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Grid-tie or battery


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Hi guys,

I was just sitting and making some calculations (like many times before). I'm not sure if this has been discussed on the forum before and please correct me where I might go wrong. Stuff like efficiency, losses, etc has not been brought into my calculations.

My system consists of an Axpert MKS 5Kva with 6 x 250w solar panels and 4 x 105Ah Energizer deep cycles. The system is connected to the grid through a standard Prepaid meter of which I pay R2,02/Kwh. The system cost me about R35000 seeing that I could do the installation myself (that's including mounting frames, BMV-700, breakers, wiring, fuses, etc)

I have already changed all my lights/floodlights to LED/CFL, my hot water comes from a LPG instant water heater and I've also got a Gas stove with thermofan oven (which I prefer the wife to use as little as possible)

I produce an maximum of 12.3Kwh per day but let's keep it down to average of 8Kwh of solar power per day (summer calculations now). I do however have problem at night. I don't use nearly as much as 8Kwh during the day but my storage capacity is just too small to store for usage at night. When connected only to the Grid (no battery charging or unnecessary consumption) I use average 12Kwh per day, not far from the 8Kwh I produce with PV.

 Thus I was making calculations to see if a full "hybrid" system was going to be cheaper than a larger battery bank.

First off, I need to go to a credit meter again. Seeing that I've still got an overhead cable supply Truman Prince's Minions want me to get a underground connection first. The connection to my house, from an outside db box to my existing db and a bi-directional meter is going to cost me just over R10k. Let's round it to R10k. THEN.......a basic connection fee of R640 per month (single phase). Yes that's correct, R640 PER MONTH.

R640 / R2.02 = 316Kwh. So I have to produce 316Kwh every month, just to break even with the connection fee. to do that I would need an additinal 2Kw Pv array to get that out of a 3.5Kw array. Okay that's sorted then. So now I've got 9 panels that must produce a minimum of 316Kwh per month just to cover my basic connection fee and another 6 to produce electricity.

To my calculations: 9 x 250 panel @R8/watt = R18000,

Meter, connection fees, etc = R10000

Upgrade to a true hybrid inverter = R30000

So that's R58000 just to be grid-tied.

For that amount I can go off-grid by adding another 6 panels, upgrade my battery bank 4 fold and still have some change?? This way I can still keep my prepaid meter for Grid backup seeing that the basic fee is included when you buy units. By not buying units (or very little at least) I won't be paying for Prince's Merc. Is my calculations correct or am I thinking like a 10 year old?


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At a service fee of R640 I'm pretty sure it's not worth it, without even looking at the rest of the calculations.

Not quite sure how you're going to fit 4 large banks of batteries into just 58k or less. The cheapest battery that can call itself an off-grid battery is the Trojan T105, and a 10kwh storage tank will cost you 40k (and then you can only use half).

Perhaps look at LiFePO4. These SolarMD (mypower24) guys have a bank that works with the Axpert, and they are just under 10k for each kwh of storage. You can discharge them quite deeply and they last much longer than lead. In theory, 60k will buy you 6kwh of storage, which might just do it.

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That's my opinion as well. I'm currently using only four 105ah's and I have to use grid power for an average of 6 hours per day so I thought that at R1500 per battery I can easily ad another 8 x105ah energizers for now. I know they will last a maximum of 3 years but that's ample time to save up for a proper battery bank then, preferably at least 5kwh LiFePO4 like you suggested @plonkster 

The one's I have now are about 4 months old and in excellent condition so I don't think I'll have problems adding more of the same batteries. Not the ideal option but it will do for now. Gives me time to look for a 2nd hand Electric forklift 48V bank ;) I understand they get thrown out very easily on forklifts that's still under warranty, most of the time, still salvageable. Only thing is to have patience......

My main goal.........DO NOT PAY FOR PRINCE'S MERC:P:D

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If these are you current batteries, you know they are not deep cycle? They are leisure batteries, or UPS batteries, have about +-250 cycles at 50%.


So if they are, I would not buy more of them. Be patient for the 2nd hand bank. 


Yes, we have thrashed this out. @PaulF007 went and did a financial calculation on this taking into account the real financial impact, replacements costs, ROI versus what if you put the monies in a investment and use Eskom (which is now a 2% increase this year) and if you did this on credit, not cash, the sums for solar and batteries are not in your favour at all.

Bottom line, no calc bar if you are off grid and have to use a generator, favours batteries at all if you have Eskom. Eskom on farms also make batteries more favorable.

On that note. I recently got a quote for under R30k - no panels - to get legally grid tied with no feedback and no meter change either.

