Energy Matters Forums

[4lex]

I'm in WA and want to install as much PV as I can fit on my modest roof, which is looking like about 6.5kwp. Thing is since WA scrapped the FiT last winter, the power company will only buy surplus power at 7c/kwh, and will only do so on systems specified with a max 5kw rated grid inverter.

I've got NE and NW aspects on the roof where I intend to fit the modules, so at a guess I suppose the system may very occasionally hit 90% efficiency, so I might conceivably hit 5.8kw. Ordinarily I'd prefer to go for an inverter with 20%+ headroom, so say 7kw.

Question is really, are there any inverters stamped as 5kw that could handle 6 easily? Or is it feasible to modify one (maybe some active cooling?) to increase its power handling capacity?

Cheers,
Alex

[Inspector]

Plenty. I'm sure Karl will be more than happy to respond

All inverters *should* handle excessive power input so long as it's within the current and voltage limits. Think of it this way - it's like connecting a 50w headlight to a car battery - the car battery can supply alot more power than the headlight needs. In this analogy, the headlight is equivalent to the inverter, and the battery equivalent to the panels.

[Tracker]

Plenty. I'm sure Karl will be more than happy to respond

Not sure that I agree ..... BUT .... It does depend on the context..
There is a significant difference between silly transient peaks that we see from freak Edge-Effects and cold winds, but when it comes to a real-day's performance, I THINK they are much the same..
YES - I have seen 2500W from a 2000W device, but that spike lasted 0.1365Seconds, and YES - I have seen 2300W for a short period of minutes, but then it settles down to 2000
YES - We can improve on things, as I showed years ago by adding cooling to CMS inverters, and we all (that are keen) do it as standard, and it ALLOWS us to get the best from the device (with improved operational safety), but not really any more than it could otherwise do..

I....want to install as much PV as I can fit on my modest roof, which is looking like about 6.5kwp......., the power company will only buy surplus power at 7c/kwh, and will only do so on systems specified with a max 5kw rated grid inverter.

With the demise of generous FIT's, it's is really a case of REDUCING your consumption.. Being ABLE to feed lots back to the grid, is likely of unfinancial value. It WILL help, but I suspect that the value will never be there..
It's actually a shame that the FIT thing does not work like a credit system.. Put 10 in and you take 10 out for free..

I've got NE and NW aspects on the roof where I intend to fit the modules, so at a guess I suppose the system may very occasionally hit 90% efficiency,

NO - your being too optimistic.. We have come to accept that 80% is the best return you can expect and then from Nth Facing panels. Naturally, we assume that you would split your 6.5KwDesign equally.
You could go to PVWatts and do the estimates for NE and NW, but in the end, the general 80% will be down to ?? (say) 75%. ie.. 4875W.
As this is within the 5KwDesign, and you have no more room for panels, then that is what you should see..
( PVWatts - Apply say 1Kw to NE, N and NW, and get the % difference )

Question is really, are there any inverters stamped as 5kw that could handle 6 easily? Or is it feasible to modify one (maybe some active cooling?) to increase its power handling capacity?

You are clearly going thru the same phases as we all have, thinking there is a better way, and I am not aware that anyone has ever said "Bud' - this Three-Headed-Dragon Brand gives 6Kw for 5Kw input..

It does not happen.. If I am trying to sell you something then I am going to exagerate it's performance, so you will buy MY product.. BUT - this should NOT happen..

MOST products give efficiency curves for Conversion Vs Input Voltage.. Either these figures are correct or they are not. You would be stupid to by old technology with 90% (conversion) efficiency, when SOoooo many others are doing 96%.

It is feasible ( ) that someone might say their 5Kw GCI is only a 2Kw, so they can get some kind of approval as a small GCI, but that is not very likely.. AND - I understand that they have to be submitted for compliance testing.

Bottom Line - Your space limit is 6500 and this will run a 5Kw GCI at a comfortable figure, so get the Highest-Efficiency GCI (@ Vmp) you can find, at the Best price. Unless you plan more roof-space, DO NOT think that a 10Kw Inverter will do any better than the 5..

IMHO
..
.

[4lex]

Thanks for taking the time to write all that mate, much appreciated.

YES - We can improve on things, as I showed years ago by adding cooling to CMS inverters, and we all (that are keen) do it as standard, and it ALLOWS us to get the best from the device (with improved operational safety), but not really any more than it could otherwise do..

So are we saying a bit of active cooling would reasonably give about 10% headroom, give or take?

