How is the Optimum overloading of an inverter determined?
rollandelliott
Solar Expert Posts: 834 ✭✭
I was playing around with Sunny boy's GT string sizer software.
It basically tells you how many panels you can have on a string.
For this example it was 11 to 14 panels each, per string. on a 5kw inverter
panels are 240w monocrystaline.
I called the manufacturer up and said, if I install this system, what is the best number of panels to go with at my zip code?
They told me if I used 14 panels I would generate more energy than the inverter could handle in the spring and fall and thus the excess energy would be clipped.
If I used 11 panels I'd hardly ever reach the maximum energy harvest the inverter is capable.
So the best number they said is 12 or 13.
That's a pretty vague answer considering there would be 4 strings and an extra 4 panels would be $1000 while an extra 8 panels would be $2000 in costs.
Their software just gives a percentage like 80% conversion effciency. But that percentage is only applicable under "ideal conditions"
Isnt' there any software that easily gives you the optimum overloading of an inverter at a certain zip/code/city?
It basically tells you how many panels you can have on a string.
For this example it was 11 to 14 panels each, per string. on a 5kw inverter
panels are 240w monocrystaline.
I called the manufacturer up and said, if I install this system, what is the best number of panels to go with at my zip code?
They told me if I used 14 panels I would generate more energy than the inverter could handle in the spring and fall and thus the excess energy would be clipped.
If I used 11 panels I'd hardly ever reach the maximum energy harvest the inverter is capable.
So the best number they said is 12 or 13.
That's a pretty vague answer considering there would be 4 strings and an extra 4 panels would be $1000 while an extra 8 panels would be $2000 in costs.
Their software just gives a percentage like 80% conversion effciency. But that percentage is only applicable under "ideal conditions"
Isnt' there any software that easily gives you the optimum overloading of an inverter at a certain zip/code/city?
Comments
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Re: How is the Optimum overloading of an inverter determined?
In general, for the "average system", 0.77 is a good number (after aging, dust on panels, some voltage drop, etc.).
If you want to see what the effects are--You can use PV watts with 0.77 (or 0.80 or whatever works for you), and use the output of Output Hourly Performance Data button:"PVWATTS: Hourly PV Performance Data"
"City:","SAN_FRANCISCO"
"State:","California"
"Lat (deg N):", 37.62
"Long (deg W):", 122.38
"Elev (m): ", 5
"Array Type:", "Fixed Tilt"
"Array Tilt (deg):", 37.6
"Array Azimuth (deg):", 180.0
"DC Rating (kW):", 4.0
"DC to AC Derate Factor:", 0.770
"AC Rating (kW):", 3.1
"Year", "Month", "Day", "Hour", "AC Power (W)"
1966, 1, 1, 01:00, 0
1966, 1, 1, 02:00, 0
1966, 1, 1, 03:00, 0
1966, 1, 1, 04:00, 0
1966, 1, 1, 05:00, 0
1966, 1, 1, 06:00, 0
1966, 1, 1, 07:00, 0
1966, 1, 1, 08:00, 1
1966, 1, 1, 09:00, 843
1966, 1, 1, 10:00, 1635
1966, 1, 1, 11:00, 2244
1966, 1, 1, 12:00, 2559
1966, 1, 1, 13:00, 2589
1966, 1, 1, 14:00, 2361
1966, 1, 1, 15:00, 1855
1966, 1, 1, 16:00, 1214
1966, 1, 1, 17:00, 203
1966, 1, 1, 18:00, 0
1966, 1, 1, 19:00, 0
1966, 1, 1, 20:00, 0
1966, 1, 1, 21:00, 0
