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Old 05-05-15, 07:46 AM   #1
stevehull
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Default Choosing correct size PV panel for inverter

Am looking at a lot of solar PV packages. Have decided to go with the Enphase microinverters for a variety of reasons.

But here is the quandry. A lot of selling sites couple the M250 inverters with panels that seem too small in wattage.

Let's look at the smaller M215 model for a bit. The guaranteed specs say that it can handle up to 250 watts in and will put out 215 watts. But performance data, from lots of people, shows that the M215 will put out 240 watts and some claim 250 watts out without clipping. Let's focus on this performance (real world) data.

We also know that the nameplate PV output and real world output are different. There are immediate losses of 10% and then degradation with time (10-20%).

So, let' take a 280 watt rated panel. Fresh out of the box it will put out (at best) 252 watts (0.9*280). The wiring drop is insignificant as the inverter is mounted behind the panel, but the inverter has a 96% efficiency. Thus, the output of the M215 would be 242 watts (252*.96). This is below the clipping value that many see in real world applications.

Furthermore, with time, the panel will degrade. At one year most panels have a further 10% loss so that panel that put out 252 watts, fresh out of the box, will now only put out 227 watts (252 * .9). Again, well within the M215s performance range.

The difference in cost between a M215 and a M250 is about $20. I will likely be putting up 35-40 panels, so this is not an insignificant amount of $. Additionally, the cost per watt of 300 watt, or higher wattage panels, is a lot higher than the 280 watt panels.

So why are sellers bundling 260 watt panels with M250s? (I suspect their bottom line, not mine is the factor).

Seems like I should couple M215 microinveters with a 280 watt panel . . .

Comments?


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Last edited by stevehull; 05-05-15 at 08:43 AM..
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Old 05-06-15, 12:21 PM   #2
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Quote:
Originally Posted by stevehull View Post
Am looking at a lot of solar PV packages. Have decided to go with the Enphase microinverters for a variety of reasons.

But here is the quandry. A lot of selling sites couple the M250 inverters with panels that seem too small in wattage.

Let's look at the smaller M215 model for a bit. The guaranteed specs say that it can handle up to 250 watts in and will put out 215 watts. But performance data, from lots of people, shows that the M215 will put out 240 watts and some claim 250 watts out without clipping. Let's focus on this performance (real world) data.

We also know that the nameplate PV output and real world output are different. There are immediate losses of 10% and then degradation with time (10-20%).

So, let' take a 280 watt rated panel. Fresh out of the box it will put out (at best) 252 watts (0.9*280). The wiring drop is insignificant as the inverter is mounted behind the panel, but the inverter has a 96% efficiency. Thus, the output of the M215 would be 242 watts (252*.96). This is below the clipping value that many see in real world applications.

Furthermore, with time, the panel will degrade. At one year most panels have a further 10% loss so that panel that put out 252 watts, fresh out of the box, will now only put out 227 watts (252 * .9). Again, well within the M215s performance range.

The difference in cost between a M215 and a M250 is about $20. I will likely be putting up 35-40 panels, so this is not an insignificant amount of $. Additionally, the cost per watt of 300 watt, or higher wattage panels, is a lot higher than the 280 watt panels.

So why are sellers bundling 260 watt panels with M250s? (I suspect their bottom line, not mine is the factor).

Seems like I should couple M215 microinveters with a 280 watt panel . . .

Comments?


Steve
I'm not sure where you are getting this 10% in the first year degradation but I haven't come across that information. Please refer to this document from the National Renewable Energy Laboratory showing >20 year outdoor exposure at .43% median losses per year. That's 8.6%. There are other figures in there that suggest a little higher but nothing over 1% with silicon panels.
http://www.nrel.gov/docs/fy12osti/51664.pdf

I can't speak with any real world data on Enphase 215 watt units and their outputs but if your sample size is high saying that they put out more than their specsheet says, it seems that's sounds.

You mentioned there aren't line losses but there are. They would be calculated on the AC side for whatever gauge wiring you are using.

Enphase inverters are expensive, I still think Solaredge has a lower cost solution for you and I don't see any disadvantages to it.
Example: SolarEdge SE10000A-US-U Inverter $2276.48
SolarEdge SE10000A-US-U Inverter
Plus $73.34 per optimizer
SolarEdge P300 300W Power Optimizer w/MC4 connectors

Total cost would be $5210.08 from that vendor, could be less from other places. You would have no clipping issues with this system so your output would be higher, even if marginally so.

Emphase is about $216 per M215 based on the cheapest price I could find on a quick search but don't forget to cost in the monitoring system that you have to pay extra for. Solaredge includes this for free.

This would allow you to have 40 panels with even some room for expansion there and you run the 8-18(limit 5250 nameplate watts per run otherwise you could do up to 25) panel wiring runs back to the inverter which would be under 14 amps on the DC side(my system runs at 370v even though the spec is 350v nominal, read SolarEdge docs for an understanding of why the voltage is fixed if you are interested). If you wanted you could use two inverters at half the size but I personally don't see the point.

A system like this would allow you to avoid climbing up to the roof and pulling panels to get access to a failing Enphase inverter when the electrolytic capacitors fail, which from solar installers I've been talking to is happening often enough to get them away from Enphase because they hate the warranty work. I think running them on the edge of clipping or at their clipping point or otherwise above their 215 watt continuous output power and 270 watt recommended maximum module nameplate wattage could be asking for a higher likelihood of unit failures.

Just my opinion, you'll go with what feels best to you.
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Old 05-06-15, 01:09 PM   #3
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The Enphase M190 have a high failure rate. The M215 and M250 have been doing great.

The M215 cost is around $118-$129 each this puts the cost at almost a wash once you add the Envoy. But SolarEdge inverter is only warrantied 12 years Enphase is 25 years.
Envoy is only warrantied for two years.
The M215 will top out a 225 watts I can show many graphs even with my own panels.
http://enphase.com/global/files/M215_DS_EN_60Hz.pdf


The panels do not degrade as fast as published I can still put out 225 watts this time a year with over 2 years old 230 watt panels. I just had my highest max power output in just over 2 years.
The M250 tops out @258 watts.

http://enphase.com/global/files/M250_DS_EN_60Hz.pdf



I am not sure where you are getting your max output numbers from but the ones I posted are from Enphase. I can confirm the M215 number is right on the money.

I would no pair M250 with a 260 watt module. I think they are doing that just to make money.

Close out modules have been 250-260 lately anyway.
They claim 1% loss a year for solar modules most people are not seeing this much loss.


Here is a screen shot on May 27 my panels are just over 2 years old.
The top row are 230 watt panels and the bottom array are 240 watt panels but are at less of a angle as the top. All have M215 microinverters.


I don't get any clipping at all but a bigger module is a better pairing with the M215 a couple years ago I went with what was on sale.
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Old 05-07-15, 07:42 AM   #4
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Hey Pinnball,

You have the answer - an extra M215. Can you do an input output curve to see where clipping occurs in an actual unit? Other sites have had posts where owners assert that clipping does not start until 250 watts input.

Not hard, just vary the input voltage, monitor current and look at the output to see when clipping happens.

Thanks!



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Old 05-07-15, 11:04 AM   #5
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On April 16 2014 my array produced 92.310 kWh
Here is a graph of one microinverter on that day. This is a 240 watt panel.
As you can see it tops off at 225 watts just like the doc says.


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