1 inverter or 3?

[mpooley]

Hi
sorry to be a nuisance but i am nearly at the point of ordering a 6kwp system.
I have 3 quotes from installers that are still left in the running.
My question concerns the way they differ in designing the system.

2 of them are quoting for 1 inverter for the whole system but 1 of them is saying that 3 x 2kwp inverters would be more efficient.

I havn't got a clue but he does sound like he knows what he is talking about.

can any body offer advice please?
thankyou

Mike

[Amy]

Not a nuisance at all Mike.

Just hang fire on deciding untill you get some 'Unbiased' advice here.

I have the same question about 1 or more inverters being the most efficient way forward.

Ive been told that I should split my ring main into 2 rings, one say up to 500w off a smaller inverter for constantly running items like fridge etc, and another off a 2500w for hungrier items that arnt used as often

[Billy]

Ames,

No doubt that that is a good idea.  Seems pointless to have 3kva inverter running a 3watt router for the solar laptop or something.  Just got to seriously plan your circuits and make sure you don't plug in something too hungry and plunge yourself into darkness like I do from time to time.   

Billy

 

[EccentricAnomaly]

Mike, can we assume you're talking about grid-tie inverters?  As Billy says, there are very clear arguments for multiple inverters off-grid but grid-tied might be different, I don't know.

[Patrick123]

Assuming mpooley is talking about a 6 kwp grid tied PV system, then one large inverter should be more efficient then 3 small ones

[mpooley]

thanks everyone

Yes it's a grid tie system.
the roof is split into 2 sections by a sticky-out-bit (technical term ) so there will be 2 strings of 11 panels on one bit and 1 string of 11 panels on the other.

He went on at great length about designing the system to be most efficient, I just wish i understood inverters 

thanks

mike

[Justme]

I would say for on grid one would be better.

My question is


Why 6kWp? the FIT rate is lower & you might have issues connecting it to a single phase
(perhaps thats why one suggest 3 inverters to fit to a 3 phase supply & also down rate the breakers & cables as the amps would be much lower)


Whilst I can see why some would see that having multi inverters is good for off grid, if you buy a quality inverter that has auto sensing power reduction & self energy reduction (like Victron's) then I would say that you are better off with one inverter. If you are using power on both ring mains then you have DOUBLE the losses for self consumption (conversions losses are also more if at the lower end of the power available so more losses on the big inverter as its not powering the smaller loads as well as the bigger ones at the same time, so if the big one is one then you should turn off the small one). Also better units tend to have better total efficiency any way.

Unless you are going to use a square / modified wave one as they do use less than pure sinewave ones, but thats apples & oranges.


In your 3 string case on separate roof pitches he might want each one separate so that each array is independently monitored & shading/angles of one will not affect the others.

Can you get grid ties inverters that can take independent strings?

[mpooley]

I would say for on grid one would be better.

My question is


Why 6kWp? the FIT rate is lower & you might have issues connecting it to a single phase
(perhaps thats why one suggest 3 inverters to fit to a 3 phase supply & also down rate the breakers & cables as the amps would be much lower)


Whilst I can see why some would see that having multi inverters is good for off grid, if you buy a quality inverter that has auto sensing power reduction & self energy reduction (like Victron's) then I would say that you are better off with one inverter. If you are using power on both ring mains then you have DOUBLE the losses for self consumption (conversions losses are also more if at the lower end of the power available so more losses on the big inverter as its not powering the smaller loads as well as the bigger ones at the same time, so if the big one is one then you should turn off the small one). Also better units tend to have better total efficiency any way.

Unless you are going to use a square / modified wave one as they do use less than pure sinewave ones, but thats apples & oranges.


In your 3 string case on separate roof pitches he might want each one separate so that each array is independently monitored & shading/angles of one will not affect the others.

Can you get grid ties inverters that can take independent strings?

Thanks for reply wish i understood it lol.

why 6kwp?  well i use about 500kw per year so it sort of balances out!  not a very logical reason i know.
As far as the FIT go's the return on investment is almost the same  (slightly more actually) as  its cheaper per KWP to install a larger system.


I honestly dont understand the rest
why does the load in my house matter?  i will be exporting any surplus to the grid. this genuinely puzzles me.

what do you mean "using both ring mains"? i thought i used them all lol

both roof pitches are the same but there might be some slight shading from aforesaid"sticky-out-bit" on different panels during a winters day. He did say that a bit of shade on one string would "pull down" the others on a single inverter.

He did say that you can get inverters to take more than one string I think.

Sorry about not understanding your reply

Mike

[EccentricAnomaly]

why 6kwp?  well i use about 500kw per year so it sort of balances out!  not a very logical reason i know.

That would be 500 kWh per year.  But, do you mean 5000 kWh per year (i.e., 10 times as much)?  That's about the output you'd expect from a 6 kWp array (equivalent to about 800 or 900 hours production a year).

As you say, though, it's not a very logical reason, particularly if you can't use the energy at the time it is produced.  If you could use it at the time it's produced then it would be good, e.g., if you work at home in a relatively energy intensive manner.  Much better than exporting the electricity at 3p/kWh during the day and replacing it in the evening at 13p/kWh.

I honestly dont understand the rest
why does the load in my house matter?  i will be exporting any surplus to the grid. this genuinely puzzles me.

what do you mean "using both ring mains"? i thought i used them all lol

That was a discussion of off-grid systems and not relevant to your case.

[Justme]

Some of my comments were about other peoples posts

IE the twin ring main & the loads & twin inverter for high & low loads.

Will your DNO let you connect 6kw to the grid as the limit is nearer 4kw. They can if they want to but dont have to so check first.

Most inverters will accept more than one string BUT handle them all as one input. If your 3 arrays will be very different I would want them handled individually.

As long as your calcs show that the FITs & return work out ok on 6kw then thats fine. I have never worked it out, just looked at the lower rates & assumed.

[JohnS]

If you have shading issues, separate inverters would be better.  It could be a 2kW and 4kW rather than 3 2's.

[mpooley]

Yes i meant 5000KWH 

it definitely works out at a slightly better return on your money! and as I don't get anything much in the way of interest in the bank at the moment. I think  it's worth it 

I calculate that it will pay my heating bill from the income which hopefully immunises me from oil price increases as it is index linked.

My wife is thinking of getting an electric car (nissan) when they come out so it will sit at home most days getting charged for free too

a pretty good investment i think.

Mike

[Justme]

it definitely works out at a slightly better return on your money! and as I don't get anything much in the way of interest in the bank at the moment. I think  it's worth it 

I calculate that it will pay my heating bill from the income which hopefully immunises me from oil price increases as it is index linked.

My wife is thinking of getting an electric car (nissan) when they come out so it will sit at home most days getting charged for free too

Better than in a bank yes but would the 4kwp array give a better return per pound spent & then save the money to add solar thermal once that gets payments too?

Just because the 6kwp array will produce the same amount of power you use does not mean that you wont get any elec bills never mind help pay the oil bill.

I dont see where FIT's are index linked? The payment per kwh produced is fixed for the 25 years. The only chance of change is the 3p per unit exported & the savings on elec not used. As most of your power (75%?) will be exported at best you will offset the cost of your imported elec. Export 3 units for every unit you import (guestimate).


If the car can be at home on sunny days then all to the good, but most would be in use (or away from home) then. Also have you looked at how many miles you get per kwh of INPUT?

[billi]

....there  are Inverters available with 2 or more inbuilt MPPT chargers to handle individual strings

Billi

[jango]

If there are 2/3 roof angles 3 inverters would be better, but would cost more i bet, thats what i would use on a install over 1 inverter