6.5K System (6.89): DC to AC Derate

jebatty

I've used PVWatts to calculate estimated average annual production for my system installed October 2013. PVWatts uses a default derate of 0.77, I have seen information indicating that 0.87 may be more accurate for a microinverter system. I did the calculation based on derate of 0.98 because my 26 Suniva 265w panels and Aurora microinverters regularly report sustained peak output in the range of 6.8K. With derate of 0.98, after 9 months I am very nearly on track for annual production of 9231 kwh, which is what PVWatts estimates for my location.

Should I expect higher initial production as I am experiencing? ... is derate of 0.87 more accurate over the long term?

Cariboocoot

Re: 6.5K System (6.89): DC to AC Derate

The problem is: where.
Up a mountain? Higher output.
In cold climate? Higher output.
Down in the valley? Lower output.
Lots of clouds? Lower output.

The 77% efficiency for panels & MPPT (either charge controller or GTI) is based on "typical" conditions average. If you plan for this and find your system does better, be happy.

Micro-inverters per se have no inherent greater efficiency over central inverters. They can do better over-all in some circumstances because of differences in panel location (each panel adjusted to its maximum power point rather than the array taken as a whole) and sometimes perhaps less wiring loss (V out from inverter over longest wire run is at a constant Voltage rather than the variable DC input to a central inverter).

How is the production per month compared to PV Watts estimate? Being as you are in Northern Minnesota you're probably at peak output in June and July. Expect that to drop off as Winter comes.

BB.

Re: 6.5K System (6.89): DC to AC Derate

Also remember that PV Watts hours of sun number is based on ~20-30 years of history (around the 1960-1990). Year over year, you can have +/- 5-10% variation pretty easily. One year with lots of cold/clear weather--More GT system production... One year with hot/muggy weather and high clouds, less production.

-Bill

jebatty

Re: 6.5K System (6.89): DC to AC Derate

This is the Enphase paper on 0.87 derate. Enphase 0.87 Derate

My rationale for using derate of 0.98 is based on maximum sustained measured output of my 26 panel Suniva 265 watt array. 26 x 265 = 6.89 kw would be rated maximum output. My Aurora CDD monitoring device has shown maximum sustained output of 6.87 kwh, an example being on March 1, 2014, with this output from 11:15 am to 1:15 pm. So I plugged into the PVWatts calculator for my location: array at 6.89 kwh, derate = 0.98, tilt = 34, aximuth = 180. This produced the monthly chart in the thread 6.5k System. The chart then shows production vs the PVWatts results.

Agreed that this is a snapshot in time, that it may not portray the long term average. Nevertheless, for this first year of operation, the chart mirrors the PVWatts result at 0.98 derate. Time will tell how accurate this is over the long term.

solar_dave

Re: 6.5K System (6.89): DC to AC Derate

jebatty wrote: »

This is the Enphase paper on 0.87 derate. Enphase 0.87 Derate

My rationale for using derate of 0.98 is based on maximum sustained measured output of my 26 panel Suniva 265 watt array. 26 x 265 = 6.89 kw would be rated maximum output. My Aurora CDD monitoring device has shown maximum sustained output of 6.87 kwh, an example being on March 1, 2014, with this output from 11:15 am to 1:15 pm. So I plugged into the PVWatts calculator for my location: array at 6.89 kwh, derate = 0.98, tilt = 34, aximuth = 180. This produced the monthly chart in the thread 6.5k System. The chart then shows production vs the PVWatts results.

Agreed that this is a snapshot in time, that it may not portray the long term average. Nevertheless, for this first year of operation, the chart mirrors the PVWatts result at 0.98 derate. Time will tell how accurate this is over the long term.

In Northern Minnesota on March 1st the ambient temp had to be quite cold, that boosts the normal output from the panels pretty significantly.

SolarPowered

Re: 6.5K System (6.89): DC to AC Derate

There is alot to consider.

If you are using polycrystaline the derate for micro inverter should stay around 85~ 86%, @ standard altitude between 0~1000ft.
Monocrystalline will typically yield 4% more output than poly crystalline 86~89%, @ standard altitude between 0~1000ft.

Regardless when I sell and build systems my derate whether it is mono or poly stays at 86%, and if my conductor runs are under 100' I use 10AWG for the installs so the systems do not voltage drop at the main panel.

You should never rate the PV watts above 90% for micro inveretrs because shade, cloud cover, and temperature play huge factors on the day to day production, not to mention soiling factors which within 3 months you can see a 2.5% drop in harvest production.

Power tolerance is as well an important factor for PV watts. (I.E) solar world panels are positive power tolerance, there is no negative power tolerance according to how they sort and arrange cells. so if you have a 275 panel for example you can actually plug the wattage in the calculator for 277watts per panel and multilply total number of panels for your kWh size if you want to adjust for a finer accuracy, as long as its micro inverter system, not string inverter system.

Solar World no longer sells and distributes the 280 module because to get the cell assortment, solar world would spend more money on production when you can just purchase a 275watt save a few cents per watt on the purchase price.

Altitude is important, the lower the altitude the more derate that will be needed. (I.E) from 0~1000ft the derate is practical for a base line at 86% whether it is mono or poly. Below sea level you want to derate further.

My recommendation is play it safe.

Review panel tolerance specs to change the actual power output according to name plate rating. If its positive/negative power tolerance keep the wattage for your system same as name plate, if its positive power tolerance add 2 watts per panel to total kwh, and this is only for micro inverter systems. When we designed for string inverters, we actually do the opposite and take the negative power tolerance to get the baseline kwh system size.

So the way finance engineers run calculations:

MICROINVERTERS: power tolerance of panel multiply number of panels, derate at 86%
STRING INVERTERS: you can as well PVwatts derate string inverters to 86% as long as you use the lower power tolerance percentage of the panels nameplate and quantify that number per string for KWh system size. String inverter systems will always react to the lower voltage of that string, unless you have multiple MPPT input such as power one/ABB inverters.

You will come to find out for string inverters due to the lower power tolerance you will have to have more panels to equal the kWh size, but the derate of 86% is consistent between both types of systems. Micro inverters are a much more solid investment. Essentially you do purchase less panels, so it accelerates ROI.

jebatty

Re: 6.5K System (6.89): DC to AC Derate

As often is true, what I thought was a relatively simple question really is far more complex. Perhaps this year has been sunnier than I thought, resulting in production much higher than PVWatts predicts, even with the power adjustment (positive only, + 2 watts; 6.94 kw; 0.86 derate). I think for present purposes I will stay with the 6.89 kw and 0.98 derate because I am tracking quite well with that predicted result. At the same time I will keep in mind a longer term view and see where my system balances out over time. The Univ of MN recently produced a Solar Insolation Tool for Minnesota. The difference in insolation between the PVWatts location closest to me and my actual site is about 1% more at the PVWatts site than my site. Meanwhile, I will remain satisfied with the results I am experiencing.

SolarPowered

Re: 6.5K System (6.89): DC to AC Derate

Solar insolation through out the entire united states hit peaks this year.

I have one system that PG&E estimated wouldn't exceed 6.10kWh, and it has had peaks of 6.98kWh and its only a 7.5kWh system. On some days the system peaks @ 93%. I also should be taking into account that this weather for this year is unorthodox as this is one of the largest drought's california has seen in years, so overall less cloud cover.