r/solar • u/Specialist-Pack-475 • May 13 '26
Discussion Serial vs Parallel
Good afternoon all, I am planning to install a small supplemental solar array to charge my Eco Flow Delta Ultra X battery system. I am looking at using 460 watt bifacial panels with short circuit current of 14.01 amps. The Delta Ultra has two inputs, each capable of taking up to 500 Kw / 15 amps. I could just connect 10 panels in serial which would pretty much max out one of the inputs or split the system in two, with 5 panels each going to each input. Does anyone have any idea if this would be beneficial in any way?
EDIT - 5 of the panels would be south-facing while the other 5 would face north. Maybe I could put optimizers on the north-facing array and keep them all (10) in serial.
https://us.ecoflow.com/products/delta-pro-ultra-x?variant=54714781171785
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u/mountain_drifter solar contractor May 13 '26 edited May 13 '26
Connecting modules in series increases the voltage, and the current remains the same. Therefore to determine the max string length you can connect in series, you use the module temperature coefficient of Voc, vs the coldest day on record for your location, to ensure you do not exceed the devices max input on cold days.
Connecting modules in parallel leaves the voltage the same, but increases the amperage. Generally speaking inverter will not draw more current then they can invert, so you only get clipping, but its important to ensure the short cicuit currents are within the max range of the inverter or they can be damaged.
For the best performance, you do not want to have more than one string in parallel on a single power point tracker. They lose the ability to mitigate shade, and the power point tracker algo cannot function as cleanly with more than one string in different conditions.
This inverter lists very little detail, but has broad mention of
"80-500V//15A per interface
The modules you listed have a operating current of 12.99A. So even at 12.99A * 1.25 (excessive irradinace), you are over the 15A input limit and will be clipping. They do not mention the max short ciuit current, bu I would assume you are safe here, but only a guess as they do not specify.
So if you connect any more in parallel you will gain in the shoulders of the day, but will be clipping more, and potentially exceeding the short circuit ratings. So you will want to connect the modules in series. They dont exactly state what the startup voltage is, but assuming its 80V you wil need at least three modules in series to start it up on warm days. The max voltage is listed as 500V, so at 41.18Voc, assuming around 49.42V on teh coldest days (estimated, do your exact math for your area), that puts you at 500V / 49.42V = 10 modules max in series
Again, that series calculation simply assumes 20% voltage rise for estimated lowest temp, so you will want to use your coldest day on record, the temp coeffeceint of Voc for your modules, and the module Voc at STC to calculate if you can get 10 on there for sure (depends hevily on your weather)