How did my local solar company do?

[SunPWR]

I recently had a 10 kW grid-tied solar system bid by a local electrical company that also specializes in solar. I live at:
48.5126° N, 122.6127° W and they specified the following system:

25 400W panels with micro-inverters:
Silfab 400HC+
Enphase IQ7+

I'm far from being a solar expert but I asked about the potential for clipping with micro-inverters rated at 295W with 400W panels. The person who specified the system said if I wanted less clipping and didn't mind spending more, I could go with Enphase IQ8H for about $100 more per micro-inverter. When I looked at the specs I noticed the latter had a higher minimum voltage. I don't know if that will limit my production under heavy cloud cover or not. How do I weigh the clipping potential during optimum conditions against the minimum voltage (which I assume might limit my production at the low-light threshold? I'm simply not confident enough that the person spec'ing the system knows the details well enough to get a solid answer.

I'm not really doing this to save money (although it will in the long run). I am more interested in helping the grid use less energy from traditional sources so low-light production early and late in the day and in the winter is a bit more important to me than peak generation or total annual production. I'm a firm believer that the current rules around net metering (at least in Washington State) are not necessarily incentivizing solar panel installs tailored to the most beneficial production. I'm glad solar is being incentivized, but I think it's important to incentivize morning, evening and winter production so we don't end up with such a huge surplus of mid-day energy while neglecting first light, late afternoon and winter production.

The town I live in is in the rain shadow of the Olympic Mountains so it's actually considerably sunnier here than even 15-20 miles away and it has a strong influence from the marine breezes that will help peak production during warm weather. I doubt this is taken into account in the models used to specify solar systems because it's a very local effect. In short, it's brighter and cooler here.

I realize no one can design an ideal system without more information than what I have provided, but I'm looking for feedback on how these particular panels mated to the two micro-inverters listed (or any other Enphase options that might improve the system). The panels themselves are manufactured locally but they are a new model, so we only have the brand name and the specs to go on.

TIA for any informed feedback about any of the mentioned aspects.

 

[foggysail]

My installation plan originally had IQ8+ inverters for each of 28 Canadian 400W panels. The more I thought about it the more I did not like it! The idea behind selecting 290W (continuous) inverters sort of matches the NMOT ratings for 400 watt panels but 400 watt panels CAN and do produce more than 290W.

So I upgraded my inverters to the ''A'' models that for all practical purposes are adequate to minimize clipping without spending for the more expensive ''H'' models. Only once did I see clipping caused by the inverters and I doubt I will see much as time passes.

I suggest you do research on STC, NOCT, NMOT and PTC to get a better understanding of what to expect from your panels. That will help you decide which inverter you will be happy with.

 

[SunPWR]

My installation plan originally had IQ8+ inverters for each of 28 Canadian 400W panels. The more I thought about it the more I did not like it! The idea behind selecting 290W (continuous) inverters sort of matches the NMOT ratings for 400 watt panels but 400 watt panels CAN and do produce more than 290W.

So I upgraded my inverters to the ''A'' models that for all practical purposes are adequate to minimize clipping without spending for the more expensive ''H'' models. Only once did I see clipping caused by the inverters and I doubt I will see much as time passes.

I suggest you do research on STC, NOCT, NMOT and PTC to get a better understanding of what to expect from your panels. That will help you decide which inverter you will be happy with.

Thanks for that, and I am trying to compare panel specs with inverter specs. I have a reasonable handle on the clipping during high irradiance periods but am kind of clueless when it comes to getting a feel for how the low voltage cutoff during low irradiance periods impacts production. I don't want to increase mid-day production at the expense of early, late and winter production unless the impact is minimal during low irradiance periods. We get a lot of low irradiance periods through the year as well as some very high irradiance periods when it's not particularly hot.

 

[foggysail]

You should look at your panel's V-I graphs. Panels are current sources, not voltage sources. For example, a battery is a voltage source. I have not studied matching V to MPP. Inverters do that. The only issue that I can think of would be an open circuit voltage that could exceed the inverter's rating ..... for instance, panel voltages change with temperature. OH back to current source......notice your panels will output almost the same current regardless of the load even a short circuit.

 

[SunPWR]

You should look at your panel's V-I graphs. Panels are current sources, not voltage sources. For example, a battery is a voltage source. I have not studied matching V to MPP. Inverters do that. The only issue that I can think of would be an open circuit voltage that could exceed the inverter's rating ..... for instance, panel voltages change with temperature. OH back to current source......notice your panels will output almost the same current regardless of the load even a short circuit.

I can't find any graphs for my panels on the manufacturer's website. There is something called a "pan" file, but I don't have the right software to open it. Will the V-I graphs be in the pan file? I'm not sure I know how to interpret it anyway. Does the V-I graph have light intensity on one of the axes?

