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Undersized inverter


Dylan
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Hi,

New here, just starting to get some ideas for a solar system for home.

Currently was thinking a ESS system with Multiplus 4000W, with a anti island device, batteries (4.8kW Li ion) on MPPT (2X 150V/35A), will do a drawing for a separate thread down the line though.

My dilemma, is it worth going for say a multiplus II but its only 2400W, meaning there will be considerable times I will use more than this, however system cost is 15k preliminary cheaper.

Things that I know pull more than 2400W are, kettle (3000w, geyser (however guess I can put 15k into a solar one), any time I use a few appliances...

Any one with a similar issue?

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Hi Dylan, welcome.

I have a Multigrid 24v, brand new, but basically the exact same scenario as you are questioning.

The costly part of your bill is not the kettle or stove, it is the loads that are on 24/7/365. They tend to be below 1kw.
And the part we tend to forget, the loads that are on when all are asleep, loads that are not needed.
And the last part, when are the loads used, like stove, late afternoon, early morning or evenings?

Let me give you my take:
- Is it worth the cost to go bigger inverter to cover a kettle on for 3 minutes each time it is used? You can get a 1kw / 2kw kettle if you want. Just takes longer to boil, same power usage.
- Stove. With ESS the big watts are off-set with all the panels can give, so you are saving (cents) each time it is used.
- Geyser, put in a smaller 2kw element, and a timer or whatnot, that it draws at the time the inverter is idling, and not when the stove / kettle is on.

ESS is the perfect balance to use all the power the panels can provide, and by staggering the loads evenly, you save even more. That is why one can use a smaller inverter and enjoy the best usage of solar power.

 

Flip side:
Go big inverter, lots of panels - and batteries - and cover all the PEAK loads that we tend to worry about, loads that are on for probably less than an (fill in time) ___ per day, or loads that probably are on in the evenings, which needs batteries, or Eskom. Batteries, where Eskom is available, with optimal daytime use of appliances, is more expensive than Eskom.

 

Another thought:
If you are Cpt based, EV tubes are more efficient in winter than any PV panel. It could be an interesting idea to check out a EV tube system for ones geyser, than adding more panels or bigger inverter, for geysers do use a lot of power.

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35 minutes ago, Dylan said:

My dilemma, is it worth going for say a multiplus II but its only 2400W, meaning there will be considerable times I will use more than this, however system cost is 15k preliminary cheaper.

Only you can answer that question. I would say that for the average house, a 3kva is sufficient. Take this chart of my morning consumption (yesterday, because today is cleaning day and a little unrepresentative).

Selection_002.png.517a06daa8dd2b0184b1bf32adb90b44.png

Draw a horizonal line around 2k then colour in the area under the chart with blue under that line, and red above that line. You only pay the grid for the red bits.

So in my system, there 3kva definitely covers more than half my loads.

Or this one which quite helpfully has a tick at 2.4:

Selection_003.png.2ef8783499b1121c57ba37209211d474.png

With a 5k I would cut 95% of loads, with a 3kva I could cut 75% (very rough estimate) if I had more battery and panels :-)

Remember also that a 5kva unit has a higher quiescent draw.

So you'll have to look at your loads and see how many hours of the day you're actually up there. I suspect not very many.

Also, if you fit a 5kva unit, you must fit a Ziehl anti-islanding relay. If you fit the Multiplus-II, no additional hardware is needed.

 

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Ditto PLONK, that is what I mysefl was looking for!!!

@Dylan my arrays combined are even smaller than Plonksters, and I'm negotiating with myself to add more panels or not, face the N/E/W ... the peak loads are kitchen, the evening loads the trick, whilst we are "negotiating" peak hours of usage, to move more to times solar performance is at it's peak. ;-)

image.png.62afc5a713c3fade779903507cd8d810.png

 

This should ideally be nicely curved, not jagged, if the use is constant, the smaller array being favoured, making me wonder if I need to move it to face East.

image.thumb.png.e4f2afafb1de41301a5e3c67ea92419d.png

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This topic has been discussed before plenty of times.

