What would cause my solar graph to plateau? It’s showing this way since march 28
How long have you had solar installed and have you seen a full standard solar curve on a sunny day ? One of two things might be causing this:
- Your inverters are clipping - perhaps undersized for your solar. Have seen several Sense users with this issue.
- Your Solar CTs are clipping - somehow calibration didn’t go right when you first set up your solar.
I would investigate the first possibility first.
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What’s your max solar output? Is this a string inverter or micro-inverters? Looks like clipping which is where the inverter has reached the max output. If you have a string inverter and that’s not your normal max- you probably have a string down. If that’s not your normal max and you have a/some micro-inverter down.
The max my string inverter will produce is 5050w and it has 6200w of modules attached. Here was the other day which was partially cloudy.
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Ok so my solar was installed in October. I have been in winter and none of my days have shown the curve peak over 4000 until now. So now that I am starting to get more sun it is plateauing at 4400.
I can’t monitor my micro invertors. The solar company never gave me the password to that device.
Should I call them?
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I would defiantly call them. Most modern micro-inverters have their own monitoring system. Sounds like your max is going to be in the 4400w range. If you’re in the US, you’ll see as we get more sun/ higher sun … you’ll hit that 4400w sooner in the morning and hold it longer in the evenings.
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The golden rule seems to be that the PV panels can be up to 125% of the (micro) inverter rating.
I assume you have Enphase micro’s, then you should also have monitoring.
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This is what the plans called out:
Suntech STP3705 B60WNHB Modules with Hoymles HM-1200 Micro Inverters
I was mostly surprised to see the clipping since I’m in a northern state, but I’m a novice to solar power. I hope the solar company did its math right and when I hit longer sunshine days the power generated compensates for the clipping. Time will tell as I have yet to go through a summer yet with solar.
Clipping occurs when the raw panel output exceeds the maximum capacity of the inverter(s), also taking to account DC to AC power conversion losses. This means there is a choice to be made for the system designer. They can dimension the inverter to have enough capacity to avoid clipping altogether, or they can design to have a certain amount of clipping during maximum panel output.
It is fairly universally true that all inverters (micro, or per panel, or string inverters) become more expensive (even on a per rated kW basis) as their capacity increases. This fact is utilized by most system designers to tolerate a moderate amount of clipping, during typical peak solar hours, under ideal conditions (for your location), to save some money there on the inverters, while making sure that the theoretical value of the lost production (during clipping) is (much) less than the cost savings on the overall system.
My system is 8.4 kW max rated output over 24 panels, using micro inverters (Enphase). I only see clipping during some spring/summer months, and even then only on some days, and for a few short hours (it requires ideal weather and atmospheric conditions). It looks like this on my graphs (this is not sense output but comes directly from the inverters):
This is a graph (for a group of 6 panels - different colors), taken on a day in April where some clipping occurred (thee 274W number applies to the vertical line in the plot, not the moment of clipping). My panels rated max output of 360W and you see some clipping in the 317W range (the rated max output of the inverter is 315 VA). During the period close to the summer solstice, the same kind of clipping would begin a little earlier, and end a little later.
The “loss” due to clipping is the area under the ideal curve (as if there were no clipping), but above the clipped level. Certainly in my case this is but a tiny percentage compared to the total under the curve and would only apply on days when actual clipping happens. Due to the application of micro inverters, each panel can have its own clipping losses. With string inverters, from the way I understand it, you would see the effect of the worst performing panel (on any given moment), across all panels on the same inverters. My estimated annual power losses due to this are in the 2-3% range. All things considered this is significantly less than shading related losses that I incurr at times (although having micro inverters as opposed to string improves that a lot).
Your clipping in the first graph seems to happen from about 1:30 through 3:30. Assuming this was while on DST it was more like 12:30-2:30 solar time, which seems the expected time frame. Since there is no scale on the graph and power output varies greatly during solar hours in this example, it is hard to discern when your theoretical peak output (without clipping) would have been (it can be calculated though, with more information about your system). It does strike me that you did not have full sun all day long, and if you had, the clipping area would have been wider. The second graph seems to confirm this. It seems a bit more excessive than mine, but it is hard to tell if it is unreasonable (which would indicate a bad system design, or some failing component(s)). Based on what I see, and without calculating, it does not seem unreasonable.
Based on your system specs it would seem that the potential for clipping with your equipment is just slightly higher (370W panels or 4*370W max, 1200 max inverter output (4 panels; about an max difference during peak of about 18%, whereas mine is more like 12%). Note you have a solution which is one inverter per 4 panels, so not quite a per panel, but also not a string.
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This has been a very enlightening conversation. I will be calling the solar installers to explain.
Clipping, now that it has been explained makes sense, but I feel like I shouldn’t be seeing clipping in April already. This feels like I’m wasting a lot of potential power if the clipping starts happening so low on the curve.
Thanks all for your input.
Although notice my example graph was from April too.
I have chosen to oversize my PV panels with my Fronius Primo 3800 watt string inverter .
So yesterday I saw this:
It maxes out at 3822 Watt for hours.
Reason is that I have a lot of dirt cheap panels available
(luxury problem! I know)
This article is worth reading imo:
They give an example with a 122% oversized enphase micoinverter:
The clipping appears to reduce the potential output of the module by approximately 12 watts during the middle of the day, however numerically the actual energy lost on this day can be calculated to be approximately 19.9Wh, or 1.01% of the total energy generated.
If we assume the energy from the solar system is being used to offset energy purchased from a utility at $0.25/kWh then the value of all of the energy lost due to clipping over a year is 94 cents per module. (or $18.8 per year on a typical 20 panel, 6kW system).
There is obviously no benefit in selecting microinverters that increase the capital cost of a system by $30 per module in order to harvest an extra 94 cents of electricity from each module each year. This extra investment would take 32 years to pay back!
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