Hybrid inverter MPPT voltages

Inverters
21

No, I am pointing out that PV inversion is your go-to money-saving option if you want to save money, not batteries.

A battery is an expensive consumable but ensures the security of supply.

A blend of both would seem to satisfy most people.

For example:
If I wanted security supply because I live in the Drakensberg and lightning takes out ESKOM 3 times a week for 12 hours, my Drakensburg batteries better be charged when I need them.
There is no luxury to predict lightning.
The price of my batteries is the cost of my security of supply. It will cost but, I am not under any illusion that it will save money on my lecky bill.
However, if I was somebody who could predict two-hour load-shedding windows 3 days in advance, I’d have a different approach. Hell, those timeframes with that predictability, I may even go with a generator instead of batteries.

My point is generalizing a universal system is pointless, there are a lot of individual variables.

But the general principles are the same: self-consume as much of the PV output as you make it as possible, and satisfy your own criteria for security of supply within your budget.

Regarding, my choice of battery lifespan:
If a battery fails within warranty the user will be compensated in some manner. So the warranty period is an objective lifespan that should attainable by all cells and all users. This is why I used the warranty period as a yardstick.

You may very well get 12 years out of your batteries, but it is a subjective yardstick, nobody actually knows.
If you are going to use wishful thinking as a basis, why stop at 12 years why not 25, or any arbitrary number that will make the maths work.

I will also add the usage you are suggesting in your equation is very ambitious and also would seem to exclude any battery capacity available to be able to endure an unexpected power cut. This would rather defeat the object of having a battery in the first place.

1 Like
22

In my equation, my bank is sized to get me off-grid on my critical loads, so I do not need to worry about loadshedding.

What I did leave out in my equation (forgot about it) is a 90% factor of DC to AC conversion loss. Doesn’t change the conclusion but does take it much closer to break even.

Isn’t Pylontech’s warranty for 10 years? But yes, it is subjective to expect it to last more than that. However, what else will you do with the R18k? Invest in some shares? The growth on that is also subjective.

I’ve made the argument before, the one you are making, that batteries should only exist to make your system viable. However, in the last year their prices came down quite a bit and my CoCT electricity prices went up quite a bit. Changed my equation enough to make it more viable. I still don’t think oversizing is a good idea, because in a few years there’ll be better tech again. But for me personally there is a place for a bank that puts my critical off-grid.

23

@karischoonbee,
As I have already pointed out, a lot depends on the life cycle of the battery.
12 years has been suggested, you are suggesting 6000 cycles.
This translates to 6000/365 which is nearly 16 and a half years.
But that is in a perfect world, 16.5 years is only if you can fully charge your batteries every day and fully utilize every drop of capacity every night. In reality, you probably could do this 3 out of 4 days maybe. Now we are up to 22 years. I think even the most fervent LiPo disciple is not going to claim 22 years of flat out daily usage.

Your ideal batteries will also have dropped to 60% capacity that’s if they meet manufacturers expectations. There are some trials in Oz at the moment that would seem to be showing that manufacturers claims are shall we say the “falling off a cliff with the wind at your back” scenario.

Again I also stress if you use batteries in this manner you will have no capacity to endure a power cut. So you are back to getting the same utility as a straight-up PV inverter, so why not just get one.

24

This doesn’t seem to be the case for Pylontech and my understanding is also that they are REALLY stressing those batteries with three full cycles per day and high ambient temps during summer and quite low winter.

I think it is safe to assume that it is a worst case scenario for realistic usage of batteries.

1 Like
25
2 Likes
26

Keeping capacity for power cuts is actually irrelevant to the discussion. You can always add more kWh for your needs and leave it charged. The point of the discussion is - if you have spare PV generating capacity (over and above what you want to store for a power cut, if that’s your requirement), is it cheaper to store that energy and use it later or to let it go to waste?

I left out some factors to simplify the calculation, but I don’t think they’re that material, compared to the rate that electricity prices have been increasing and battery prices are plummeting. Even if you’re only breaking even now, in a year or 2 the equation will solidly be in favour of storage. It’s why solar is winning on utility scale too.