It was for a 1500w SMA system with +-1600w array.

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I know the energizer (mine is the EC36 Sealed Lead Acid) are just temporary but they do the job for now. I had to get something to get the Axpert working and they were the cheapest choice at that moment. I do however seldom take them past 80% SOC so I think they will buy me some time. I just need to store a little more for night usage.

I would go Grid-tie if it weren't for the fact that that our service fee's were so high. It's actually bloody ridiculous. I haven't been using any municipal water for more than a year now, yet I get a monthly bill for R382 "basiese heffing" and up to R682 "Interim Lesing"

So let's recap that, up to R1000 Municipal bill for water while my supply tap is shut. And don't think complaining will do you any good, I've tried that for more than a year now. You just have to comply with "Die wet van Transvaal". Even under DA management (and Truman's watchful eye) things do not go much better than before. Well noted that it's not easy to get something working that's been broken for so long.........


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34 minutes ago, Czauto said:

My main goal.........DO NOT PAY FOR PRINCE'S MERC:P:D

I thought Prince was out after the elections last year? Clearly I missed something, probably to do with a DA-Icosa coalition. I have family in Prince Albert, similar situation. The locals were so fed-up with the "Aalwyn" party that some of them actually preferred a DA/ANC coalition, but the local party managed to make a deal and so many of them remain in charge.

39 minutes ago, Czauto said:

I understand they get thrown out very easily on forklifts that's still under warranty, most of the time, still salvageable.

If you find a good source for these... do let me know. No guarantees that I'd actually have money at that point, and I can't say I've been looking very hard, but they don't seem to be as easy to get hold of as I once thought.

33 minutes ago, The Terrible Triplett said:

So if they are, I would not buy more of them. Be patient for the 2nd hand bank. 

I agree with TTT. Those 105Ah jobbies have a storage cost of R3/kwh or more. Use them for backup, no more.

There is another reason for going hybrid which should not involve a meter change. Even if you don't push back energy into the grid, a hybrid still has an advantage. Using a grid-limiter device, you can push energy out of the input side of the inverter to appliances that would normally consume grid power, appliances you purposely left on the grid side because they are heavy consumers. If you have such devices (for me that's a tumble dryer and air conditioners), you could potentially make some gains.

Since I went from my makeshift Axpert-esque self-consumption to ESS (Victron's lingo for Energy Storage System) in December, my harvest improved and my DoD on the batteries are much lower. Where I used to cycle the batteries down to 70% SoC, I now cycle them to no more than 92%. Where I used to harvest around 4.2kwh a day, I know do 5kwh almost every day (with a 900Wp array!). Where my peak currents were over 50A, I now rarely need more than 20A, because I spread the energy flatly around the baseload and let the grid handle the peaks.

I'm not sure if it is worth the money I spent on it, but who cares... I love my system. I don't drive a fancy car and a man must have some kind of pleasure in life besides his wife :-P

I'll have to see how it goes after I add the much vaunted next phase of PV. Being employed full time, and with the kids taking over weekends (yet another birthday party tomorrow) I get about one hour a day to work on the installation. Frame's almost done. Must post some pictures some time.

I expect that I will be able to use the extra 4kwh I think it will generate. See attached consumption chart for yesterday, with a nice 1.2kwh spike (the bit above the yellow) circled. The red is grid, the yellow is solar.

This info really is worth it's weight in gold :-)


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You see they way I look at it.If you are on a 4kw system and want to run off batts at night you can max pull about 3kwh per night out of your batts and stay within the 20% soc sweet spot. To do that you will need a 400ah bank that will set you back about R 40k (Sums are really rough thou ) Now for the lekker part:

3kwh per night gives you 3 x 31 = 93 kwh or units a month.
93 units at R 2.02 gives you R189.86 per month to run the equivalent of a 400ah battery bank.

So if you took your R 37 000 and invested it at ultra conservative interest rate of 7% and Eskom's escalation stayed only with inflation, 6%(This year so far only 2% bat that could change) and you spent R200 a mount prepaid instead of batteries you will outlast the batteries shelve life well over 10 years.
Infact after 10 years you will still have about R 24k left over from your investment.
See below graph:


If you went and borrowed the money to buy the batteries it gets even worse since you will pay a odd 12% interest instead of earning interest and will never be able to recoup the money spend on savings.

