Hi,
Asking for practical experiences from those of us who know what we are talking about, because Googling this seems to be an infinite source of elementary level (+ often wrong even at that) bullshit.
So I have a 3kWp rooftop system installed by the previous owner of the house. One series chain, one MPPT tracker in the inverter. Ten panels of approximately 300Wp. Last time I checked today with dirty (spring; pollen, no rain for weeks) panels, production with blue skies and nearly optimal solar angle is 2.2kW at 299V so around 7.4A. All-time high peak record is at 3700W.
The system is installed in 2017 and I doubt I could get the exact same panel.
There is room for 5 panels more on the roof (10 -> 15), and besides, the inverter is grossly oversized (5kW) so it would be just obvious to add those 5, for a 4.5kWp system.
My understanding is, of course, that I need to match
* Most importantly, rated current at maximum power point
* But also, temperature coefficient thereof
* And also, having similar response to stray light vs. direct light
The last two bullet points so that maximum power point current would be similar not only in optimal conditions, but over wider operating area. But I guess finding similar enough panels would be a true challenge - so matching rated current is likely the best I can do.
Would you agree so far?
Since 5 < 10 (i.e., less than 100% increase in system size), the old panels should dominate, and it would be a smaller monetary loss to buy panels with a tad larger current rating than the existing ones - only the new ones, 5 out of 15 would be operating at <100% current. If I buy panels with too small current rating, then the old ones - 10 out of 15 would be operating with too little current: more wasted resources. Again, would you agree with this?
MPPT obviously should be able to find the global optimum for the whole chain.
Any other practical comments?
As it is now, both roof area and inverter power capability is wasted resource, and I do want more PV. But of course if 50% of the investment goes to waste, I buy 1500Wp worth of panels but only get 750Wp more due to the whole system operating outside optimal power point, then it does not make sense to extend the system. But my gut feeling says, match the rated current and add maybe 10% and it'll be good enough?