question

TL;DR:

Optimal MPPT is around Vmp = 1.5X battery nominal. If you have long PV wires, more in series will likely be the best option as reduced wiring losses will offset reduced conversion efficiency. If your array must be configured in a given XSYP configuration to stay within MPPT PV input limits, then you just get what you get.

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I don't have hard data for you. I have found nothing published by Victron, but I use this as a guideline (Outback FM80):

I doubt that Victron has any "magic" that allows their performance to be significantly better. The bigger the step in DC-DC conversion, the lower the efficiency.

Using the above, optimal efficiency is approximately Vmp = 1.5X battery nominal. Each step loses 1-2%.

Considerations:

Array in general. In order to fit an array on the MPPT and stay within limits, it may demand a certain XSYP configuration.

If you're overpaneling a unit due to poor solar conditions, more in series ensures you stay within the PV input current limit.

If you have long wires between the array and MPPT, more in series (higher voltage, lower current) will reduce wiring loss, and this is usually more beneficial than any efficiency losses the higher voltage might introduce.

If you typically have good solar and short wiring runs targeting Vmp about 1.5X battery voltage is likely the most efficient option.

My case: I originally had a 150/100 for a 6kW array @ 48V. For this basis alone and the 70A current limitation, I went with 3S6P. Unfortunately, I didn't allow for cold temp effects on Voc, and I was dangerously close to the 150V limit. I upgraded to a 250/100 to allow enough Voc margin. I could have reconfigured the array for 4S5P (adding two panels), but that created implications with the physical installation.

thanks for answer!

I am wondering why victron doesnt give any futher info on efficiency....But why do you say the best should be higher Vtg and low Current if on the other hand the efficiency of the inverter falls at higher Vtg compared to otpitmumg Vtg.

PS: the maximum efficiency of the 250/60 is 99%

Optimal MPPT is around Vmp = 1.5X battery nominal.

2X battery nominal = about 1% loss

4X battery nominal = about 2% loss

LONG PV wires may have very high losses > 2% and more. In that case, HIGHER Vmp (results in lower current) will likely be the best option. Even though you might lose 2% due to MPPT efficiency, you might gain < 2% in wiring efficiency.

Example 1: short PV wires. 1.5X battery voltage = 99% efficient. Wiring losses are 0.5% = 98.5% eff

Example 2: long PV wires. 1.5X battery voltage = 99% efficient. Wiring losses are 5% = 94% eff

Example 2A: long PV wires. 3X battery voltage = 97% efficient. Wiring losses are 1.5% = 95.5% eff

Example 2A is the same as 2, except 2X the Voc. Same wiring. Even with 2% loss due to MPPT efficiency, you have 3.5% less wiring losses, so overall efficiency is higher.

Sometimes the size of your array determines how it needs to be configured (Voc must be less than 250V and Isc must be less than 60A), and you don't care about losing 1-2% efficiency because getting the whole array to fit is better than leaving off panels.

I have calculated an efficiency of 95/96% on my victron 30A with 12v battery and 40v MPP voltage at 50 watt.

And of 97/99% at 250/350 watt.

I'm based on victron data not an external amperometer. But victron smartshunt and victron controller have a great accuracy for waht they are.

My old epever tracer have 93/94% at 50w and 95/96% at 250w..but with a 29v MPP (more efficient voltage situation).

Epever controllers get worse at high MPP voltages.

A expensive Morningstar pro star mppt got the same results graph of epever an average but they have a better performance with high MPP voltages like 93v with 12v battery.

It is sacandalous that victron don't public the efficiency curves of their controller but only the max efficiency (98% until 30A and 99% above 30A).

A relatively new,small very peculiar back boost American mppt have a incredible efficiency.

It is the SOL BACK BOOST 20a mppt.

Here https://www.diysolarforu.com/store/p11/Solar_Charge_Controller_Buck_Boost.html

They say that have significantly better performance respect to ALL COMPETITORS especially in case of multiple falses MPP,with a global MPP tracking

while it makes sense that "Optimal MPPT is around Vmp = 1.5X battery nominal" it seems to be contradictory to victron listing its MPPT 150/X at max 98% while the MPPT 250/X has 99% max efficiency?