1966, 1, 1, 22:00, 0
1966, 1, 1, 23:00, 0
1966, 1, 1, 24:00, 0
1966, 1, 2, 01:00, 0
1966, 1, 2, 02:00, 0
1966, 1, 2, 03:00, 0
1966, 1, 2, 04:00, 0
1966, 1, 2, 05:00, 0
1966, 1, 2, 06:00, 0
1966, 1, 2, 07:00, 0
1966, 1, 2, 08:00, 0
1966, 1, 2, 09:00, 248
1966, 1, 2, 10:00, 1514
1966, 1, 2, 11:00, 1558
1966, 1, 2, 12:00, 1701
1966, 1, 2, 13:00, 1703
1966, 1, 2, 14:00, 1535
1966, 1, 2, 15:00, 797
1966, 1, 2, 16:00, 734
1966, 1, 2, 17:00, 12
1966, 1, 2, 18:00, 0
1966, 1, 2, 19:00, 0
1966, 1, 2, 20:00, 0
1966, 1, 2, 21:00, 0
1966, 1, 2, 22:00, 0
1966, 1, 2, 23:00, 0
1966, 1, 2, 24:00, 0
1966, 1, 3, 01:00, 0
1966, 1, 3, 02:00, 0
1966, 1, 3, 03:00, 0
1966, 1, 3, 04:00, 0
1966, 1, 3, 05:00, 0
1966, 1, 3, 06:00, 0
1966, 1, 3, 07:00, 0
1966, 1, 3, 08:00, 0
1966, 1, 3, 09:00, 417
1966, 1, 3, 10:00, 571
1966, 1, 3, 11:00, 1082
1966, 1, 3, 12:00, 1434
1966, 1, 3, 13:00, 1072
1966, 1, 3, 14:00, 812
1966, 1, 3, 15:00, 852
1966, 1, 3, 16:00, 547
1966, 1, 3, 17:00, 1
1966, 1, 3, 18:00, 0
1966, 1, 3, 19:00, 0
1966, 1, 3, 20:00, 0
1966, 1, 3, 21:00, 0
1966, 1, 3, 22:00, 0
1966, 1, 3, 23:00, 0
1966, 1, 3, 24:00, 0
1966, 1, 4, 01:00, 0
1966, 1, 4, 02:00, 0
1966, 1, 4, 03:00, 0
1966, 1, 4, 04:00, 0
1966, 1, 4, 05:00, 0
1966, 1, 4, 06:00, 0
1966, 1, 4, 07:00, 0
1966, 1, 4, 08:00, 0
1966, 1, 4, 09:00, 1
1966, 1, 4, 10:00, 3
1966, 1, 4, 11:00, 22
1966, 1, 4, 12:00, 123
1966, 1, 4, 13:00, 163
1966, 1, 4, 14:00, 91
1966, 1, 4, 15:00, 31
1966, 1, 4, 16:00, 3
1966, 1, 4, 17:00, 0
1966, 1, 4, 18:00, 0
1966, 1, 4, 19:00, 0
...
And you can use Excel to "clip" the output at 5kW and sum up the 365 days of data and see what happens (this is 365 "average" days with estimated power production for each of the days).
-BillNear San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset -
Re: How is the Optimum overloading of an inverter determined?
Actually, that's a very good question and one for which I do not have a definitive answer. Latitude, climate, and module orientation are all certainly factors to be considered. Some clipping during peak production times is acceptable if it is more than made up for in extra energy production during other times. I have seen inverter "overload" numbers approaching 40% and heard of them going even higher.rollandelliott wrote: »I was playing around with Sunny boy's GT string sizer software.
It basically tells you how many panels you can have on a string.
For this example it was 11 to 14 panels each, per string. on a 5kw inverter
panels are 240w monocrystaline.
I called the manufacturer up and said, if I install this system, what is the best number of panels to go with at my zip code?
They told me if I used 14 panels I would generate more energy than the inverter could handle in the spring and fall and thus the excess energy would be clipped.
If I used 11 panels I'd hardly ever reach the maximum energy harvest the inverter is capable.
So the best number they said is 12 or 13.
That's a pretty vague answer considering there would be 4 strings and an extra 4 panels would be $1000 while an extra 8 panels would be $2000 in costs.
Their software just gives a percentage like 80% conversion effciency. But that percentage is only applicable under "ideal conditions"
Isnt' there any software that easily gives you the optimum overloading of an inverter at a certain zip/code/city? -
Re: How is the Optimum overloading of an inverter determined?