Besides, I’m also not quite sold on the idea of keeping your battery charged up for the event of a power cut, rather than using it for free power. It assumes the worst case that it gets cut right at the bottom of the battery’s cycle and that you can’t run your essential loads off PV during daytime. In contrast, if the late afternoon thunderstorm hits the cables, you’ll just be using your battery during the night as per usual. By the next morning it’s flat, but then hopefully the sun is out even if power isn’t back on. Maybe the cut happens at 9pm and you’re sitting at 60% rather than 100% and it’s still overcast the next day. Just manage your loads more carefully or top up the battery with a genny in that rare case.

27

I grant you Pylontech are fairing better than most, I know it is an accelerated test,my understanding is that charge discharge rates although 3 times a day are strictly within the manufacturers recommendations.
Also the temp swings reflect OZ winter-summer as within the manufacturers specs and they are not too dissimilar to SA conditions.
The test is ongoing, so linear projections have a big caveat.
That said the linear projection is that 60% capacity End-of-life will be reached at 4470 cyles, that is 25% short of 6000 cyles.
Don’t get me wrong I’m not saying these test batteries brands are bad at all, I think some of the batteries even outlasted the companies that made them.

28

I think it’s very relevant, I wouldn’t entertain batteries if the grid was reliable. I don’t understand how you could justify it.
Stating that you can handle power cuts because the battery bank is actually bigger and
excluding the capital cost of the bigger battery out of your calculations is creative accounting.

Your battery bank costs what it costs, this idea that 2 cells are being fully cycled and two cells are fully available is false thinking. You are selectively applying the cost of the free car that comes with the million dollar lollipop.The correct calculation would reflect that your entire battery bank cost twice as much and it is being utilised half as much as optimum.

In the true calculation, it would reflect the user is 4 times worse off than the best-case scenario. And the best-case scenario doesn’t cut it to start with.
I am glad you brought it up though because that more closely represents the more typical set up out there, and it further highlights my point.

29

Even if they are doing it within manufacturer’s spec, that it is worse for the battery than doing it once a day is undeniable.

In SA, the temps might well be comparable, but surely you won’t keep you batteries outside? My batteries for example are kept effectively underground in a south facing garage. The ambient temps there, even in summer, are much more mild and stable. Interestingly, most heat enters that room from driving around and then parking a warm engine there. :sweat_smile:

Furthermore, you wouldn’t cycle the batteries during daytime, then you have PV, so when you cycle them it will be cooler than the max temp of the day. There’s therefore a gap of a few hours between charging and discharging giving the battery time to get rid of some of the charging heat. They also use the max temp of a month to cycle the batteries at the entire month where in reality, not every day, except a few, would actually reach the max temp.

To cycle batteries three times a day I am assuming that they will also charge them at the max current they would accept. This is much worse than what would happen in reality. A properly sized bank as per Victron recommendations, would be twice the size of what your inverter can discharge in an hour at max. Assuming you’ve got a 5kVA inverter, that means 10kWh of batteries. People typically pair such a setup with about 100A (in a 48V setup) of MPPTs. So 0.5C will be your absolute max charging rate, but you will use much of it in the house as well, and also won’t just charge during your peak.

Lastly, end of life to me isn’t 60% SoH, it is when it can no longer hold a charge and voltage drops off a cliff when I pull anything. 60% original capacity is still alright, I’ll just drop my nightly max discharge to a lower amount.

So long story short, I really appreciate those Aus tests and have been following them for about a year, but I view them as the absolute worst use case a battery would have to endure.

30

You almost have to wonder why the OZ government is spending millions doing these tests?
You’ve told them they got it all wrong, I told them that they should sacrifice a virgin on the summer solstice over the positive terminals.
It seems our unsupported opinions and intuitive insights are being summarily dismissed.
What ARE they playing at?

31

Haha I don’t think they are doing it wrong, they are doing it as objectively as possible. Producing a reasonable minimum is a very useful result to have.

32

I agree and we have to be careful about what seems to make sense but is totally unsubstantiated by critical analysis.
The Aztecs had a daily human sacrifice to ensure the sun rose the following day, it always worked.

33

Just keep in mind that everybody’s use case differs. No one is an average. So it is your responsibility to take in all the information available and make adjustments, hopefully objective, but some will have to be subjective, to make it make applicable to your use case.

As per my calculation above (12 x 365 x 3.5 x 0.75 x 0.8 x 0.8 x 0.9 x 3 > 18,000), the only input you can really fault is me using 12 instead of 10 (the 0.8 average SoH is covered by the warranty, as is 10 years). I could just as well say that 0.75 is an underestimate and it should be 330/365, which is a figure I remember from somewhere (30 days of the year in SA is very low yielding PV days - but this obviously differs from place to place).

