Solar Panel and Inverter Rating - Page 1
Forum section: electronics / general

author: fastbike
author_rank: Regular Contributor
timestamp: August 17, 2024, 06:48:23 am
content: I'm looking at theLongi LR5-54HTH-440Mpanels. The specs show a max output current of 13.24A.I'm also shopping for a 3 phase hybrid inverter and theDeye SUN-12K-SG04LP3-AUis shaping up to work for my system spec (I will be including a battery bank).However the specs for the inverter show Max. Operating PV Input Current (A) 26+13Confusingly, page 20 (the solar PV input says "Max. DC input current should be 20A. if exceeds, it may damage the inverter and it is not covered by Deye warranty."I'd be splitting the PV bank across two MPPT inputs (10 panels each). Does the second input require a lower current to get within spec ?If so, how do I make this work ?Or is it ok because page 20 means a small excess is fine

author: nctnico
author_rank: Super Contributor
timestamp: August 17, 2024, 03:12:29 pm
content: It looks like the inverter can deal with 3 strings of solar panels where 2 strings are connected in parallel somehow to use only two MPPTs. Hence one MPPT channel can handle twice the current you'd expect from a single string. However, putting solar panels in parallel does not seem like a good idea to me either way. I'd look for an inverter which has an MPPT for each individual string.

author: Siwastaja
author_rank: Super Contributor
timestamp: August 17, 2024, 04:57:15 pm
content: 26A input current is ridiculous because you can't buy panels anywhere close to that. And as nctnico says, you cannot parallel panels. (You can of course, just like you can hit your head on a brick wall). Having connectors which are internally paralleled is waste of good connectors (and it might encourage people to do things you should not be doing). Actual MPTT trackers (per-input power converters) do not cost that much anyway, products that use extra bogus inputs are ridiculous.Market is saturated with sanely specified inverters without this kind of weird trickery.Anyway, I'm pretty sure that 13.24A is OK with that inverter. After all, it's the inverter which regulates the current, and can also limit it.

author: fastbike
author_rank: Regular Contributor
timestamp: August 17, 2024, 11:38:57 pm
content: Quote from: Siwastaja on August 17, 2024, 04:57:15 pmInteresting, a quick google search for 10kW 3 phase hybrid converter in New Zealand shows mainly high voltage battery systems. I suppose the advantage is a thinner DC cable when current is only 25/30A from the battery cf 200A from a 48V battery.The downside is the higher voltage.Anything else I'm missing.

author: Siwastaja
author_rank: Super Contributor
timestamp: August 18, 2024, 08:21:07 am
content: Yeah, those transformer isolated safety extra low voltage battery systems are "nice" to install using any random lead acid battery pack by any random hobbyist, but as a result they are less efficient, more expensive per power, and have relatively smaller battery power."Real" hybrid inverters use higher voltage non-isolated battery packs (often LiFePO4) with all integrated BMS and safety contactors. The difference is similar to buying a super expensive e-bike vs. an actual car.

author: EEVblog
author_rank: Unknown
timestamp: August 18, 2024, 08:48:26 am
content: Quote from: fastbike on August 17, 2024, 06:48:23 amI had to limit the number of panels (7) on each string of my Deye 5K inverter due to the max power input (not max current) per string. I wasn't going to risk going over the label max string power limit. I took the label on the side of the unit as the absolute reference.

author: Siwastaja
author_rank: Super Contributor
timestamp: August 18, 2024, 09:05:01 am
content: Quote from: EEVblog on August 18, 2024, 08:48:26 amIt is completely normal to use higher panel power than rated inverter power, there is no "risk" of any kind unless the inverter is broken-by-design piece-of-shit. The limit in question is active power limit by the inverter. Higher panel-than-inverter power was quite common at some point when inverters were expensive; while it wastes some of the panel investment, it does so only during times when plenty of power is available anyway. This way one can have decent winter-time power without investing into a larger inverter.Of course today I would recommend getting an inverter which at least matches the panel power or is slightly larger, but there is nothing unsafe or iffy doing otherwise.