Wow that is awesome, I don't understand why the PVwatts data gives years from 1970's to 1986, but here are the results:5443 443 5419 419 5267 267 5216 216 5208 208 5168 168 5122 122 5037 37 5203 203 5068 68 5067 67 5006 6 5288 288 5261 261 5225 225 5181 181 5160 160 5108 108 5067 67 5022 22 5018 18 5047 47 3601
with 6.72KW worth of panels on a 5kw GT inverter there were only 22 hours out of the entire year that clipped over 5000. this is the first column.
the second column is the amount of watts over 5,000
At the bottom of second column is the total amount of watt hours clipped or wasted 3601w
Which is 3.6kw
At ten cents per kw that is 36 cents????
I found this result to be surprising. I thought much more energy would be wasted.
edited to add I just ran the numbers again at 0.85 derate factor and yearly wasted energy jumped to 38.4KWH or around $3.80.
I guess I was worrying for no reason. -
Re: How is the Optimum overloading of an inverter determined?
You won't get better accuracy until someone comes up with software that can accurately predict the weather at your house every day for the next twenty years. -
Re: How is the Optimum overloading of an inverter determined?
PV Watts instead of a "made up" / average solar power per day--They actually give the entire measured data for the "average day" out of a ~20 year period--The year and date is the day of the entire data set type picked as "average".
Remember if you have other effects (such as reflections from snow on the ground adding to total power) will not be part of the above data.
It is not perfect--But better than anything else we have to answer these type of questions.
-BillNear San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset -
Re: How is the Optimum overloading of an inverter determined?
In a related story...
Does anyone here know of calculations and/or software which can show what the Imp for a module/array will be with the sun at a given angle displacement from perpendicular? This would be very useful in figuring out if, when, and how much an inverter will clip. For example (a real world one for me), if an array is tilted at 10 degrees with an azimuth of 210 degrees at a latitude of 40 degrees, what is the maximum current that a string will produce? I can figure out the closest the sun's rays will ever get to perpendicular to the array. -
Re: How is the Optimum overloading of an inverter determined?
ggunn;
Interesting question. You realize it is also dependent on the panel used? Certainly the amorphous/poly/monocrystaline factor is involve, but there are probably differences within those types from one make/model to another.
But you should be able to calculate the difference in insolation at angle as a percentage and draw a fairly accurate conclusion from that. -
Re: How is the Optimum overloading of an inverter determined?
PV Watts will do it (set the derating to 1.0, 1.25 or 1.56 depending on your assumptions then divide by ~Vmp-array as appropriate--possibly use array power as Isc*Vmp to create the "safety rated current" for the array?), by using the hourly output log.
Otherwise is it the Cos of the angle to the sun * safety factor (with sun moving through the sky--would you not assume full sun at less than 10 degrees of misalignment to sun--would give very close to 1.0 due to angle).
-BillNear San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset -
Re: How is the Optimum overloading of an inverter determined?
The different types of modules (single crystal, poly, amorphous, thin film) could very well have different Imp vs solar angle response curves, but I would expect modules of a type to be very similar to one another.Cariboocoot wrote: »ggunn;
Interesting question. You realize it is also dependent on the panel used? Certainly the amorphous/poly/monocrystaline factor is involve, but there are probably differences within those types from one make/model to another.
But you should be able to calculate the difference in insolation at angle as a percentage and draw a fairly accurate conclusion from that. -
Re: How is the Optimum overloading of an inverter determined?PV Watts will do it (set the derating to 1.0, 1.25 or 1.56 depending on your assumptions then divide by ~Vmp-array as appropriate--possibly use array power as Isc*Vmp to create the "safety rated current" for the array?), by using the hourly output log.
Otherwise is it the Cos of the angle to the sun * safety factor (with sun moving through the sky--would you not assume full sun at less than 10 degrees of misalignment to sun--would give very close to 1.0 due to angle).
-Bill
I'm not concerned with deratings or absolute numbers; just the relative current values as the angle of the sun changes. PVWatts makes no distinction between module types; I don't know if that is a problem.