You might call this unsubstantiated. Fact is, no investment is a sure thing. If it is a sure thing, the efficient market hypothesis dictates that arbitrage exists and demand would increase until the arbitrage is cancelled out. Now, this is very theoretical but still, if there is an investment with a close to 100% probability (including the warranty terms) to at least break even, then that is a flippen good investment in my opinion.

Remember that “break even” in this instance isn’t the payback term, that will be shorter than 10 years if Eskom keeps going on with its increases. It is breaking even against Eskom’s electricity price inflation. Time value of money isn’t easy to take into account objectively, unless you can hedge yourself out, which you can’t do because as far as I am aware, no forwards exist on Eskom’s electricity price inflation.

1 Like
34

Very cleverly debated, respect.

For me, as I’m no “clever” person, two things stand out:

  1. Letting batteries sit there fully charge so that they can die of old age, is a waste. Use them.
  2. If your system cost R150 000 all in and because you did it right it shaved off over 12 months R2500pm off your Eksom bill, that sums says that in 60 months you would have recouped your initial outlay. Basta Eskom increases, that just shortens the period.

The 60 months, or less, are within the warranty period of the Victron equipment as well as within the warranty of the brand name lithium bank and panels - assuming that the companies survive the warranty period of 10 years, 25 with panels, because for a company to last 10+ years today is quite a mean feat.

Give the lady a Bells! :raised_hand:

35

I understand @Phil.g00’s argument to mainly be that batteries worsen the overall ROI (not that the total investment wouldn’t provide a positive return) rather than earn some return themselves. I used to be in that camp, but with the latest prices I’m not so sure it is clear cut anymore.

36

Yes, I agree with Phil.

Eskom is cheaper than batteries.

BUT one must also consider “how is Eskom cheaper”?
Cause you switched off more at night being on Eskom having no batteries i.e. your monthly cost already like R200?
Or because you use more power because you have batteries?
Or is it the hardcore average 24/7 loads that you are shaving off your Eskom bill wisely that generated the saving because you have batteries?

That sum differs for each solar system and should not be hypothetical, it must be hardcore fact at the end of each month, after 12 months, the savings made by cutting out the 24/7 loads from Eskom supply.

37

Then do so when it makes absolute clear sense that it is cheaper :slight_smile:

I’ve seen so many calculations. For us CoCT people, who pay R2.56 on the top end, the answer is a bit clearer to direct Eskom customers who pay R1.70 or thereabouts.

Then there are grid connection fees. My father for example (who is a Nampower customer) pays a total bill of R1700 a month, of which R1100 are connection fees and R600 is actual consumption. If he goes completely off-grid, that’s over 12k a year immediately… and he only needs to store between 6kWh and 10kWh to have a day’s backup… he’d get ROI very very quickly, and even in his case we calculated that if he spends 80k on batteries, over a somewhat pessimistic but probably realistic estimate of lasting 8 years, that’s 10k a year, which is not too far off what the grid connection costs. In his case, since the grid connection in Namibia is still stable, and he likely won’t see a return in his life… he is not going to do it. He’s sticking with the Fronius that is already there.

But it gets even more complicated. What if you take that 80k you were going to spend on a large battery and invest it, you know somewhere that has a bit of risk to it, like the stock market. Something about on par with a battery made on the eastern side of the planet :slight_smile: Odds are you’ll make more money than you would have saved on electricity.

Nevertheless… not everything we do is based purely on the numbers. If it was… nobody would be driving those expensive Jaguar SUVs (complimented a guy on his really nice-looking SUV… he said he doesn’t think it is worth the 1.3 he paid for it… I realised there is only one way to interpret 1.3 :slight_smile: ).

38

The latest rates I see are R2.37 at the bottom end, and R2.89 at the top end… So the inflation seems nice and steep…

To think, when I was little, all I really wanted was a Porsche Boxer (I’m modest, don’t need a Carrera) one day when I’m old. And now, all I really want is a VW Caravelle. Both of which are now more than “1” and not really “sound” investments. But man, a Caravelle would be so amazing. Space for my daughter, dogs and baggage!

And also, is this forum called “Economics Talk”? :stuck_out_tongue: The cyclists I know spend more on bicycles than I could ever spend on a reasonable PV system. Even on an unreasonable one. I bet “Cycling Talk” do not discuss the ROI on bicycles to the n-th degree! Let’s all just be normal enthusiasts and recklessly spend money on hobbies! At least there’s a case to be made that you might have an ROI with batteries, even if you might have to hope they last you 10 years.

