Oversizing inverters

[garybeck]

Hello, I know this subject comes up a lot.

and I understand that a 400 watt module never puts out 400 watts, so some oversizing is logical.

but don't you think if you are oversizing at 150% that you are going to get some significant clipping?

one question I have - what exactly is "clipping?" when an optimizer/inverter is "clipping" does it shut down completely or does it continue to put out the max output?

If it shuts down at the max inverter output, i would think this is very undersireable because it's right at the time when you would be getting the max output. wouldn't it just make sense once you get over 125% or so, to just get the next size inverter?

gary in vermont

[Mike 134]

I'm following this post because I don't understand the logic behind oversizing if all the panels are mounted on the same roof plane or are ground mounted and are very close to a due south azimuth. Multiple mounting planes (roof sections) I can understand as the sun moves from east to west you'd never generate the full wattage of the system.

I would think you'd also have to look at the size options of the inverter(s) to proper design the system. In my case I have 7.29KWs of panels on a single mounting plane with inverter size choices of either 6KW or 7.6KW. On any sunny day I'm always producing more than 6KWs for 1-1/2-2 hours either side of "solar noon", so had I not chosen the 7.6 I'd have been clipping all that energy.

my 2 cents

[J.P.M.]

I'm following this post because I don't understand the logic behind oversizing if all the panels are mounted on the same roof plane or are ground mounted and are very close to a due south azimuth. Multiple mounting planes (roof sections) I can understand as the sun moves from east to west you'd never generate the full wattage of the system.

I would think you'd also have to look at the size options of the inverter(s) to proper design the system. In my case I have 7.29KWs of panels on a single mounting plane with inverter size choices of either 6KW or 7.6KW. On any sunny day I'm always producing more than 6KWs for 1-1/2-2 hours either side of "solar noon", so had I not chosen the 7.6 I'd have been clipping all that energy.

my 2 cents

Mike:
Apologies if I've asked this question: What was the price differential between the 2 inverters ?

Thank you.

J.P.M.

[Mike 134]

Mike:
Apologies if I've asked this question: What was the price differential between the 2 inverters ?

Thank you.

J.P.M.

No worries about $400 dollars

[J.P.M.]

No worries about $400 dollars

Thank you.
Depending on how much clipping is involved and how the time value of the lost revenue is calculated, it may take a while to recover that lost revenue.

[bcroe]

Clipping is when the inverter puts out less energy than the panels
have available at that time. The inverter is not shutting down, it is
just moving to a less optimal operating point on the panel curve, so
less energy is delivered.

Clipping may be lost energy, but not always bad. Here the inverters
are set at 15KW by contract with the PoCo. More inverter capacity
is not an option, but the more hours a day I can keep them in clipping,
the more energy is delivered. Here they just hit clipping for 8 hours
a day. Bruce Roe

[J.P.M.]

Clipping is when the inverter puts out less energy than the panels
have available at that time. The inverter is not shutting down, it is
just moving to a less optimal operating point on the panel curve, so
less energy is delivered.

Clipping may be lost energy, but not always bad. Here the inverters
are set at 15KW by contract with the PoCo. More inverter capacity
is not an option, but the more hours a day I can keep them in clipping,
the more energy is delivered. Here they just hit clipping for 8 hours
a day. Bruce Roe

If everything in a system is operating normally and the inverter is unable to handle what the panels are capable of producing, the inverter can be said to be undersized.
If so that undersizing may be a design consideration based on the application for several reasons, some of them - such as your application - may be mandated by the POCO in an NEM setup.

Your situation mandates that your LCOE is lower than would be possible if you were not size limited through the max inverter size mandated by the POCO.
So you have multiple panel orientations that helps keep your max system output below 15 kW but does so for a longer time during a day.
Your output curve be said to be platykurtic (flatter over the day but with a lower maximum and shorter tails than an optimal system orientation might produce).
The penalty you have is that your cost goes up because you need more panels to produce the same quantity of electricity that one smaller but single and more optimally oriented array would produce. So, your cost goes up but your annual production per installed STC kW of panel is lower than a single optimally oriented array would produce.

Or, where my question to Mike was going, an undersized inverter may be an economic decision such as when, for example, an array is occasionally capable exceeding max inverter output by a small amount and the calculated differential NPV of the next larger inverter cost is greater than the calculated NPV of the lost electricity as when, for example, the bigger inverter will allow, say, $10 more/yr. in electrical generation bill offset, but cost, say, $400 more up front.

Every application is somewhat different.

[garybeck]

Thank you.
Depending on how much clipping is involved and how the time value of the lost revenue is calculated, it may take a while to recover that lost revenue.

It depends on who's perspective. If you are the homeowner, your lifetime savings from solar is usually in the tens of thousands of dollars. I'd have to do the math but i would bet the savings far outweigh the cost over the lifetime

[garybeck]

does anyone know if Solaredge's online design tool takes into account clipping? I mean if I play around with the design and try two different inverters, and leave everything else the same, will it show me the estimated affect of clipping?

