Deleted

ABrass

As for the lower ROI for larger systems. Yes. If you don't export power to the grid then you won't get paid for it and so batteries make more sense. But you also don't pay twice as much for twice as many panels.

Increasing a PV system from 2kW to 3.2kW (three extra panels) costs around £600 in materials. Assuming your numbers in the OP are right that would save an extra £50 (a bit more but rounding down to 3kW) a year.

Those extra panels pay for themselves in 12 years. Then give you extra savings for the life of the system.

Going to 4kW is probably going to be  £1000. That give an extra saving of £90 a year over the 2kW array. It also pays off in under 12 years.

ABrass

What's your source for this idle draw btw? I haven't found anything to support it.

ABrass

More evidence for a larger system.

ABrass

The idle draw of the inverters don't scale linearly with size. So if you're worried about idle draw then a larger array and Inverter is better. 20W is a much bigger proportion of your generation for a 2kW array than 40W is from a 10kW array.

Why do you keep using the term 'generator', an Inverter is not a generator so it's not applicable.

Hexane

Deleted_User said:

ABrass said:

Your point is that you should turn your inverter off when it's dark or when there are clouds. Fine, it's an interesting concept. Utterly impractical but maybe it will save you a few joules.

Then you should buy a battery, work out the maximum output of that and the inverter and run everything off the battery at once. Fine, utterly impractical but maybe it'll save you a few joules.

If your time is worthless then it might make sense. And it completely ignores the normal situation where people get paid for export or have no batteries. It's advice for a very niche use case

So those for whom it doesn't apply COULD simply skip this thread?

All very well saying that, but a thread title "Solar Arrays - Diminishing Return on Investment" seems to imply some general rule that has widespread application, intended to pull people in. Clickbait?

We've had similar thread titles before like "Solar panels - not worth it?", with similar woolly thinking and similar muddled terms (and similar argumentative discussion and justification) from those posting them.

An easy way out is that whatever information is being provided here, isn't applicable (or isn't accessible) to anyone who sees the phrase "5kw of electricity per day" and doesn't have a clue what you're on about. And still doesn't know what you're on about, 42 replies later. That's me, so I'm out. Thank you for trying to enlighten us about... whatever it was.

70sbudgie

My inverter switches itself off if the panels output less than 20W, so I'm not really aware of it "idling". Is this not a common feature for PV inverters?

I don't disagree with the comment 

The most efficient way of doing things is to use the SMALLEST inverter possible for the job in hand. 

I believe this is why it is common practice to increase the size of PV array compared to the inverter e.g. my 4.35kWp vs 3.6kW inverter.

Solarchaser

I've read it all, with varying levels of engagement if I'm honest. 

Thread title suggested solar, in reality thread seems to be about battery powered mains inverters, and so probably the actual terms used should be in amp hours of depreciation of the battery.....

Sorry, sorry, it was too easy.

Anyway just to throw something random into the thread, cos it's quite random anyway, I have an ene/wsw array, powered by 3kw on the former and 3.6kw of panels on the latter, and an inverter for each side with two strings for each.
In the summer I wire them as such. So the 3kw has 2x1.5kw strings and the 3.6kw has 2x1.8kw strings
When it hits October I switch the 3kw inverter off and wire the 3kw ene array (connecting both strings together) into one side of the 3.6kw inverter, and take the 3.6kw array (with both strings connected together) and attach it to the other side if the 3.6kw inverter.
Is this for efficiency?
Not really, the inverter is a solar inverter so is only on when there is sufficient panel voltage to do that.
So why do it?
Kinda ish what the op was talking about, but in reverse, in the winter the panels don't generate anywhere near their peak power, so I make the array double the size and have more chance of generating some power through one inverter, rather than potentially never reaching the switch on voltage for either inverter. 

Boffinboy24

Scarter you are talking about generators (which can have solar input) for off grid use - things like Bluetti make, right? If so you perhaps should stick to the term generator because the vast majority of people will read inverter on this forum and believe you are referring to standard solar inverters made by SMA, Growatt etc that do not operate off grid. I suspect this a big cause of the confusion and worth bearing in mind for future. There is little off grid or DIY solar discussion here, for reference. 

