as low as you can for not only is this the largest part of your electricity bill, it also costs a wee bit to put all in place / replace older equipment. Not the stoves and kettles and microwaves, more like geyser and fridge/freezers / TV's / Computers etc.
2) Invite SWAMBO (She Who Always Must Be Obeyed) and her little ones (if you have) to further assist to reduce the average consumption - if possible = Lifestyle Changes and a lot of shouting.
3) Now get prepaid as suggested - or start with this.
4) After all is said and done, only then make the call for going onto solar or not.

Solar is not cheap, it is a lifestyle change ESPECIALLY if you have been on Eskom since the beginning.

[Chris Hobson]

Just a word of warning the KS Axpert models are PWM so are going to disappoint you about 30% less solar production than the MKS (MPPT) models.

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3 hours ago, Askari said:

In general are purely off grid inverters cheaper than hybrids?

Yes. Off-grid is generally cheaper than hybrid. Not all off-grid inverters come with an internal charger (the Axpert does though). It depends on what sources you will use to charge. Back in the day, we had a Hoffberg generator/alternator combo driven by a Hätz Diesel engine, and then we had a solar charger. If we got behind, we had to start the engine, but then we charged the batteries directly using the generator half of the Hoffberg. If you don't have such a device, you'll need a way to charge when there is no sun. Usually you use a internal combustion engine driving some kind of generation device, and if this doesn't include a DC charging side, you'll need an AC charger (or it has to be included in the inverter).

2 hours ago, The Terrible Triplett said:

- Batteries are more expensive than Eskom - you need to fully understand the costs and implications over 5-10 years.

On farms this is sometimes not true. With the standing cost included, even without, you could pay as much as R4/kwh, as @Chris Hobson also confirmed. It is possible to do solar at less than R4/kwh, but then you have to go all out. Small systems don't beat Eskom. Especially if you buy the wrong batteries...

Bu first, you'll have to work out how much you use, and for what. If you use electricity for pumping water (for example), it's an entirely different ball game compared to just some lights and TV on the weekend... :-)

[Chris Hobson]

I have decided that in a off-grid situation you decide on your batteries first (probably your biggest cost) and then select a compatible inverter and round off with some good panels.

[DeepBass9]

And completely understand what you energy needs are before you start. Also factor in the cost of a decent generator for multiple cloudy days and high load applications. With a generator for occasional large loads, you can get away with a smaller system in terms of both panels and batteries. You don't have to have a battery bank that will last a few days with no sun, rather run the generator to top up the batteries as required.

[Guest]

As we keep on hammering, once you know the exact load required, have decided the batteries type and AH you require, have already or have chosen a generator (make sure the generator can work with the Axpert), all that is left is to confirm your Eskom connection fee per month (just the fee to be connected) to the farm.

This fee can be quite a "bonus" saving towards solar.

Am pretty sure that if you have reduced your load, know the limitations and benefits of solar for your particular needs, have adjusted accordingly (like peak sun hours etc) your usage and you have a generator and have confirmed how to get the maximum value out of each liter of diesel used like charging whilst powering select loads to run gennie at peak optimal performance, (I assume diesel on a farm), then solar would be quite attractive on a farm, even with batteries and replacements.

There are quite a few farmers here whom have had huge successes and can advice you in detail what NOT to do.  

City folk on the other hand, it is a wee bit more complicated.

[Askari]

Much food for thought. Thanks for all the input.

[maxomill]

damn I was in Underberg the whole of last week

 I could have helped you a bit

sorry

 im guessing you have 3 phase power there .

a 100 grand is a nice figure to start with

[Askari]

Hi @maxomill coming through again anytime soon? 

 

Right, getting to the point where I am going to likely have to get a system going. 

Looking as follows:
Axpert MKS Plus 3KVA  24V (about R7k)
Probably 2 x 300w panels to start with, will try for 4 (about R4k for 2 or R8k for 4)
Two 100-105aH batteries at about R2300 each.

Now batteries. Seeing as though its a 24v system will need to start with 2 x 12v in series. I have been reading threads and have seen something about single strings better than 2 or more? I understand that connecting in series results in the sum of the voltage but the same aH. Connecting in parallel same voltage but sum of aH.

So if i have two strings of two 100aH batteries I would have 24v and 200aH? Is that how it should be done?

Peak daytime  requirement if I manage properly will be just over 2kw, will fire up the gennie for welding etc.  Night time usage will be max 200w/hour x 8 so about 1,6kwh, call it 2kwh. Recommended battery capacity for this? 
 

