It’s a delicate balance. And for sure, us saffers tends to be helpful by providing more of the backstory
So you appear to be asking, hypothetically, what the round trip efficiency would be of charging from the grid, and then taking it out again. As a round number, something like 80%. I think the Multi charges at 95% efficiency, but it inverters at max 93% efficient, more like 88% on average. So just say 0.9 times 0.9 is roughly 81%, or just round it down to 80.
So nobody in their right mind would do this willingly. You’d only do it if you have PV to charge it with, or if you are forced to. For example, anything above stage-6 load-shedding would cause me to charge from the grid (if we want to live normal lives), which means a portion of my power technically becomes 20% more expensive.
In fact, without harping on this, but I want to give you an example of how hard it is. Take this item, from a recent list of release notes:
Change the “Total of all phases” mode to be symmetrical: all phases are adjusted to convert the same amount of power from, as well as to, DC. In the past, this option primarily avoided passing power through the DC-bus to avoid inefficiency, but didn’t take full advantage of the billing arrangement to use all the available power when there is a shortfall on another phase. Now it divides the work equally across the phases, thereby making the full capacity available, and with no impact on billing.
That was the final “concise” version. It took half a day to write. The native tongue of the writer is not English.
One day I see 3.2kw from panels for maybe 1 hour. Next day 4.2kw for 3 hours, very unreliable benchmark that we “solve” by oversizing.
Now, where is enough ENOUGH!?
Right, so in essence, due to inverter inefficiencies, effectively it costs me R2.45 + 20% = R2.94 if I use Eskom to top-up the batteries like say once a week.
Then, when I use them at night, I save R2.45 per unit from Eskom again.
So It costs me 0.49c per kwh more?
IF that is the case … 0.49c goes a hell of a lot into R20k.
See, the thought came where at THIS point I open up the “taps” to boost the charging using Eskom. Panels are currently at max already … so to add 4kw of unreliable panels more, or 6kw, which is more like R30k thumbsuck …
To where I open the taps with some amps from Eskom …
I can’t understand why you ever would want to top up the batteries from Eskom, unless you want a heck of a lot of backup?
Get more panels if you need more power during the day. If you currently don’t have enough batteries to get you through the night, add more batteries (if you think it is a wise investment). If you then do not have enough PV to charge the batteries during the day and meet your demands, buy more PV? At the end of the day, you should do the following:
How much do you purchase from Eskom every month?
If you buy more PV, how much would that reduce?
Would that saving from Eskom pay off the PV cost, taking into account interest that you’ll lose out on if you kept the money in a type of monetary investment?
I’m at that point, with just a 18 cell 280ah bank and 4.2kw array, that I cannot reduce much more on Eskom no. I have a plan for 3 x 350w panels IF I can find the right ones. Have one, but the specs changed, and then the stock shortages, and mounting …
560ah bank, now that needs more panels … but I don’t want to haul out more for panels, hence the musings on "What if … " or it is like cents on the rand more … bah, who cares.
Sums says 250/100 can go to 7kw with 350w panels maxed, clipping peak daytime hours … OR I have to put up a West/East arrays … which I cannot justify really financially.
Or I take the geysers off the system … back to low-pressure flat-plate solar heating … Solar system as backup.
It all boils down to long-term sums.
More panels, cost X, the benefit is Y.
Same array, geysers back to flat-plate, costs X- aa, Benefit is Y - bb.
Solar panels will last longer than flat-plate heating BUT inverter will work harder for longer. So there is that too.
Think the test would be, if I switch off the geysers for one day, can I get the bank full on 4.2kw and normal house loads? I don’t care to cover cooking times.
Charging batteries faster usually shortens their life slightly, so I still don’t quite understand the motivation.
The only reason is if you have frequent outages and you have to top-up for the next outage, or if you are on a grid where you can buy low and sell high.
250/100 can go to 100 x DC voltage. It looks to be 54.6V in your picture above. Maybe when they are full they sit at 56.8V, so don’t think it’ll go to 7kW, unless you have a 70V battery pack. Or you mean that you will put 7kWp of panels on it, but it would clip a lot during peak production time?
As @plonkster says, emergency charge, or me, just to “top-up” now and then, maybe, if we are talking cents on the rand.