The cosine function is a very rough approximation and gets increasingly inaccurate as you get farther away from the perpendicular. -
Re: How is the Optimum overloading of an inverter determined?
Back on original topic:
"with 6.72KW panels on a 5kw GT inverter there were 22 hrs out of the entire year that clipped
Total amount of watt hours clipped or wasted 3601w= 3.6kw.
Using PV watts a 5KW system makes 6723KW/year
using PV watts a 6.72kw system makes 9036KW/year
Difference of 2313Kw which is $231.3 more savings per year, while ONLY loosing 36cents for "overloading" the inverter. At today's prices of around 90 cents a watt the extra 1723 watts of panels would cost around $1500 more. The payback period would be reasonable.
I've read a couple of solar books and I've been reading this forum for a while and I've never seen this brought up before. It seems like Solar array sizing and inverter sizing is something not talked about much. Given the extra potential of energy production it should be get much more emphasis.
Any why is it that SMA, and power one, as well as a few solar panel sellers have always told me to make sure the inverter rating and the amount of solar panel watts I buy should be close to the inverter rating. For example, get 5000 watts of panels for a 5000 watt inverter? These solar companies should know that you get more bang for your buck by overloading inverters. -
Re: How is the Optimum overloading of an inverter determined?
We (I) use the 1/0.77 as an "overloading" factor all the time. And in California, the rebates were based on the PTC derating of the system (~0.77 to 0.80 or so derating factor).
-BillNear San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset -
Re: How is the Optimum overloading of an inverter determined?
The problem is that it is not the same everywhere. Differences in both climate, utility agreements, and installed prices make it highly variable as to how profitable or not it is to "over panel" a GT system.
Around her GT is a complete waste of money. Elsewhere it can make you a profit.
"Amount of solar panels should be close to inverter rating" is, as a general rule, true. Off-grid it always pays to have a bit more than minimum because not every day is perfectly sunny. I always recommend that if you have a 5kW GTI you should have 5kW of panel yet so many people/installers come short of that number.
As for how much you should go over, extra panel is like adding hours to a sunny day or adding sun to a cloudy one. By the same token, too much extra panel is money invested that will take a long time to recoup.
This is one of those tweaking issues; a matter of fine-tuning the system design to suit the particular circumstances. As such it's impossible to make a rule-of-thumb about always have 'so much' more panel than inverter capacity.
Along these lines, you may notice some of the panels approved for certain micro-inverters are up to 20% larger than the inverter's rated capacity. -
Re: How is the Optimum overloading of an inverter determined?
Yes I understand it is location dependent, but for most of the Southern USA, it's going to be good to overload the panels.
I'm guessing the closer you get to the equator the less overloading will benefit.
Good point about micro inverters I always wondered about that till now. -
Re: How is the Optimum overloading of an inverter determined?
My opinion is that 10% maybe 15% overloading is ok. The next size larger inverter often does not cost much more and running an inverter closer to its max will negatively affect its reliability. Also, inverters achieve their max efficiency at about 75% efficiency. -
Re: How is the Optimum overloading of an inverter determined?running an inverter closer to its max will negatively affect its reliability.
Is this just a general theory, or do specific manufacturers back this up? Testing data? Etc?
Since energy production during the day is a bell curve, even with an "overloaded" inverter, it is only running at Max for a couple of hours a day.inverters achieve their max efficiency at about 75% efficiency.
So a 10KW inverter performs best at 7.5KW worth of panels. SMA, Powerone, Schneider, etc all are similar in this regard?
I wonder if there are any historical studies showing inverter failure rates, etc? -
Re: How is the Optimum overloading of an inverter determined?
A grid-tie inverter's output rating should be stated for continuous use as defined by the NEC regs, as that is how it would be considered for installation.
Note I said "should", not "absolutely without a doubt and no exceptions". Check with the manufacturer if you need to be certain about a particular one.