I appreciate what you are saying about making sure a generator is correctly sized or it won’t be efficient, eg if running too low a load, like a lightbulb. That’s a potentially helpful  point - bigger is not necessarily better unless it’s going to be used. I wasn’t aware they had a standard draw as well even when not used. But then I am unlikely to go off grid!

Solarchaser

I agree about a sweet spot, and in general agree with most of your last post.

I have several inverters.
I have 3 solar inverters for solar power which as you say are switched on by the dc power of solar, and switch themselves off a short while after the dc voltage has fallen below their startup voltage.

I also have a couple of battery powered ac inverters which are more akin to what you have been talking about here. 

On the usage if kw, kwp and kwh, sorry, but you are not correct which is what abrass and others have been saying.
Energy itself is pretty much meaningless without the time it is based on.
If I switch on a 9.6kw shower for only 1 second you could argue it consumed 9.6kw but energy consumption is only relevant over time and the actual consumption is 0.0026kwh, you can talk about theoretical energy generation and even power in terms of kw, but as soon as it becomes a tangible unit, it has to be based in time.

I fully understand what you are saying with your statement of kw =kwh/hours, but that just makes it theoretical power, not useable energy. 

A few pages back you talked about the battery and stated I think 460... but that should have been 460ah because that's what storage is measured in also.

I guess what I'd say about your post, is I understand what you are driving at, and yes if you want ultimate efficiency of your battery inverter then running loads less than or equal to the inverter consumption is not efficient and should be avoided.

I think most folk would agree with that.
The solar point you have made, I get where you are coming from, but its relevant only to your particular circumstances. 

If you had a larger solar inverter,  then having more solar panels would make more sense, but if you have a 2kw inverter, there's no point having a 10kw array supplying it.

And in *Your particular circumstances* you would have diminishing returns by adding more solar.

There could also be an argument to say you already have dimishing returns in purchasing a battery system when you have such tiny tiny usage of only 1500kwh/year.

I could and have waxed lyrical about my particular circumstances in other posts and threads I've done, but realise that there are not many folk running their house *exactly* the way I am, in the same way there are not many doing it the way you are.

In short, you are trying for best efficiency of your stored energy,  but also running the house as dual supply, grid and off grid, for alot of people, me included that's too much day to day hassle.

I'm personally happy to run my battery inverters less efficiently running house loads, as its still cheaper for me to fill my batteries from cheap rate leccy, or free solar, than run the house loads on peak rate leccy.
Perhaps the fact that you are running on single rate leccy is causing you to investigate the efficiency more.

Horses for courses.
I'm not sure many folk would have dug out 4 or 5 cubic meters under their house to fit water tanks, but this idiot done it.

Solarchaser

Deleted_User said:

Boffinboy24 said:

Scarter you are talking about generators (which can have solar input) for off grid use - things like Bluetti make, right? If so you perhaps should stick to the term generator because the vast majority of people will read inverter on this forum and believe you are referring to standard solar inverters made by SMA, Growatt etc that do not operate off grid. I suspect this a big cause of the confusion and worth bearing in mind for future. There is little off grid or DIY solar discussion here, for reference. 

I appreciate what you are saying about making sure a generator is correctly sized or it won’t be efficient, eg if running too low a load, like a lightbulb. That’s a potentially helpful  point - bigger is not necessarily better unless it’s going to be used. I wasn’t aware they had a standard draw as well even when not used. But then I am unlikely to go off grid!

If people think adding more panels will help them through the winter - even doubling won't make much difference as 2 x sweet FA is still sweet FA. All I've been pointing out for umpteen pages is that often there is more to be gained in winter by optimizing the use of your system than there is from even doubling the size of your array. 

Just on this point. 
Solar inverters will usually have a switch on voltage, usually around 150v, but some can be much larger.
If your 2kw array is generating 125v, you get sweet FA, but if that 2kw array becomes a 4kw array, you are now generating 250v, and so you now have useable power, and that CAN be the difference between generating 0wh or 1kwh during a winters day, which in your case is 1/5th of your house usage.

I mean yeah, you can extrapolate that 1kwh to 60 days of 33p, who gives a ... but its all return on your outlay if that's the minutiae you are chasing