The inverter is set:
6pm - 8pm: Supply 2kw to the house, maybe even 3, will see, from batts.
8:01pm - till next morning: Supply 1kw from batts.
Result, after a few days, I run between a SOC of max 80% - down to maybe 45% the next day on this monster bank.
560ah bank, in my case, is HOBBY Level, as I don’t NEED it and with more costs to be incurred, don’t WANT it either. 18 cell 280ah bank will do the job, I recon.
Haha! If there’s one thing I’ve learned it is that the cost of governances is great… but not nearly as great as the cost of poor governances. Half a day is worth it!
My thoughts on your setup:
Check what your current solar array can produces.
Then set your min SOC to that. That will enable your batts to charge fully the next day.
Example: if your array can produce 20kw, then work out what SOC 20kw is on your setup. You then use eskom to charge it full on days you cant get the bats full.
Or Adjust your SOC to match your panels production?
@TheTerribleTriplet
I can see that you are trying to find an angle to optimize things.
Here is my take on why I will expand for a long time yet. It isn’t because I want to curtail usage it is because I want to use more.
I’ll give you two scenarios:
A domestic set-up, like yours where you are trying to save money on expenses and minimize the inconvenience of load-shedding. A noble cause.
Your first line of attack is to rid yourself of all your power guzzling appliances. Everything is a hole in the pocket. Every capital expense has to be carefully thought about.
A commercial scenario, where production is linked to the power input. If the power guzzlers don’t guzzle, you don’t make money.
A 30kW irrigation pump is 30kW. There is no AAA+++ rating for a 25kW pump – it just does less work and makes less money.
The bigger the better for the large part.
In this scenario, once the initial capital assets are paid for, the operating costs for production power guzzlers using solar will beat prohibitively expensivee grid power charges of a heavy user now and in the future. The bigger your operation the bigger the competitive edge over conventional large power users. Especially, if your operation is daytime power use and can skip the odd day or two during bad weather.
My retirement plan is to go off-grid with a massive scheme (relative to most forum users) that will run predominately as scenario 2, but will be flexible enough to ensure that the domestic scenario 1 is an incidental bonus.
YES! Thank you @Phil.g00 . I KNOW I need more panels BUT, is that the only answer every time?
Like you, along came @neliuszeeman and drops that little gem of his: Adjust the loads to the panel production.
We always balance the loads to the batteries to accommodate weather in any event, with automation and all the other efforts we do.I just have to have more batteries than panels … for now.
Thinking about it all some more, IF I did what you plan, or what @Gman is doing, I can tell myself TODAY, I’m off-grid, I “could try” and justify using Eskom to “top-up”, same as off-gridders would do using a generator on bad weather days. Eksom is cheaper than diesel, and I don’t have to buy a gennie, I have batteries!
Yes, that is stretching it. But where does one draw the line?
This is my line: Would LIKE 4 more +350w panels, have one, but that is IT!
Sunday is a rainy day, so lets check how the system operates after Saturday use and see how the array can handle it. I will say add 4 more panels.
Yes, you got more storage now, but remember, you already got a program that you sticking to how you run your loads in the day, doing this and doing that and I need to warm this, but I can’t charge my batteries because I need to run that first.
If you got more panels, then you can run this and that and charge the batteries full, because with bigger storage comes more loads in the night time that you can run, so what you use there you need to put back between your this and that loads in the day.
I don’t even have a small bit of stress with my setup now, if I want to run the dish washer in the night time or want to switch on the geyser, then I press the button and go on with my life because I know I have the backup storage and array to do what I want.
Jip, I went bonkers with my setup and yes “my gat jik” to get a extra 10kwh storage just to have it but will check how winter will turn out with the new system.
I made so far 1144kwh in 65 days with out any grid support and that gives me R2802.80 at current rate “one inverter is already paid off on the old system that was at the other house”. No, I don’t export to the grid like I spotted some guys are doing pushing 2kw plus back to the grid and say they make this massive production for the day. I charge my batteries and run my loads that I need to run.
What I’m seeing … 18 cell 280ah bank will do the same job, will see today and tomorrow how far I get get it recharged.
Also got quotes to take a geyser onto a flat panel/heat pump … at those prices, more solar panels are by FAR the better spend cause I have 2 x 150l geysers.
And panels last far longer than any water-heating solution.
