It mostly depends on the microinverters used and how much they clip power. It is common to pair up a 230-watt module with a 190-watt microinverter (Enphase IQ6 or 7). If this person is only getting 28kW output for 40kW of installed capacity, there will be a bit of ungenerated power due to the sun angle unless the tilt gets them sunlight that is perpendicular to their array at this time of year. We also don't know if there are shading affects that would chip away at the max potential power output. 40/28 = 1.42 which is kind of high for a DC-AC ratio so I have to think there are other things at play besides just power clipping by the uinverter.
Personally, despite it being a common industry practice, I am not a fan of DC/AC ratios greater than 1. Why? Because it keeps the power electronics, the weakest link in product lifetime, working at their max rated power for ectended periods. I have observed 5kW inverters with 3.5kW of PV modules and they are running great after more than a decade. Everyone is free to bump up that DC/AC ratio (although most residential customers have no concept of what is happening so its not a free and informed choice on their part) but be prepared to replace or repair your inverter more often.
Equations to answer this question are still being researched but general rule of thumb a system should operate within a 2 % efficiency drop when converting from DC to AC, but as the scale increases more factors are involved as location, sun hours, wire size, trackers, storage, equipment, distances, and loads involved. Found a few web sites that goes into this research more listed here as this group are working with equipment within that scale of range. " Industry Standard " is a variable that will fluctuate over time as solar technologies evolve.
A 40kW Solar Kit can require over 2,300 square feet of space. This 40kW system provides 40,000 watts of DC direct current power. This could produce an estimated 3,200 to 5,600 kilowatt hours (kWh) of alternating current (AC) power per month, assuming at least 5 sun hours per day with the solar array facing South.
Micro inverters also have 95% maximum efficiency like central inverters even though they are a newer technology. Another factor weighing in favor of the micro inverter market is that micro inverters are much safer than central inverters, since micro inverters convert the electricity to AC at the panel level.
Choosing Between a Central Inverter and a Micro Inverter
The efficiency of an inverter refers to the amount of AC output power it provides for a given DC input. This normally falls between 85 and 95 percent, with 90 percent being the average.
Efficiency of Inverter: Calculation & Equation Guide
- 40kw DC to 28kw AC around 70 % seems to be to low an efficiency rating but of course the other related factors are not mentioned but in a perfect scenario with a full Sun Hour day the AC side should be much higher like 35 kw plus. I would suggest certainly not a standard of efficiency potentials.
Quote from Sophy Cui of Tanfon Solar when asked this same question.
" In fact, we calculate the electricity generated by the solar panels with the sunshine times 3.5hours-6hours, different country, different season, the sunshine times are different, if we calcualte with 6hours best sunshin, then 40KW solar panel can generate 40kw*6hours*80%=192kwh electricity power. if we calcualte with 3.5hours best sunshine, then 40KW solar panel can generate 40kw*3.5hours*80%=112kwh electricity power. 80% is the factor that we consider not all solar panels face totally to the best sunshine, some solar panels might be shaded sometimes."
------------------------------Timothy McbrideCEO/OwnerSol-Era R & D------------------------------
------------------------------Ella NielsenMembership & Engagement DirectorAmerican Solar Energy SocietyBoulder CO------------------------------
That's terrible. I don't like micro inverters anyway, unless there is an ABSOLUTE STRONG case for shading. But if you are going to use them, their rated capacity should be at least 115% of rated peak panel output. Installers undersize them to cut costs, just like geo-thermal well drillers "short drill" wells to cut cost, causing the backup resistance heating to run more often on the heat pumps.
Its all about costs.... and not making the quote too big, possibly shunning off a sale...
Peak sun time doesn't give information about the sun power hitting the PV panels so it is hard to tell if the inverter output power is within the expected range.
As already stated in the answers, season, time of year, sky clearness, location, panel orientation, shading....all affect greatly the sun power received by the panels.
The simplest way to make sure everything is fine may be to measure sun power at the panel location (same angle) with a handheld solar irradiance meter and then compare the value to the inverter output.
For more accurate troubleshooting, factors such as temperature, air flow must be considered but at that level, it is better to call a professional.
Given the level of detail in the question posed, the only way you can answer it is to assume that the STC output was the base reference to the max output of the micro's.
As you noted, anything else would open the door to a whole bunch of variables.
At the same time though, when sizing a micro on any given panel, under sizing max micro output against panel STC is a bad idea, and that is the "frame" I used to evaluate the question, 400W at 28KW, rather than ~39.2KW desired...
I assumed the author of the question just read the inverter output at noon time (approximately) and worried that the system is underperforming. If the author was a technical person who is able to read the inverter output and adjust it to STC, the author would have knowledge about these DC to AC ratio, and system DC to AC conversion efficiency (just a guess). I may be wrong about the assumption, but anyway, the given answers will certainly help the author make the right interpretation of the system performance.
For those looking to downsize the inverter for whatever reason, you can do it by reference to the PTC rating instead of the STC rating. If the panels are 1000 watts (STC), they may be 850 watts (PTC), then use an inverter that is 850 watts at least. A too-small inverter may not be the optimum one when considering the life cycle cost (loss of production, early failure ...)
Thank you all so much for your insight! This is extremely helpful. I have shared your information with this person and will let you know if they have any additional information.
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