Note Rolland's point about the bell curve output is correct, and that adding or subtracting panels alters the shape of that curve: if the unit is "over paneled" it will produce its peak power for longer, with more midday clipping. It should also produce more power on less-than-perfect days.
And we go back to Enphase to point out this company has the longest warranty for GTI's and is the one that okay's the 20% over-sized panels. They don't seem to be worried about it, and other manufacturer shouldn't be either. A lot depends on where you factor in the derating; if it's in the design then you can get a true 100% output rating. But if your promoting the max power out in order to gain sales and then have to put in the footnote about "not continuous rating" you are fooling people, including yourself. -
Re: How is the Optimum overloading of an inverter determined?
Where do you get that?...running an inverter closer to its max will negatively affect its reliability. -
Re: How is the Optimum overloading of an inverter determined?Where do you get that?
Basic principles: the closer to maximum current any electrical device is kept the more heat generated. The more heat, the shorter the lifespan.
But as per my previous post a GT inverter should have a continuous rating of 'X' Watts, which means it is okay to run it there all the time and it won't have a negative affect on the expected lifespan. Can't say this is true of all of them, of course; just how it should be. A good indicator used for off-grid systems is their surge rating: sustainable surge of 2X the rated output shows it will be in no danger from running at its limit for most of its life. The caveat here is "sustainable"; a 1 second surge rating doesn't count. :roll:
Unfortunately GTI's don't have surge ratings because they don't need them. We just have to assume they can take their rated output for the long haul. -
Re: How is the Optimum overloading of an inverter determined?
That's conjecture; I've seen no data specific to inverters to support it. But even if it were true, in the financial analysis it might make sense to load an inverter to the gills even if that makes it wear out sooner if the added electrical production offsets the cost of the shortened inverter lifespan. Everything wears out eventually.Cariboocoot wrote: »Basic principles: the closer to maximum current any electrical device is kept the more heat generated. The more heat, the shorter the lifespan.Cariboocoot wrote: »But as per my previous post a GT inverter should have a continuous rating of 'X' Watts, which means it is okay to run it there all the time and it won't have a negative affect on the expected lifespan.
No inverter runs at 100% capacity all the time. At best its output is at maximum for a couple of hours per day, though a bit longer for tracking systems. -
Re: How is the Optimum overloading of an inverter determined?
There is the old engineering rule of thumb of ~1/2 life for every 10C increase in operating temperature...
From my experience, I would rather have a "loaded" inverter in a cool room with good ventilation than a 1/2 loaded inverter in a hot room with poor circulation/in direct sun.
Besides the 10C 1/2 life rule, there is simple thermal cycling--Again keeping temperature swings low (cool basement/shaded outdoor installation) will certainly increase the life of your electronics (and batteries) too.
-BillNear San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset -
Re: How is the Optimum overloading of an inverter determined?That's conjecture; I've seen no data specific to inverters to support it. But even if it were true, in the financial analysis it might make sense to load an inverter to the gills even if that makes it wear out sooner if the added electrical production offsets the cost of the shortened inverter lifespan. Everything wears out eventually.
Not conjecture, reality. Proof is in my five decades of dealing with this stuff. Run it hot, it gives up quicker. End of argument.No inverter runs at 100% capacity all the time. At best its output is at maximum for a couple of hours per day, though a bit longer for tracking systems.
True. But here we're talking about the NEC's definition of "continuous use" which is, the last time I looked, 3 hours. So therefore this qualifies, especially when you consider we're talking about "over-paneled" inverters which would run closer to peak for longer times than one with a "standard" amount of panels. -
Re: How is the Optimum overloading of an inverter determined?
It isn't an argument, but that doesn't support the premise that a fully loaded inverter will fail sooner than one that is less so. I know that heat can cause electronic devices to fail but that is a generality. An inverter with zero loading might last forever. So what?Cariboocoot wrote: »Not conjecture, reality. Proof is in my five decades of dealing with this stuff. Run it hot, it gives up quicker. End of argument.