[maxomill]

sorry once in a lifetime

 

Big big question . do you have a electric or solar geyser . you need to have a hot bath / shower to maintain sanity

gti system not going to work for you with all the power outages you have

rather go for 4  x 300w panels ( I have a 1200 w array which is not perfect and I only get 1000w at best , sometimes in certain parts of summer)

for only a little more  , rather go 48v system ( you were going for 4 batts anyway)

http://fullcirclesolar.co.za/wp-content/uploads/2017/09/5KVAunit.jpg

200 w = 8a x 25 v (24v system ) 50% discharge every night will eat your batts

200w = 4 a x 50v (48v system ) 25% discharge is much better for the batts

going solar is a lifestyle change  so unless you have a solar/Gas geyser and a gas stove   be warned

 I like @Chris Hobson  theory of pre paid  but yes still get the system for back up

cheers

 

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19 hours ago, Askari said:

So if i have two strings of two 100aH batteries I would have 24v and 200aH? Is that how it should be done?

Correct. But you'd do better going with a single string of 200ah batteries instead. Us old timers usually recommend Trojan T105RE 6V batteries, they are 225ah each and have a very decent cycle life. Or as @maxomill said, if you're going to buy 4 batteries anyway, go 48V. There is not a big difference in the cost of inverter (though I may have to qualify this a bit, I'll do that below). Also, if you do go 48V, you can still get away with just two panels, provided they are 72-cell modules (count the little squares). Two such panels will make a good 72V at their power peak and will be sufficient to charge a 48V bank.

Re cost of inverter. I don't think the argument is going to work with the Axpert, because I don't think they make a 48V 3kva model. With Victron kit, you can get a 3kva model in either 48V or 24V, and the cost is very similar. A 24V 3kva costs exactly the same as a 48V 3kva unit. Also, they tend to cut the price more aggressively on their 48V units because they are so popular, so you can often get a better deal that way. You're probably not in the market for this one though... but I had to qualify what I said :-)

[Askari]

Ok so today I got offered 3 brand new solar panels from a friend.  Providence! I tell you. We will do a barter deal- sheep for power. I do love a good barter deal....

 

Anyways so he sent me a photo of the label on the panel which I have attached. Now I am confused again. Please could you awesome people tell me if:

1) Can I put 3 of these in series?

2) Can I used these with the 24v Inverter I was planning on getting (http://www.bonanzatech.co.za/index.php?id_product=7845&controller=product)

Looking at the specs of the panels Im getting the feeling these are for a 48v system? 

 

 

[PaulF007]

Ok I am going to make an attempt this on but @Chris Hobson please would you verify  

You will see that your Voc = 48.84v so if you were to put them in series you will get 48.84 x 3 =  146.52 v . On the Axpert they are rated on 145 Voc max so I for one would not be comfortable with that , one reason is that you get from time to a phenomenon where by the sun's rays are "magnified" on a partly cloudy day. I once had about 10% increase on my output so that could burn the inverter if you are running it on the dge . Then you also need to have a look at what your current array is. If they are running at 112v Voc and you now introduce  an array of 97.68v it will "pull down" the rest of your array and as a result you will loose generation capacity. So you might get more watts but end up having less. 

But again I am saying this as I see it. Chris will confirm or set it right if I am off the mark so just hold for him or @plonkster or @The Terrible Triplett top confirm . 

[Chris Hobson]

Something troubles me about the the specs on that Bonanzatech unit. My neighbour has one and  we have opened it. It has exactly the same SCC as the 3000W SCC  4kW units. The specs are usually 1500W and VOC max of 75V (since it is 24V the amps are double and that is why I think they have the limitation).  Bonanatech have it as 1500W but with VOC of 145V.  I would just confirm that the inverter's VOC is indeed 145V DC.

If it is 145V you could safely put 2 in series.

If it is 75V like I think it is then you could parallel the 3 units.

 

[pilotfish]

6 minutes ago, Chris Hobson said:

Bonanatech have it as 1500W but with VOC of 145V. 

The MKS Plus version is indeed 145Voc as per the attached brochure, but I would still stay away from 3 panels in series as you have a real possibility of exceeding the max 145Voc under certain conditions.

I believe that the best option would be to obtain a 4th panel of the same or VERY similar spec, and have 2 strings of 2 panels. You could use the 3 panels in parallel, but may run into problems with another spec on the brochure...

MPPT Operating Range - [30v to 115v] or [60v to 115v]

From the above I assume that the equipment comes in 2 versions, if you get the [30v to 115v] version then you can run single panel strings, if you get the [60 to 115v] version then it may not work with single panel strings as the panel voltage will be below the 60v minimum.

If I was you I would forget about the Axpert 3k and go for the Axpert MKS 5K for very little extra money (and also get the 4th panel) - it is far more versatile and will allow much better options for expanding the system later, which you will definitely need to do if your goal is to kick Eksdom as you mentioned in an earlier post.

 

Axpert MKS Plus.pdf

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WARNING ELECTRICALHAZARO...

Got to love the Chinese.... :-)