First you need to determine your amperage usage for all your different implements - not forgetting that the female needs are far more complex than men.

There are many energy savers at the present time - including washing machines, dishwashers, hobs etc.

When you analyse this make an intelligent estimate when you may need to replace them as they might be near their replacement time.

For a difference of R15000 you can replace quite a few of them. 

Also look at the use of gas for ovens and hobs - we don't really use them so much. My wife totally refuses to use a gas hob and I've given up on that argument. 

I just read an interesting article of washing clothes with beads to save water - it seems the future is closer than we think.

At the end of the day it will come down to what you can afford. Unless you see a lifetime of 30 odd years solar cannot come close to being more affordable than Eskom - unless the run out of coal and start implementing increases of 20% per year.

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49 minutes ago, Johandup said:

This topic has been discussed before plenty of times.

We have, and you are right, all of it.

But that was before grid tied (feeding all the power into the DB board) like ESS software does. ESS takes grid / panels / batteries and cleverly balances it all by pushing all the possible power back into the DB when needed, even back into the grid if you want, this is optimal, pushing into the grid, but that has some T&C's.

Or go grid tied like with a Solis and no batteries. That is THE cheapest approved grid tied inverter I know of, sending all the panel power into the house on demand. Even on a 1kw unit you start winning.

So this changes everything, can now go smaller inverter and have more savings.

Edited by Guest
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@Johandup makes a good point. It would be silly so shell out on a 5kva inverter 6 months before you get rid of the appliance that you needed it for.

Additionally, something I didn't mention: You can put inverters in parallel. Put in a 3kva. If you really need to... add another one later.

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Thanks for the reply's,

It is going to be a hard question for me to answer however your usage graphs do give some insight, if you could pm me some more i'd appreciate with some more info on kW used etc.

Looking at your diagrams, its about 1.2 to 2.1 kWh usage over 2.4kW, which is still money to Eskom...

 

3 hours ago, Johandup said:

At the end of the day it will come down to what you can afford. Unless you see a lifetime of 30 odd years solar cannot come close to being more affordable than Eskom - unless the run out of coal and start implementing increases of 20% per year.

I have been running these numbers, a 100k isn't a nothing amount and it is a fine line to being cheaper or more expensive, however if Eskom pulls a 2008 to 2012 24% increase cycle agai

2 hours ago, plonkster said:

@Johandup makes a good point. It would be silly so shell out on a 5kva inverter 6 months before you get rid of the appliance that you needed it for.

Additionally, something I didn't mention: You can put inverters in parallel. Put in a 3kva. If you really need to... add another one later.

Have thought about this, problem is in a few years for it to work need to match hardware + software versions.

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54 minutes ago, Dylan said:

in a few years for it to work need to match hardware + software versions.

The Multiplus-II is brand new, I'd say you have a good 3-5 years before you need to worry too much about that. Victron makes firmware for older models for many years, you can run the latest 430 version on almost anything going back to late 2013.

Also think about when the loads will run. If you have large loads running during day time, then maybe use the 5kva. If you have larger loads running in the evening, then the kind of battery investment you will need to keep even a 3kva running full speed for extended periods of time will far outweigh the other costs.

Remember also that you still have to make a little bit of effort to match the PV and the inverter to the batteries. Lithium batteries do allow a bit more room, but if you're putting in a 5kva inverter, you're going to need at least 25kwh of lead acid battery, or 10kwh of LFP to handle the peak. You can go a bit smaller and rely on PV a bit more during the day... but spare yourself the pain of such mismatched systems.

I'd say the tipping point is around 25kwh. If you use less than that in a day, go 3kva. If you use more than that, you may consider a 5kva. If you can get yourself down to 25kwh a day, even better.

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The coming trade war between the USA and China might (or not) work in our favour when it really gets hot.