But even if it were so (which has not been established), the ROI benchmark should not be the chronological lifetime of an inverter but the number of kWh's it produces before it fails. -
Re: How is the Optimum overloading of an inverter determined?It isn't an argument, but that doesn't support the premise that a fully loaded inverter will fail sooner than one that is less so. I know that heat can cause electronic devices to fail but that is a generality. An inverter with zero loading might last forever. So what?
But even if it were so (which has not been established), the ROI benchmark should not be the chronological lifetime of an inverter but the number of kWh's it produces before it fails.
The issue goes back to my explanation of how the manufacturer is rating their devices for output. A good company will consider the heat generation and dissipation in the design and rate accordingly. A Xantrex XW6048 can produce 12 kW on surge - but only for 15 seconds. If it were forced to run at that level any longer it would suffer immediate component failure because the heat generated can not be dissipated fast enough. This is the same with all electronic components, even wire, regardless of the application. Read Bill's post. This isn't something made up to scare people; it is in fact a result of laboratory and real world testing on many types of devices over the decades that I've lived through and beyond. I've seen it myself with "identical" components where the only difference was the ability to dissipate heat. (There was even an instance where one unit got afternoon sun on it and the one next to it didn't. Guess which one quite working.) If this were not a real concern there would be no difference in Ampacity rating for the same wire in and out of conduit.
Rating them according to the kW hours produced before failure would also have to have the qualifier of the Watts output as in the above example where the XW can be pushed but at the expense of its lifetime. In general, giving an inverter an output rating of 'X' Watts and a warranty of 'Y' years pretty much does what you ask once you factor in daily production based on 'Z' hours of equivalent sun. So a GTI of 5kW rating that puts out that amount over 4 hours is 20kW hours per day or 7300kW hours per year * a 10 year warranty = 73 mW hours of lifespan. It's a numbers game and it depends on how the maker decides to apply the math. Warranties always have caveats about "typical usage". This is one reason why some people are skeptical about the Enphase 20 year warranty.
BTW, components that never get used also fail with time. Sometimes sooner, sometimes later. -
Re: How is the Optimum overloading of an inverter determined?
Yeah, it's different for a GT inverter, and that's all I deal with. Our design procedures call for them to be as loaded up as possible; a GT inverter's maximum output is what it is no matter how "overloaded" it is. Some clipping at peak production times is acceptable if increased off-peak production offsets it. -
Re: How is the Optimum overloading of an inverter determined?Yeah, it's different for a GT inverter, and that's all I deal with. Our design procedures call for them to be as loaded up as possible; a GT inverter's maximum output is what it is no matter how "overloaded" it is. Some clipping at peak production times is acceptable if increased off-peak production offsets it.
See that makes sense!
A good company would build a 7 kVA inverter and limit its current so the output was 5kVA and know that it would last at full capacity for years. That's how it should be done.
But as you know there are a lot of "shadow" companies out there that would build a 5kVA inverter and expect it to be operated at less than full capacity and so limit the warranty returns.
This happens all the time in off grid systems. One advantage GTI's have over OGI's is in this area; they supply current according to what's available from the panels rather than the batteries (except hybrids) so there is a sort of 'self limiting' factor there.
All of which goes to show there are specifications, and then there are specifications. A reputable company's you can believe. -
Re: How is the Optimum overloading of an inverter determined?
The way that they do it is to clip the DC current input. If an inverter's DC current limit is 100A, for example, when the available Imp from the array exceeds 100A, the MPPT function of the inverter pushes the load point to the right and down the IV curve to the point where I=100A.Cariboocoot wrote: »See that makes sense!
A good company would build a 7 kVA inverter and limit its current so the output was 5kVA and know that it would last at full capacity for years. That's how it should be done.
But as you know there are a lot of "shadow" companies out there that would build a 5kVA inverter and expect it to be operated at less than full capacity and so limit the warranty returns.
This happens all the time in off grid systems. One advantage GTI's have over OGI's is in this area; they supply current according to what's available from the panels rather than the batteries (except hybrids) so there is a sort of 'self limiting' factor there.
All of which goes to show there are specifications, and then there are specifications. A reputable company's you can believe.
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