Underspecifying the MPPT in relation to panel size

[sharpener]

I have just had a quotation for 8 x 380W panels with a Victron 150/45 MPPT which I think will be too small.

8 x 380 = 3040W
45 x 48V nom system voltage = 2140W
45 x 53.2V battery voltage limit = 2394W

The supplier tries to justify this as saving unnecessary expenditure on the MPPT "We can give him a larger MPPT, but its more money and will not be used to its maximum efficiency"

Is this usual? What do others think/do?

[Oldgreybeard]

It will exceed the maximum input voltage if only one MPPT is used. The "150" is the input voltage maximum rating, and the panels will typically run at around 45V to 50V each, so 8 panels is close to 400V. If they are dividing the system up and using three MPPTs, then that's just within the limit for the Victron 150/45 (which is rated at 150V maximum input, 45A maximum output, I believe).

[nowty]

Are you planning 4 strings of 2 panels ?, as 2 strings of 4 will probably kill it.

And are you planning to use this (I presume) charge controller to tie into your existing Pylontec 2 x 3.55 kWh Force-L2 batteries because they appear to have a pretty naff charge / discharge spec.

[sharpener]

The installer is proposing 2S4P. I am not happy with this because of voltage drop as there is a 25m cable run. The battery is OK to take 74A, the new panels will just free up the existing AC solar inverter to supply the house loads, which is a more efficient topology.

But that was not the question. To re-state it, what do ppl think about the power rating of the MPPT being undersized in relation to the panels? They have cited the 2600W data sheet output of the 150/45 - which would not be too bad - but that is only achieved into a 57.6V LA battery which is not relevant to me.

[nowty]

The installer is proposing 2S4P. I am not happy with this because of voltage drop as there is a 25m cable run. The battery is OK to take 74A, the new panels will just free up the existing AC solar inverter to supply the house loads, which is a more efficient topology.

But that was not the question. To re-state it, what do ppl think about the power rating of the MPPT being undersized in relation to the panels? They have cited the 2600W data sheet output of the 150/45 - which would not be too bad - but that is only achieved into a 57.6V LA battery which is not relevant to me.

To answer the question, it's fine to load up the charge controller as it will simply current limit but please confirm if your talking about an MPPT Victron Charge Controller and are you ONLY going to use these panels to charge your batteries ?, or, are you ALSO going to use your multiplus to charge the batteries as well ?, because you would risk exceeding recommended charging rate of your batteries.

And 2 x 4 standard 380W panel strings would be around 160V open circuit voltage which may kill the charge controller. So you would need either 2S3P and only use 6 panels or 4S2P if using 8 panels.

Although your batteries will do 75A continuous, their datasheet ONLY recommends 30A with 2 x L2 modules.
https://zerohomebills.com/wp-content/up ... -Sheet.pdf

[sharpener]

I was asking about the power mismatch from the point of view of the economics.

My Victron supplier (who is content with the proposed configuration technically) tells me it is common nowadays to specify panels that are 30% oversized in relation to the MPPT. The rationale is that it is not economic to size the MPPT for the absolute peak power as it is rarely achieved and the value of the generation foregone does not justify a bigger MPPT. At over 60p a kWh you only need to win an extra 500 kWh to pay for the upgrade so I am not sure I agree.

[Oldgreybeard]

It seems commonplace to have panel ratings that are higher than the inverter or MPPT. Our system is like this, albeit with only a 4% difference, and the new system I'm putting in has panels that are rated about 17% higher than the inverter. All I've read suggests that this improves overall efficiency, as panels rarely deliver their full rated power and the electronics all run a bit more efficiently if run closer to their upper limit.