So for my money I would say keep those energy batteries for a backup  maybe use 1 kwh per night of 10% soc just to keep them working and the rest of the night stay on eskom. Day time stay on solar , if need be add more panels so that you only use panels during day time. If your peak load is higher than you solar output you will draw a little bit from the batts but since you don't really use them at night it won't be a problem. If things really gets bad with Eskom then you can consider getting more bats to give you a longer load. 

Going grid tie also looked interesting but if you would like to do it "leagal" it sort of loses its charm. Especially with what the Axperts are going for.

Hope it helps a bit :) 


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@PaulF007 I think you must start a thread, Batteries are NOT cheaper than Eskom (T&C), and post the above there as the first post.

The T&C are if you have to use a gennie or are on a farm with huge Eskom connection fees.

Lets settle this for all newbies once and for all for no-one I know has ever done it comparing investment vs Eskom vs batteries.

Because if I saw the above, and there where no Eskom failures, I would never have bought batteries.


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19 minutes ago, Mark said:


My fixed costs for eskom on the farm are more than 50% of my bill. Darn it...


Depending on your general load @Mark you should consider going prepaid. I got mine today and yes the price per unit is quite steep ( R4 per unit) but it is full under your control..

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Good advice.

I think i must just get something to scratch my arse with. My system works great and i have to buy minimum prepaid electricity every month.

I'll rather invest that money into more energy effecient appliances for now.

Sent from my S60 using Tapatalk

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8 minutes ago, PaulF007 said:

Depending on your general load @Mark you should consider going prepaid. I got mine today and yes the price per unit is quite steep ( R4 per unit) but it is full under your control..

I feel for you farm boys. I calculated my average cost for the last financial year ending February 2017. Average Eskom rate for the year = R1.40 per unit.

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2 hours ago, PaulF007 said:

That's just wrong! We need to look after our own infrastructure , security , roads and even waste disposal and we pay more!

Yes, and you had better transfer some of your land to your employees too!

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7 hours ago, plonkster said:

Frame's almost done. Must post some pictures some time.

As of this afternoon. You can see part of the old steel frame at the back with 3 150w panels on it. Its angled at 36 degrees and points NNW. Needs a bit more neatening-up but the hard work is done.


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2 minutes ago, Don said:

justify taking your farm without compensation,

Interesting to listen to the analysts talk about that. It seems what the governing party meant, or how most of them is taking this, is to get the transformation through economic growth and job creation. That is something I can easily go with. There is also talk of expropriation "within the confines of the constitution". That is also something that doesn't seem threatening. But then there is the other crowd, the EFF and co...

I know I'm veering off-topic again, but the one question I'd like to ask this bunch, the one thing that is left unsaid: Is such expropriation without compensation supposed to be punitive in nature? Is it meant to punish the guys who "stole" the land? The way these guys are talking, it seems that they see this policy (that they are pushing) as a two-edged sword: You get the land, and you get back at the enemy.

The problem, as it seems to me, is that the guy who is farming there now is probably third- or fourth-generation (since the 1930s when most of this went down). A large proportion of them are also old. This is their home, their retirement policy, their life's work. Regardless of how much you might hate them... there is simply no way you can take away a man's home, livelihood and retirement and not compensate him for it. You are going to run into the constitution so many times your head will spin!

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9 minutes ago, plonkster said:

You are going to run into the constitution so many times your head will spin!


Before they do anything crazy, they open it up for public debate and input. People protest like hell and draw up a petition and get 2 mil signatures. After 21 days they look at it, smile, and push it through in any case without taking any comments or recommendations into consideration, as is.

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19 minutes ago, Don said:

Looks nice Plonkster. You not making it adjustable? 

No. I considered it, but I don't see much point. In summer I really want a 30 degree angle. I winter I need to go as high a 45 degrees (because this catches late-afternoon sun). But in winter it's generally cloudy, or if it isn't, there is usually clouds on the horizon, so that last bit of sun is likely worthless anyway.

How I ended up at 36 degrees? In dividing up the 6-meter lengths of aluminium, the support at the back had to be 750mm. The front is 1.5m, the cross-bar at the bottom is 1.4m. Now whenever the support is half of the long edge, the angle is 30° (because sin(0.5) = pi/6 or 30 degrees). That seemed too low to me, so the easiest way to angle it up was to pull it in a bit (so the support is at 1.2 meters rather than 1.4). That makes it an isosceles triangle rather than a right-angled one... and that was the point where my trig left me so badly I had to google how to calculate it! :-)

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4 minutes ago, plonkster said:

and that was the point where my trig left me so badly I had to google how to calculate it! :-)


I know, it is dangerous for the wife too to get up there 4 times a year and adjust the angle while you check the output on the monitoring software. :)

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