Confusion re module rating, microinverter rated output power, and PVWatts.

[wexcellent]

Hi, I am a newb trying to design and install my own grid-tie system. I have seen this addressed in some other posts but it is still confusing to me. I have a quote to install Enphase M215 microinverters with Hyundai 290w modules from a reputable local company. The M215 spec sheet says that they are rated for 215W continuous power (225W peak). I watched an Enphase video on their newer M250 microinverters which said that they were best matched to a 330W module, even though they are rated for 250W. I don't understand why this is. I am assuming that when I go to PVWatts and put in my DC system size that this is based upon the rated wattage of the module I am using (290W). But how does this make any sense when the microinverter I am using has an output of 215W (AC)?

Thanks for your help, you guys rock!

[J.P.M.]

Understand that a module's STC rating will rarely be achieved in day/day operation and why, and you will be on the way to ending your confusion.

Read the PVWattts help/info screens, including the part about DC to AC size ratios. Also, download a free but somewhat outdated copy of "Solar Power Your Home for Dummies" from the net, or get an updated hardcopy at bookstores/Amazon for ~~ $20. Some of the scales of solar confusion will fall from your eyes.

All that said, an M215 micro seems undersized for a 290 STC Watt module. The 250 Watt micro seems like it would be a better choice for a 290 Watt module, but still undersized for a 330 W module.

[wexcellent]

Thanks, I will have a look. Here is the Enphase video that recommends pairing the M250 with a 330W module: https://www.youtube.com/watch?v=Vx0dzxvfw4M. I don't know why the reputable company I have the quote from paired the M215 with a 290W module, but that is what they did.

[wexcellent]

J.P.M., did a search for Solar Power Your Home for Dummies in a free format and it looks like all the links have been disabled! Just an FYI

[ButchDeal]

Thanks, I will have a look. Here is the Enphase video that recommends pairing the M250 with a 330W module: https://www.youtube.com/watch?v=Vx0dzxvfw4M. I don't know why the reputable company I have the quote from paired the M215 with a 290W module, but that is what they did.

That is over 3 years old. It would be far better to par a 330w with the newer Q series or even better to go with solaredge.

[J.P.M.]

J.P.M., did a search for Solar Power Your Home for Dummies in a free format and it looks like all the links have been disabled! Just an FYI

OOPS ! You seem to be correct. Guess we'll need to spend 20 bucks for the 3d ed. The free ride is over.

Thank you for the correction and information.

Even at $20 or so, still a value.

J.P.M.

[J.P.M.]

Thanks, I will have a look. Here is the Enphase video that recommends pairing the M250 with a 330W module: https://www.youtube.com/watch?v=Vx0dzxvfw4M. I don't know why the reputable company I have the quote from paired the M215 with a 290W module, but that is what they did.

Perhaps your opinion of their reputability is not warranted. Happens all the time.

[inspron]

from a reputable local company.

I would question the assertion this installer as being reputable. How big is your system and what city are you in?

If you are very far north, it is possible that M215 could work since you are unlikely to achieve outputs in excess of 215W even in the summer with the 290W panels.

But if you are in the south, this pairing sounds like your installer trying to dump old inventory of microinverters now that IQ models are out.

[ImInPhxAZ]

I'd ask for an updated quote with iq6. Remember the panel rating is peak output also. Perfect conditions, perfect angle. Pop one panel in pvwatts with the orientation you plan to see some real world and look at hour by hour. Chances are you won't miss much.

[J.P.M.]

If you are very far north, it is possible that M215 could work since you are unlikely to achieve outputs in excess of 215W even in the summer with the 290W panels.

For arrays with close to an optimal orientation with respect to tilt , latitude has little to do with it until the location gets ~ > 60 N. or S. latitude.

One example: Portland ME:

PVWatts

M215 on a 290 W panel will clip 330 hrs. of 4510/yr., 78 kWh of 417 kWh/yr., or ~ 18% loss of total possible output.

M250: on a 290 W panel will clip 66 hrs. of 4510 hrs., 17 kWh of 417 kWh/yr., or ~ 4 % loss of total possible output.

Azimuth 180, tilt 45 deg.

Unless I had a lot of shade, I'd go with a string inverter.

[sensij]

For arrays with close to an optimal orientation with respect to tilt , latitude has little to do with it until the location gets ~ > 60 N. or S. latitude.
...

Unless I had a lot of shade, I'd go with a string inverter.

While I appreciate the point you'd like to make, I'm having trouble replicating your results.

Max continuous output of an M215 = 225 W. With a 290 W panel, that is a DC to AC ratio = 1.29.
Max continuous output of an M250 = 250 W. With a 290 W panel, that is a DC to AC ratio = 1.16.

PVWatts for Portland, ME TMY3, 180 deg Az, 45 deg tilt, roofmount, premium panels, 8% loss, 1.00 ratio = 422 kWh / yr
PVWatts for Portland, ME TMY3, 180 deg Az, 45 deg tilt, roofmount, premium panels, 8% loss, 1.16 ratio = 421 kWh / yr (0.25% loss to clipping)
PVWatts for Portland, ME TMY3, 180 deg Az, 45 deg tilt, roofmount, premium panels, 8% loss, 1.29 ratio = 417 kWh / yr (1.2% loss to clipping)

Have I made different assumptions in the model than you did?

[J.P.M.]

While I appreciate the point you'd like to make, I'm having trouble replicating your results.

Max continuous output of an M215 = 225 W. With a 290 W panel, that is a DC to AC ratio = 1.29.
Max continuous output of an M250 = 250 W. With a 290 W panel, that is a DC to AC ratio = 1.16.