 

[DThames]

What tilt angle are you on your panels? That far north, if you are on a rather shallow roof angle, you may not see a great angle most of the year, so clipping wouldn't be as much as a ground mount with a better angle.

 

[foggysail]

What tilt angle are you on your panels? That far north, if you are on a rather shallow roof angle, you may not see a great angle most of the year, so clipping wouldn't be as much as a ground mount with a better angle.

Roof angle is 28 degrees. What bothers me most is my neighbor who will not allow me to cut her trees that block the sun starting around 1 in the afternoon beginning at the end of September.

My panels went online 20 October and so far have produced 766KWH. During this short period our daylight hours have dwindled from 10h 54m to 9h 37m with lowering sun angle. I just paid my October electric bill most of the time was without activated panels......$6.50. The shortest daylight hours will be 9h 5m beginning on 18 December and starting to increase on the 27th with 9h 6m. Things will rapidly change beginning then.

 

[DThames]

Do you have module level data and if so can you see clipping? At that tilt angle and as far north as you are, I wonder what you see on a good day, per panel.

I am running APSystems QS1 units that have 300 watts per channel/panel. I have 340w and 355w panels. Those units report more than 300w at times and don't seem to clip. However, they shut completely off at times when the production is very high. It has to be pretty cool and the sun pretty well aligned to get over 300w with those panels. I am at 35degrees with a well tilted ground mount.

 

[foggysail]

NO to individual panel's live KW, I can see module individual performance but only for produced power not producing (Wh) power. My system is Enphases so if you are familiar with it you know exactly what one can see. Why is your system shutting down??? That is strange.

I am stuck getting the best I can with low sun angles, shorter days, neighbor's trees but overall it is doing what I need. That is helping me to avoid paying for electric power consumption.

 

[Hedges]

I could go with Enphase IQ8H for about $100 more per micro-inverter.

You haven't told us the price of the originally proposed system.
$900 to $1200 per panel? In that case a 10% price increase; will it increase power output 10%?

Ignoring the minimum voltage issue, if higher wattage microinverter harvests more power during hours and months when panel output is greater, that is worth some amount of money. But over-paneling and allowing clipping is another way to get more power in exchange for money.

In summer, higher temperatures will reduce power output, vs. winter colder temperatures increase it.
Tilt of panels will affect output vs. season, also orientation vs. time of day.
You can find insolation calculators that consider your location and panel angle, show power through the day. Estimate area of curve which will clip.

An inverter rated 75% of panel STC rating won't clip that much.

https://silfabsolar.com/wp-content/uploads/2022/10/Silfab-SIL-400-HC-Data-Generic-20221017-Final.pd

"STC 400W, NOCT 298W"

https://solarequipment.energy.ca.gov/Home/PVModuleList?ManufacturerID=Silfab+Solar+Inc.

Similar model SIL-400HU "PTC 375.6W"

The PTC is surprisingly close to STC.
If it produced NOCT, you'd rarely see clipping, but PTC would have some more.
Still, that only happens part of some days.


Would you want to keep you electricity on during power outages in the future? If so, the time to plan such a system is before you buy PV. Know the capacity and price, not just that it is available. Various brands offer alternative equipment or add-on equipment for battery storage and transfer switch. Some brands may play together.

Do you understand what utility rate schedules you will have once you install solar, how they will credit you for power exported to the grid?
Some are net metering, get back 1 kW for every 1 kW exported (often zeroed once per year.) Some credit and charge $$ at different rates throughout the day (time of use.) Some credit at 25% of retail, charge 100%. In California, a time of use net metering schedule is available for systems granted permission to operate by April, then grandfathered for 20 years. Systems missing the deadline will have 25% credit.

Your electric company doesn't care whether or not this system will result in you subsidizing your utility company and low-income customers, only that they make a sale.

 

[foggysail]

I deliberately spent extra $ for both inverters and panels. No way do I require 11,400 panel watts with one exception...... TREES! Yeah, trees that are not mine, I already got rid of mine and I am stuck with an unfriendly neighbor. And yes, Wifey was adamantly against my installation but life is short.

General comment-- NOCT is panel cells temperature which gets complicated IMHO if related to power unless seasons and locations are also considered.

 

[SunPWR]

You haven't told us the price of the originally proposed system.
$900 to $1200 per panel? In that case a 10% price increase; will it increase power output 10%?

Ignoring the minimum voltage issue, if higher wattage microinverter harvests more power during hours and months when panel output is greater, that is worth some amount of money. But over-paneling and allowing clipping is another way to get more power in exchange for money.

In summer, higher temperatures will reduce power output, vs. winter colder temperatures increase it.
Tilt of panels will affect output vs. season, also orientation vs. time of day.
You can find insolation calculators that consider your location and panel angle, show power through the day. Estimate area of curve which will clip.