[J.P.M.]

It depends on who's perspective. If you are the homeowner, your lifetime savings from solar is usually in the tens of thousands of dollars. I'd have to do the math but i would bet the savings far outweigh the cost over the lifetime

It's any potential system owner's perspective who is examining potential clipping loss from an inverter who's capacity is less than the panel's maximum capacity is likely to be for the application.
I'm not referring to total system savings, only the NPV of the cost differential in the 2 inverters vs. the NPV of the electricity not lost by spending additional $$ on a larger inverter that would eliminate the clipping loss.
Doing that math is where most folks get lost. Using the moron method, if the value of that eliminated clipping is worth, say, $10/yr. and the larger inverter costs $400 MORE, it'll take $400/($10/yr.) = 40 yrs. to break even, and so perhaps/probably not a wise investment.
Off topic, potential total system savings over a system's lifecycle may be substantial but probably not the way most residential systems are (not) sized, designed and assembled.

[Mike 134]

It's any potential system owner's perspective who is examining potential clipping loss from an inverter who's capacity is less than the panel's maximum capacity is likely to be for the application.
I'm not referring to total system savings, only the NPV of the cost differential in the 2 inverters vs. the NPV of the electricity not lost by spending additional $$ on a larger inverter that would eliminate the clipping loss.
Doing that math is where most folks get lost. Using the moron method, if the value of that eliminated clipping is worth, say, $10/yr. and the larger inverter costs $400 MORE, it'll take $400/($10/yr.) = 40 yrs. to break even, and so perhaps/probably not a wise investment.
Off topic, potential total system savings over a system's lifecycle may be substantial but probably not the way most residential systems are (not) sized, designed and assembled.

Don't forget location, place like Phoenix has 310 days of sunshine, my area 180 days. With a going rate of .15 KWH, lets guess I gain 2 KWH on a sunny day which is not unreasonable seeing my bell curve hit 6KW 2 hours before solar noon on the "good months" of cooler temps and higher sun angles, 1 hour on the hot summer days, so rough guess $54/year gained by not clipping. Note about my system, all on the same 33 degree roof plane, with an azimuth of 176 degrees.

The Best (and Worst) Cities for Endless Sunshine | Move.org

[J.P.M.]

Don't forget location, place like Phoenix has 310 days of sunshine, my area 180 days. With a going rate of .15 KWH, lets guess I gain 2 KWH on a sunny day which is not unreasonable seeing my bell curve hit 6KW 2 hours before solar noon on the "good months" of cooler temps and higher sun angles, 1 hour on the hot summer days, so rough guess $54/year gained by not clipping. Note about my system, all on the same 33 degree roof plane, with an azimuth of 176 degrees.

The Best (and Worst) Cities for Endless Sunshine | Move.org

Understood. Headed for my 2d home in the La Quinta near Palm Springs that has weather and insolation that's the twin of Phoenix.

Since 10/13/2014 (1 yr. after startup and 1st yr. burn-in) my array in N.County San Diego has produced 80.9% of the energy that it would have produced if every day was cloudless with otherwise TMY weather data.

There's a way to get a decent 1st est. of clipping and lost revenue by using PVWatts and the appropriate POCO tariff schedule. Hollar if you want details.

[nerdralph]

SMA Sunny Designer and OpenSolar both will estimate clipping losses.
I target 0.2% to 0.5% clipping losses for most PV systems.

[foggysail]

Mine is an AC system using Enphase's IQ8A inverters. I moved away from the IQ8+'s just prior system's installation which was completed last September and activated in late October AND AM VERY PLEASED that I did. I cannot monitor individual inverter performance and do believe that I might be getting clipping at times. Half of my panels face east while the other half face south. My home is in Massachusetts, about 30 miles west of Boston.

Now that the sun angle is getting higher I frequently see around mid day my 28 400 watt Canadian panels producing 10.1-10.2 KW. That is an average/inverter of 360-364KW!! And yes, I understand that the IQ8A inverters are max rated at about 366W, 349 continuous. Very happy with my system.

[J.P.M.]

Mine is an AC system using Enphase's IQ8A inverters. I moved away from the IQ8+'s just prior system's installation which was completed last September and activated in late October AND AM VERY PLEASED that I did. I cannot monitor individual inverter performance and do believe that I might be getting clipping at times. Half of my panels face east while the other half face south. My home is in Massachusetts, about 30 miles west of Boston.

Now that the sun angle is getting higher I frequently see around mid day my 28 400 watt Canadian panels producing 10.1-10.2 KW. That is an average/inverter of 360-364KW!! And yes, I understand that the IQ8A inverters are max rated at about 366W, 349 continuous. Very happy with my system.

A question please: What's the slope of the array orientations ?

Thanx.