PVWatts for Portland, ME TMY3, 180 deg Az, 45 deg tilt, roofmount, premium panels, 8% loss, 1.00 ratio = 422 kWh / yr
PVWatts for Portland, ME TMY3, 180 deg Az, 45 deg tilt, roofmount, premium panels, 8% loss, 1.16 ratio = 421 kWh / yr (0.25% loss to clipping)
PVWatts for Portland, ME TMY3, 180 deg Az, 45 deg tilt, roofmount, premium panels, 8% loss, 1.29 ratio = 417 kWh / yr (1.2% loss to clipping)

Have I made different assumptions in the model than you did?

I might have screwed up. More later.

[J.P.M.]

While I appreciate the point you'd like to make, I'm having trouble replicating your results.

Max continuous output of an M215 = 225 W. With a 290 W panel, that is a DC to AC ratio = 1.29.
Max continuous output of an M250 = 250 W. With a 290 W panel, that is a DC to AC ratio = 1.16.

PVWatts for Portland, ME TMY3, 180 deg Az, 45 deg tilt, roofmount, premium panels, 8% loss, 1.00 ratio = 422 kWh / yr
PVWatts for Portland, ME TMY3, 180 deg Az, 45 deg tilt, roofmount, premium panels, 8% loss, 1.16 ratio = 421 kWh / yr (0.25% loss to clipping)
PVWatts for Portland, ME TMY3, 180 deg Az, 45 deg tilt, roofmount, premium panels, 8% loss, 1.29 ratio = 417 kWh / yr (1.2% loss to clipping)

Have I made different assumptions in the model than you did?

Later.

I made some errors with respect to clipping losses. Thank you for catching them.

My apologies to Inspron and Wexcellent for any confusion and waste my errors may have caused.

When I calc'd an annual clipping estimate I measure against a system (0.290 W in this case) with an assumed string inverter using PVWatts default DC to AC size ratio of 1.0. Then, for any hourly output that's > the micro equipped max output, I subtract the lesser (225 Watts) micro output from the string inverter equipped output. The sum of the 8,760 hourly diff. calculated in that fashion is the annual clipping estimate. That's perhaps the Luddite way, and I do understand DC to AC ratios as PVWatts works, but this way allows an easy read of clipping hours and it's just easier to keep straight in my head.

My error was reading the wrong column in my calculations - the one with the hourly total before the micro max. capacity subtracted, and partly, but certainly not entirely a consequence of not having someone to check my work as was a requirement when I was gainfully employed.

Redoing the numbers, my estimate, using PVWatts, 10 % system losses, 180 deg. az., 45 deg. tilt, 417 kWh/yr. output is that the M215's will clip ~ 210 - 220 hours/yr. and gather ~ 4 kWh less per 290 Watt panel than a string inverter equipped system. The M250's will clip ~ 65-70 hrs./yr. and overall produce about 1 kWh less over a year, or essentially no clipping.

Those estimates seem to agree with yours.

I do still maintain however that an array's annual performance will be mostly unaffected solely by how far it is from the equator. Off grid performance will be affected by seasonal irradiance variation as it may affect battery performance (or lack of it) brought on as f(latitude) and the shorter days that go with higher latitudes and how short days/long nites affect batteries, but grid tie systems in sunny northerly climates (like the Yakama valley in WA, or northern NV for example) will, probably outproduce similar systems on an annual basis in, for example, the southeastern U.S.

Provided each array is optimally oriented for the location it's in, a higher local solar clearness index and lower temps. will make the difference more than latitude.

[wexcellent]

So I read the relevant parts of "Solar Power Your Homes For Dummies" and found little that helped me understand how clipping works.

Because of cost and reliability I plan to stay with the Enphase M215 so I ran different scenarios with PVWatts at my location. I will have two different rooftop arrays, location is Troutdale, OR. Here are my results:

SSW Array, minor spring/fall shading, 18% loss, standard module, roof mount
(21) 290W rated panels = 6.09 kw, 96.5% efficient M215 microinverters rated for 215W continuous results in a DC/AC ratio 1.35
22.62 tilt, 193 Azimuth
Annual output 6587 kwh @ 1.5 ratio
Annual output 6603 kwh @ 1.4 ratio
Annual output 6,604 kwh @ 1.35 ratio
Annual output 6,603 kwh @ 1.3 ratiio
Annual output 6,600 Kwh @ 1.25 ratio
Annual output 6595 kwh @1.16 ratio (this would be the ratio with the m250 microinverter paired with 290W panels)

ESE Array, no shading, 13% loss, standard module, roof mount
(10) 290W rated panels = 2.9 kw, 96.5% efficient M215 microinverters rated for 215W continuous results in a DC/AC ratio 1.35
22.62 tilt, 103 Azimuth
Annual output 2,917 kwh @ 1.4 ratio
Annual output 2.918 kwh @ 1.35 ratio
Annual output 2,917 kwh @ 1.3 ratiio

Like I said, I don't really understand how this whole clipping thing works but according to this it seems like the sweet spot for me is indeed pairing an M215 with a 290W panel, as the bid I obtained suggested. If I pair a M250 with a 290W panel my DC/AC ratio would be 1.16 and my annual output would be less.

Please help me understand where I am misguided.

[sensij]

18% and 13% loss? That is really high. Most properly designed systems are modeled well by 10% or slightly less.

M215 produces 225 W continuously.

I don't disagree with the conclusion you've made... For the right price, M215's are a reasonable choice. Just think through the long term implications of 31 eventual inverter failures on your roof, with no inverters still in production that are compatible out of the box with the interconnect the M series uses.