And the Tesla effect might also work for us as they are talking of producing vast numbers of batteries and developing new technologies.

The Tesla installed giant solar farms are not really discussed in the media.

I think their products are the way to go if you need to make your investment future proof.

Unfortunately we also know that Trump regards us as a sh$thole country. 

So for the immediate future it will be "made in China" for us at a dumped price.

 

 

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I decided to do the work. From the last 24 hours, total consumption just under 30kwh, 84% could be covered by a 3kva inverter.

Selection_256.png.94eb99ee83fbcfdbd219b3338d5de2df.png

3.9kwh of that 5.7kwh is between 6pm and 8am, so it will have to come out of batteries if I wanted to cover that. It seems to be mostly related to cooking and hot water, things I could deal with by using a heat pump and cooking on gas (except that I've already argued in the past that gas is nice but doesn't actually save money).

So to save 2kwh a day I'd have to install a larger inverter. To save 5kwh a day I need to install a larger inverter and a more battery storage.

I'd have to double my PV panel installation to get close to that sort of production too. And getting more than 3.5kwp signed off will take even more money.

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Edit link is gone again...

I made a mistake in my math. The number is 80%. I'm taking 20% from the grid, or I would be if I had the generation capacity. If I install the full allowed 3.5kwp for my 60A breaker, I can make around 20kwh a day. I cannot even generate enough power to warrant a larger inverter.

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3 hours ago, plonkster said:

Edit link is gone again

@Energy, I think it's now on a setting such that you only get about 10 minutes after a post is written during which you can edit it. After that 10 minutes, or however long it is, the edit link disappears.

If that's a deliberate forum policy, that's fine, but I think that people like @plonkster and myself, and probably many others, value the ability to edit a post when we spot errors, or opinions that change in the light of later experience, sometimes months after a post is made. Maybe a recent forum software update has introduced this "feature" and it's set at a short time limit by default? This seemed to change about 4-6 weeks ago, at a very rough guess.

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8 hours ago, plonkster said:

I cannot even generate enough power to warrant a larger inverter.

This is an interesting point... it would be nice to be able to determine (not by calculations, i.e. how much roof space, etc)  just how much you will be generating before you buy a specific inverter.

We can already calculate what our usage is energy meters, etc. 

 

So my question is, if I should buy 2 panels without any inverter (why does inverter always come up as a spelling mistake? :() how would I be able to measure the actual power generated?

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Then again if you wanted or should I say needed to run an appliance that required a ( 48v )5kva inverter then get it . Yes a lot of what we do in a solar house is shaving power off where it's not needed but if you want to run the washing machine which is able to run with the inverter , Problem is that you can't switch on anything else. It's not so much as being able to use more of the inverter power all the time, it's only for those brief moments when more than 3kva is needed. In the morning I need coffee and my better half needs to dry her hair and the two often result in me having to reset the inverter thus I need a larger inverter . If I had a 3kva unit and paralleled up a second one then the pair would use more standby power than a 5kva unit and then that might result in a larger battery pack and extra pannels. Granted if it's a 24v system then that presents diffrent set problems but for me , a 5kva inverter makes sence.

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3 hours ago, Antony said:

This is an interesting point... it would be nice to be able to determine (not by calculations, i.e. how much roof space, etc)  just how much you will be generating before you buy a specific inverter.

Take array capacity and multiply by 5.5. In winter it will be lower, in summer it can go as high as 7 depending  on where you live. 3.5 * 6 = 21kwh.

It does depend on how you look at it of course. If you want to be fully self-sufficient, in other words, you want to be able to run all your loads (or you have specific high-consumption ones you want to run) even during an outage, then there is no way around it: Get the larger inverter.

If your emergency loads are relatively small, and your primary goal is to save money, then all you want to do is take the 20 odd kwh and use it to offset the consumption, and then it won't matter if you do it in a tall 5kw rectangle or a flat 2kw rectangle, visually speaking. Save the money and go with the smaller one :-)

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