author: nctnico
author_rank: Super Contributor
timestamp: August 18, 2024, 12:14:07 pm
content: Quote from: Siwastaja on August 18, 2024, 09:05:01 amYou are correct about the inverters capping the input power.IMHO it can still be wise to see if spending more on an oversized inverter is wise. Prices have dropped considerably but the difference can be in the 100 to 200 euro ballpark which you likely won't recoup as panels won't output maximum power most of the time. My solar panels are rated 4500Wp. They can output about 5kW on a cold & sunny day for about 2 hours during the spring. But it doesn't justify spending 100 euro on an inverter rated for 5kW instead of the 4.2kW I have installed. When the panels output too much, I see it peaking to 4.6kW and then throttling the MPPTs back. On warmer days the output drops to around 4kW which is just right for the inverter. Most inverters also have settings to limit the output power below their rating in order to prevent overloading the wiring (in case the inverter is connected to a regular mains outlet for example).In addition, if the panels are oriented such that they never can reach maximum power, then it makes zero sense to buy an inverter capable of handling the theoretical maximum output. A better approach is to check a graph for the location which shows solar panel output versus tilt (roof angle) & orientation (roof direction) and use that to determine the power rating for the inverter. For example: if your panels have a maximum output of 1000Wp and are at an orientation that receives 70% of the sunlight, then a 700W to 800W inverter will do just fine.

author: EEVblog
author_rank: Unknown
timestamp: August 18, 2024, 10:43:41 pm
content: Quote from: Siwastaja on August 18, 2024, 09:05:01 amYes, it's common as mud to have an undersized inverter, but there is a limit. My nominal 5KW Deye is rated 6.5kW total max, or 3.25kW max per string (which is also specced). 8 x 415W would have been 3.32kW, and I'm in Australia where I definitely will get full rated output or more on my panels on a good day. So I limited mine to 7 panels per string. Also, in terms of warranty, I would be going over that maximum string spec. My solar installer also would not sign off on the install if it was over the max spec. As always, YMMV.

author: nctnico
author_rank: Super Contributor
timestamp: August 18, 2024, 10:57:20 pm
content: That is kind of odd. Growatt inverters (for example) allow for twice the Wp rating compared to the maximum inverter AC power output rating. That gives way more flexibility.

author: The Soulman
author_rank: Super Contributor
timestamp: August 18, 2024, 11:13:43 pm
content: In all cases keep within the inverters limits including:Maximum total pv-array power (different from maximum inverter power)Maximum short-circuit current per input (different from current at mppt in pv-panel datasheet)Maximum open circuit voltage (number of panels in the string times Voc from pv-panel datasheet)Imagine this inverter blowing up one day and causing mayor damage to your house (lets keep it at that) you don't want to have any liability on yourselfand don't wan't to give the lawyers any way to shift the blame and all consequences on to YOU.What the inverter can handle beyond its datasheet ratings is besides the point, follow its ratings and mounting instructions to the last letter.

author: The Soulman
author_rank: Super Contributor
timestamp: August 18, 2024, 11:28:05 pm
content: Quote from: nctnico on August 18, 2024, 12:14:07 pmBigger inverters may have a lower efficiency on lower pv-power levels, so it is often better to choose a slightly smaller inverter to maximize annual yield and performance under low-light conditions.Sma sunny design is a great tool for that (although limited to Sma inverters) it takes nearly all variables into account.

author: EEVblog
author_rank: Unknown
timestamp: August 18, 2024, 11:44:04 pm
content: Quote from: nctnico on August 18, 2024, 10:57:20 pmNot in Australia.3.5kW max recommended for a 2.5kW model etc.

author: Siwastaja
author_rank: Super Contributor
timestamp: August 19, 2024, 06:29:38 am
content: It's a recommendation, the idea is to ease design when design software is not available and people making the decision do not have good understanding how to actually calculate/simulate these things.Actual calculation is done based on temperature-compensated worst case open-circuit voltage of the panels. That must remain below the maximum input voltage of the inverter. In the past it was common to actually design PV installations using design/simulation software, where you choose panel type, inverter model, location of install, and it would immediately say PASS/FAIL, today it's more usual just to sell people something which probably works or not, so mistakes happen, like here a company sold hundreds of installs with so few panels that they don't ever reach minimum MPP voltage and sometimes not even startup voltage, it would have been a 15-minute napkin calculation for any professional to see it is not going to work.