Mine started off as a 2.5KW and later updated to 4KW with 4.2kw of panels , It was a pain in the butt only 8 months after original install , I had to reapply for approval for the larger inverter and the panels I first used were already obsolete , so after a lot of searching I found the same type of panels to expand.
The original inverter was a 2.5kw Transformer type inverter and the new one is a 4kw SMA4000TL Transformerless type ., basically it was like starting again with new mounting rails , double wiring for the extra array , larger wiring back to the meter box , new inspections and approval for REBS etc , it was just as difficult the second time as the first , but still well worth the effort now that we get credits instead of bills
Panel pricing has dropped significantly over the last 12 months and especially since the AU Dollar has been going crazy , I can't imagine their being a better time to go as large as you can while making sure your supplier is passing on the benefits of the high AU Dollar.
There are a few issues with starting off with say a 3KW system and then adding later even though just about every salesmen will tell you it's easy.
1.If you buy a 5000KW inverter now using only 3kw of panels the inverter will not be operating anywhere near it's most efficient conversion rate., Inverter efficiency is designed and rated at it's maximum operating limits so ideally you should have at least 15% more PV output than the inverters maximum rating , about 3.5kws of panels on a 3kw inverter is perfect.
3kw of panels on a 4 or 5kw inverter is like driving with the handbrake on.
2. Have you got ample space in the same orientation to add more panels without shading issues ?
3. Will you still be able to purchase the same type of panels as you are already using ? not so important if you have a Dual MPPT inverter and can put new panels on a separate input.
The problems start if you want to add new panels to an older string , in order to add them in series they need to be identical , ( you wouldn't put a carbon battery ,and new alkaline battery and new lithium battery in series ) and when adding in series you need to watch you are not exceeding the inverters maximum open circuit string voltage.
You also face the same problem connected a new parallel string , the voltage needs to be identical to the existing string ( you wouldn't connect a car battery and new motor bike battery in parallel ) .
4. Consider that 99% of people start out with the idea of buying a larger inverter for later expansion , less than 1 % of people ever proceed to expand on their original system , the ones that do face the above problems and the remainder either give up or add a new complete system beside the existing one .( much better idea ).
5. from a performance and return on investment point of view , if you have 3kws of panels and a 3kw inverter and are thinking about small scale future expansion , spend your money on a couple of extra panels , don't waste it on a larger inverter. , you can always add another smaller inverter later if you need it but those extra panels will start making you money straight away.
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