An inverter rated 75% of panel STC rating won't clip that much.

https://silfabsolar.com/wp-content/uploads/2022/10/Silfab-SIL-400-HC-Data-Generic-20221017-Final.pd

"STC 400W, NOCT 298W"

https://solarequipment.energy.ca.gov/Home/PVModuleList?ManufacturerID=Silfab+Solar+Inc.

Similar model SIL-400HU "PTC 375.6W"

The PTC is surprisingly close to STC.
If it produced NOCT, you'd rarely see clipping, but PTC would have some more.
Still, that only happens part of some days.


Would you want to keep you electricity on during power outages in the future? If so, the time to plan such a system is before you buy PV. Know the capacity and price, not just that it is available. Various brands offer alternative equipment or add-on equipment for battery storage and transfer switch. Some brands may play together.

Do you understand what utility rate schedules you will have once you install solar, how they will credit you for power exported to the grid?
Some are net metering, get back 1 kW for every 1 kW exported (often zeroed once per year.) Some credit and charge $$ at different rates throughout the day (time of use.) Some credit at 25% of retail, charge 100%. In California, a time of use net metering schedule is available for systems granted permission to operate by April, then grandfathered for 20 years. Systems missing the deadline will have 25% credit.

Your electric company doesn't care whether or not this system will result in you subsidizing your utility company and low-income customers, only that they make a sale.

The unsubsidized system price is just under $1200/panel. The more powerful inverters will increase system cost by 8% and I'm pretty sure I won't get 8% more total generation. I would fork out the extra money if I knew it wouldn't impact the generation during marginal solar production hours, but I think the specs of the higher output inverter with a higher minimum voltage indicates it would "clip" on the low end (but I still haven't received a clear answer to that). That's mostly what I'm looking for.

Most of the panels will be due E and W facing at a relatively shallow angle (about 25 degrees) which is one reason I want to maximize production during marginal hours when other panels in the region are not contributing. It's not all about money savings, I will be happy if I break even but since I have full net metering with an annual reset I will do better than that. I don't have room for more panels so I will only be able to offset about 75% of my annual consumption and I'm not interested in generating during outages.

How does the minimum voltage spec of the inverter affect generation during marginal hours of generation? I can't seem to find graphs specific to these panels that will answer that question for me. I think I would like the Enphase inverter that will perform the best under low light levels (assuming the clipping on the high end is not too excessive).

 

[Shimmy]

Personally, I think over-paneling is a good investment. It is helpful to have a fairly long peak production window on the AC side, and it is nice to make sure that cloud coverage still produces a consistent minimum amount of energy. It really comes down to your weather patterns... and there isn't just one type of location that microinverters work really well.

 

[Hedges]

Most of the panels will be due E and W facing at a relatively shallow angle (about 25 degrees)

That can make a string inverter more attractive. If East and West facing strings are same length, they can be paralleled onto a smaller MPPT without clipping, because each peaks at different time. Biggest downside of string inverters is you need RSD modules, and added cost, but string inverters cost less than microinverters.

How does the minimum voltage spec of the inverter affect generation during marginal hours of generation? I can't seem to find graphs specific to these panels that will answer that question for me. I think I would like the Enphase inverter that will perform the best under low light levels (assuming the clipping on the high end is not too excessive).

On a hot day the voltage from PV panel will drop lower. Study PV panel data sheet curves and adjust with temperature coefficient, see if they will drop too low. Some planning tools I've used (for SMA) calculated cold and hot performance. See what you can get from data sheets.

One of the things I like about string inverters is you can adjust number of panels in series to fit voltage limits.

 

[400bird]

I'd skip the extra cost. If your panels are east/west at a shallow angle, it's unlikely you'll see much of those perfect days where the 295 watt micros will clip.

Even when/if you do see some clipping it's not too to be huge loss of overall production. If it clips for 2 hours, down from 330 to 295 watts, that is 875 watt hours. 0.875 kWh
Not much loss there.

I'm not an Enphase expert, there may be other benefits to the newer IQ8H inverters. But, not knowing those and going by the information provided, I'd stick with the less expensive inverters.

 

[svetz]

...10 kW grid-tied solar system...48.5126° N, 122.6127° W ... 400W panels 295W microinverters...live in is in the rain shadow of the Olympic Mountains...
How do I weigh the clipping potential during optimum conditions against the minimum voltage?

TL;DR: Easy. Use a program like SAM, it's free (well, paid for by U.S. Tax Payers).

Long Answer:

I doubt this is taken into account in the models used to specify solar systems because it's a very local effect.

