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Help with the maths of an off grid system

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Started 11 years ago by johnpao
Latest reply from rsigmund

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johnpao
Member

I'm doing some back of the envelope figures for an off the grid system, based on the government announcement that instead of the old 50% rebate for these systems to a recs multiplier - 5 x up to a 20kw system.

So for example (and remember figures are pretty rough!) and I'm using the same figures for the batteries etc, just altering the panel array size.

Say a 2.1 kw system (12 x 175 panels)

total system cost say $47500
roughly translated to:

panels $9000
batteries $25000
mmpt reg $1500
invertor $7000
installation $5000

230 recs, lets say $9200.

total cost $38300.

__________________

say a 4.2 kw system (24 x 175 panels)

total system cost say $56500
roughly translated to:

panels $18000
batteries $25000
mmpt reg $1500
invertor $7000
installation $5000

460 recs, lets say $18400.

total cost $38300.

___________________________

say a 8.4 kw system (48 x 175 panels)

total system cost say $74500
roughly translated to:

panels $36000
batteries $25000
mmpt reg $1500
invertor $7000
installation $5000

920 recs, lets say $36800.

total cost $37700.

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ok - crazy scenario

say a 20 kw system (114 x 175 panels)

total system cost say $124000
roughly translated to:

panels $85500
batteries $25000
mmpt reg $1500
invertor $7000
installation $5000

2190 recs, lets say $87600.

total cost $36400.

Can you see where this is going or am i missing something, apart from an increase in the installation and regulator costs as the system increases in size, and whether a 10-20kw array can actually fit into this battery setup - or is it apparent to go as big as possible for no extra cost?

Posted Monday 16 Aug 2010 @ 11:19:14 pm from IP #
franks
Member

As much as its a good thing to get some rebate on a RAPS system, its still a scam to get a RECS credit based on 15 years deeming and 5 times the renewable power it produces
all your doing is helping power compays meet there RET target with fake renew power generated.
Its also distorting the large scale renew power investements the 5 times RECS is flooding the RECS market and keeping the price down that is stopping large scale renew power projects!

Posted Monday 16 Aug 2010 @ 11:52:05 pm from IP #
johnpao
Member

True - and not unlike the NSW gross feed in tariff (which I admit I'm a recipient)whereas we get paid top dollar for our green solar electricity, even if we use more than we produce, when doesn't make a lot of green sense (as opposed to a net system).

But if that's the offer...

Posted Monday 16 Aug 2010 @ 11:58:25 pm from IP #
franks
Member

Also your assumption of having a large PV install keep the same battery and inverter, how will your excess power generation help, not running a genset or using more kilowatts each day.
I think your ballance of components need to be checked eg 20kw PV array, say 4 hours peak sun(year average) means you generate 70kw hours per day into your battery bank lets assume your using a 48V battery bank thats 1400 A/h charge per day you would need a very large battery bank (5600 A/h 48volt battery) normally desigened for 4 days autonomy, not to mention when the battery is at full charge you must dump or turn off PV current.
not sure how a RAPS and Grid Feed PV systems reach the same goal.

Posted Tuesday 17 Aug 2010 @ 12:10:24 am from IP #
LanceTurner
Key Master

Yep, you have to do calcs with accurately configed system design, leaving the battery bank the same size skews everything. When designing a system, you should allow for 5 days autonomy and also make sure that the battery is never discharged more than 50% (many manufacturers recommend this now). This means that the battery bank should be around 10 times the actual used capacity each day. I would recommend redoing the calcs more accurately, with properly sized system specs for each system...

Posted Tuesday 17 Aug 2010 @ 1:23:57 am from IP #
CNCandy
Member

Supplementing with a Wind System would also give a steady stream of juice into the batteries depending on how high up the mountain you live...

Posted Tuesday 17 Aug 2010 @ 2:59:05 am from IP #
johnpao
Member

OK - here's a better real world scenario.

Say I need and base the system on daily usage of 6 kw.

A 1.75KW system would be ideal - so the rough maths goes:

panels (10 x 175w) - $7500
batteries 48volt 1800 (amh) $22000
mmpt reg $1500
invertor $7000
installation $5000

A 1.75 kw system off grid gets 191 recs, lets say $7640.

cost is $43000 less $7640 = total cost $35360.

but the bigger the panel array the more recs (disregard whether I actually need it)

so let's say we go to a

4.9KW system:

panels (28 x 175w) - $21000
batteries 48volt 1800 (amh) $22000
mmpt reg $1500
invertor $7000
installation $5000

A 4.9 kw system off grid gets 536 recs, lets say $21440.

cost is $56500 less $21440 = total cost $35060.

see my point? (that's if it is correct) basically same price (or maybe marginally more for installation) for a much bigger system, thanks to the x5 recs and in the above case both the systems would technically work - obviously the 4.9kw system is going to be fully charged a lot quicker and dumping excess power but it gives scope for shunting or more usage later on etc.

obviously my 20kw scenario in the original post is over the top technically in relation to the same battery size but am I reading the off grid recs calculations correctly?

So therefore why not go with a much bigger system if it is effectivly free...? maybe plug the car in etc

Posted Tuesday 17 Aug 2010 @ 4:24:31 am from IP #
rockabye
Member

You actually do need a bigger array in my view, even with the same size battery bank. I would go for more PV definitely if the end price is the same or similar.

Solar systems rarely produce 'average' output. That is during winter and on cloudy days you can get a lot less than your calculated average daily output.

My 1kW system has produced anywhere from 0.3kWh to 5.56kWh per day over the past 12 months but I don't rely on it, unlike someone who is off the grid. Any excess can be either used to carry out high energy tasks during periods of surplus and when weather is poor you will have sufficient for your average daily needs. Your batteries will last much longer with more PV also as the cycling will be much less.

Posted Tuesday 17 Aug 2010 @ 4:36:53 am from IP #
Benny
Member

I agree with you franks that the 5xREC's multiplier is a bit of a scam, but at least the new RET legislation has separated small scale REC auditing from large scale. So the big power producers are no longer affected by the excess of small scale RECs.

Posted Thursday 19 Aug 2010 @ 4:53:26 am from IP #
rsigmund
Member

I made a spread sheet based on the tables and calculations in "AS4509.2 Stand Alone Power Systems Part 2" and other Aus Standards.

The spreadsheet I made would automatically calculate all of the values you are looking for plus lots more!

For example would automatically calculate the necessary battery size/number based on the number of days of autonomy and number/size of the panels.

There is a LOT in AS4509 but if you can get hold of it - it has more than you need to answer your questions (nothing about RECS - but the rest is relevent)...

Posted Friday 20 Aug 2010 @ 2:06:02 am from IP #

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