With the demise of generous FIT's, it's is really a case of REDUCING your consumption.. Being ABLE to feed lots back to the grid, is likely of unfinancial value. It WILL help, but I suspect that the value will never be there..
It's actually a shame that the FIT thing does not work like a credit system.. Put 10 in and you take 10 out for free..

Damn, I should have read that more closely, I had thought that was how it worked. So I guess batteries might be a way to go - is there anyone here gone completely off grid? A few quick sums makes me think if I would use on average 10kwh/day and didn't want to cycle my battery array below 1/3 depth of discharge I'd need 26 100ah batteries (based on 12.8v and 90% efficient system) - so about six grand worth of batteries and have to start replacing 20% of them each year after about year 3 - which makes them about 2.5k per annum over 25 years in todays dollars but allowing 5% inflation in battery cost yoy.

NO - your being too optimistic.. We have come to accept that 80% is the best return you can expect and then from Nth Facing panels. Naturally, we assume that you would split your 6.5KwDesign equally.
You could go to PVWatts and do the estimates for NE and NW, but in the end, the general 80% will be down to ?? (say) 75%. ie.. 4875W.
As this is within the 5KwDesign, and you have no more room for panels, then that is what you should see..
( PVWatts - Apply say 1Kw to NE, N and NW, and get the % difference )

You are clearly going thru the same phases as we all have, thinking there is a better way, and I am not aware that anyone has ever said "Bud' - this Three-Headed-Dragon Brand gives 6Kw for 5Kw input..

It does not happen.. If I am trying to sell you something then I am going to exagerate it's performance, so you will buy MY product.. BUT - this should NOT happen..

MOST products give efficiency curves for Conversion Vs Input Voltage.. Either these figures are correct or they are not. You would be stupid to by old technology with 90% (conversion) efficiency, when SOoooo many others are doing 96%.

It is feasible ( ) that someone might say their 5Kw GCI is only a 2Kw, so they can get some kind of approval as a small GCI, but that is not very likely.. AND - I understand that they have to be submitted for compliance testing.

Bottom Line - Your space limit is 6500 and this will run a 5Kw GCI at a comfortable figure, so get the Highest-Efficiency GCI (@ Vmp) you can find, at the Best price. Unless you plan more roof-space, DO NOT think that a 10Kw Inverter will do any better than the 5..

IMHO
..
.

But ultimately without some kind of storage system I'm giving the power company effectively 3 units for every one they give me back? Mind you if the system can generate a reliable 30 per day (say six hours average - is that realistic?) and we can organise stuff like the washer and the pool to come on when it's sunny it might still work?
Cheers,
Alex

[Tracker]

YES - We can improve on things, as I showed years ago by adding cooling to CMS inverters, and we all (that are keen) do it as standard, and it ALLOWS us to get the best from the device (with improved operational safety), but not really any more than it could otherwise do..

For anyone seeing those words "Cold Turkey" you might like some clarification.
ALL inverters are designed to work as they do, under ALL weather conditions..
Some have forced-cooling in-built and this activates when the temperature reaches a set figure.. Others rely on natural ventillation, AND most all have internal protections controlled by temperature.. ie.. if it gets too hot, then they Self-Protect by cutting back on the amount of POWER that they process from the PV Panels..
Meblourne-Cup day three years ago, was a STINKER in Sydney and after a Lunch at the local club, I came home to the smell of "Cooking Bakelite" -- the smell/sense of electronics over-hot..
As an old engineer, I know that you normally design to NEVER get that hot.. If you MUST accept those conditions, then you use dramatically better quality "MIL-SPEC" components, and we can be sure that our Chinese Inverters (and all others) would NEVER use Mil-Spec..
So - adding cooling fans was a logical choice, and meant that out inverters NEVER get too hot. Mine never go above 47c, even on the hottest days.
I leave home , happy in knowing that a STINKER will not damage my inverters, and also that they will not throttle back on generation, as a self-protect mechanism.
I know that the boss of CMS was most put out when I suggested that I could improve on his product, but
that's life.. I do know that I get ALL the power that the GCI is capable of safely producing..

So are we saying a bit of active cooling would reasonably give about 10% headroom, give or take?

SO - if you read those words about adding fans, understand that it is not mandatory, by any means, but you can be equally sure that it can't hurt, if you have the imagination for adding cooling.
Whether you get EXTRA (10% or otherwise) will be determined by YOUR inverter.. You should not get MORE power, but you might not LOSE as much when it's a stinker of a day..

NO INVERTER should be mounted in the sun, and IF your's has been (because it's the only practical place), then you would be wise to try and shield it from the sun via a screen, or awning or ?????
..
.