SAM can do it if you have the right local data. As you said the only way to answer your question is to have a lot of experience with local light conditions which is more dependent on shade and weather than latitude and longitude. But, SAM handles both and has weather files for most places in the U.S., so it's just inputting your data to see what the average annual clipping will be. You can also very easily swap the IQ7s for IQ8s inside the program and rerun it to see if the extra cost is worth it. SAM is the kitchen sink of programs and very intimidating to get started with, I highly recommend the video tutorials. I haven't done it, but if you can't get compatible local weather files, or narrow enough for your mountain-shadow/rain area, you should be able to build it from local weather station data with solar data. We get ~44" of rain annually here, I've posted comparisons of my setup vs SAM in Predicted Accuracy: Sam Vs. Reality.

Minimum Voltage
You might also be interested in these posts: Power from panels during shade events, post#2, to sum them up - on my 340W panels when ambient light is < 40W/m² my IQ7s shut down. Efficiency starts to fall off < 200W/m², but it's not that much (e.g., ~6% loss at 40 W/m²). I'm at 25°N, have almost no tilt, and hardly ever see clipping (but it's also stormy here through summer when you'd expect clipping). Don't forget that the IQ7s are guaranteed for 295W, but given tolerances they typically are over-achievers and will go over the 295W, I've seen mine slightly over 300W.

Hope that's of some help!

 

[Elexide]

It is very common and beneficial to to oversize the amount of panels wattage compared to your string inverter or your micro inverter for this reason: you actually end up getting better system efficiency and better production because, like you touched on, even when it's super cloudy you're going to be running at a higher wattage from your panels which allows the micro inverters to operate as intended . What you have there with the 295 watt micro inverter to the 400 watt panel is a peak production of panel to micro inverter rating of 1-.75 this is a normal and beneficial design choice. Sure, you'll get some clipping but that waste is less than it would be to have underpowered solar feeding your microinverters when its cloudy.

 

[foggysail]

It is very common and beneficial to to oversize the amount of panels wattage compared to your string inverter or your micro inverter for this reason: you actually end up getting better system efficiency and better production because, like you touched on, even when it's super cloudy you're going to be running at a higher wattage from your panels which allows the micro inverters to operate as intended . What you have there with the 295 watt micro inverter to the 400 watt panel is a peak production of panel to micro inverter rating of 1-.75 this is a normal and beneficial design choice. Sure, you'll get some clipping but that waste is less than it would be to have underpowered solar feeding your microinverters when its cloudy.

I agree it is best to over panel-----

 

[foggysail]

TL;DR: Easy. Use a program like SAM, it's free (well, paid for by U.S. Tax Payers).

Long Answer:

SAM can do it if you have the right local data. As you said the only way to answer your question is to have a lot of experience with local light conditions which is more dependent on shade and weather than latitude and longitude. But, SAM handles both and has weather files for most places in the U.S., so it's just inputting your data to see what the average annual clipping will be. You can also very easily swap the IQ7s for IQ8s inside the program and rerun it to see if the extra cost is worth it. SAM is the kitchen sink of programs and very intimidating to get started with, I highly recommend the video tutorials. I haven't done it, but if you can't get compatible local weather files, or narrow enough for your mountain-shadow/rain area, you should be able to build it from local weather station data with solar data. We get ~44" of rain annually here, I've posted comparisons of my setup vs SAM in Predicted Accuracy: Sam Vs. Reality.

Minimum Voltage
You might also be interested in these posts: Power from panels during shade events, post#2, to sum them up - on my 340W panels when ambient light is < 40W/m² my IQ7s shut down. Efficiency starts to fall off < 200W/m², but it's not that much (e.g., ~6% loss at 40 W/m²). I'm at 25°N, have almost no tilt, and hardly ever see clipping (but it's also stormy here through summer when you'd expect clipping). Don't forget that the IQ7s are guaranteed for 295W, but given tolerances they typically are over-achievers and will go over the 295W, I've seen mine slightly over 300W.

Hope that's of some help!

You able to monitor individual inverter status? My Enlighten's Envoy allows displays of the system's real-time status but the IQ8A's arrays only show produced WH not realtime W.

 

[svetz]

You able to monitor individual inverter status? My Enlighten's Envoy allows displays of the system's real-time status but the IQ8A's arrays only show produced WH not realtime W.

SAM is a simulation tool for the prediction of power generated based on things like location, tilt, hardware. Enlighten provides actual system data.

With my IQ7s, "Live Status" shows Watts in Enlighten.

Or, with a token (blog on getting a token starts with this post), this query returns current power: https://envoy.local/api/v1/production (or if you want it per panel use https://envoy.local/api/v1/production/inverters). If you complete the free courses at the university to be an installer you can get access to tools more advanced than Enlighten that can provide High-Resolution data like this:

​

Here's data for an individual microinverter:

​