4 Likes
39

Use cases. This reminds me of my QA check lists.

Let me add my use case and maybe a different perspective in here:

We currently only have a small trolley inverter to give me internet and keep the TV and home office up and running during a power outage. After having had the trolley for ~2 years and seeing how convenient it is to have a couple of things running and not sitting in the dark or scampering to get a movie on to a laptop to survive a load shedding stint my wife and I had a long conversation.

What it came down to is that we want the luxury of being able to have the whole house (bar the water heater and pool pump) to be able to run as normal during a load shedding stint or a short unplanned power outage. We want to not have to worry about looking at the load shedding schedule, flipping a change over switch and starting the generator while also ensuring that the generator runs every now and then, has petrol or diesel and gets maintained.

So for our wants we decided that we need :wink: an inverter with batteries specced to last a load shedding stint. This was 4 hours in CoJ before they decided to go to 2 hours not too long ago.

For us this is something that we are willing to spend money on as a luxury (but not make debt to finance it). Those of you that have an inverter with batteries must surely remember how it felt the first time there was load shedding and you didn’t have to lift a finger or didn’t even realize that the power is out?

We then took the conversation a step further. Having an inverter with batteries is quite a large capital outlay for (like Phil said) ~80 hours a year of battery backup.

We then said what if we add some PV into the mix. You get the good feeling of running off solar energy and by using PV during the day (by shifting loads like the water heater, pool pump, dishwasher and laundry to the daytime) your monthly electricity bill will drop and you can then start seeing a return on investment for some of the costs. Add onto that the fact that lithium batteries can discharge to 20% on a daily basis and that you can now charge them up with PV and you start smiling a bit.

We won’t necessarily make a 100% return, but the way we’re looking at it is that we will be able to get a return on the solar panels, MPPT’s and overall installation costs. For as long as the solar panels & MPPT’s stay working after their return you have the bonus of now getting a return on the inverter & batteries. We also don’t know exactly how long any of the parts will work for before needing replacement. IF we manage to get a long life out of the parts then we may even have the added bonus of making a 100%+ return which can then be used to offset the next big thing that will come out in the next 10 years.

But of course the math isn’t just as simple as that. For a battery backup only inverter we could go for a cheap inverter. It might not necessarily last as long, but it will only be used for the ~80 hours a year and the time to charge after an outage. Throwing PV into the mix and daily battery charge & discharge made us want something more robust. After adding an ET112 energy meter hooked up to a Raspberry Pi running VenusOS and thus getting a feel for Victron did it for me. Yes, it adds quite a cost onto it, but something my wife mentioned was reliability and after sales support as we shouldn’t be buying something that may turn into a paper weight when something goes wrong. Don’t get me wrong. I haven’t ruled out something like the Sunsync just yet. I’m just leaning more towards Victron mainly based on the feedback in various forums over the last 3 years that I’ve been reading up on and also some real world feedback from people I know and also conversations I’ve had with @JacoDeJongh.

To be honest though I envy my colleagues in Germany & Australia who don’t have load shedding or power outages and simply have an inverter with solar panels connected, moved some loads to during the day and don’t have a need for a large and expensive battery and thus have a very short ROI.

1 Like
40

This is how my journey started as well. We wanted to have backup power as both my wife and myself work from home often and we were expecting a baby and didn’t feel like trying to deal with the baby in the dark or aren’t able to warm something during loadshedding.

So we bought an inverter and battery backup. The inverter I got was a Growatt and it was horrible. It had no idea what to do with lithium batteries and based on that I got the supplier to take it back. During that time I did a lot of research, chatted to Jaco and Izak, and realised that I’m not going to replace Eskom with something that is equally unreliable or not well supported. So I went for the Victron inverter and Venus at first.

After anyways spending the money on a quality inverter and a reasonable battery bank, adding PV and MPPTs was a simple investment a few months later.

Regarding other countries, I am firmly of the opinion that people need adversity to really become useful humans. So I don’t envy those in other countries. I don’t want to live in a country where the government takes care of my every need. What meaning do I have left then? Obviously I don’t want to live in a warzone, but having to look after yourself is much more satisfying than just watching netflix all day because there’s not much for you to take care of. :stuck_out_tongue:

1 Like