Question about PV panels output

That SMA document says that it is safe to use the inverter when the array DC power to inverter AC power ratio is 1.42. At that ratio, losses from "clipping" may become noticeable, but it is good to know that inverter life is not expected to suffer. A ratio of around 1.2 will have very little clipping in most installations.

Panels are not likely to sustain production at or above their STC ratings because of the thermal effects you noted. Transient spikes at or above STC are possible on cool days with some clouds that can refract/concentrate irradiance at their edges to levels above what would be received in a clear sky.

Thanks for reply
that means pv panel don't reach its peak power OR exceed it in 99% of weather conditions
my thoughts were that all over the winter pv panel will produce more than peak power

In winter, the angle of the sun is usually such that it is hard to get anything close to STC power. For arrays with orientations that have been optimized for winter exposure, especially if there is snow cover increasing the albedo, you could spend some time at or above STC. You can play with PVWatts hourly output to get a feel for what you might see in your specific case. Make sure to set the DC to AC ratio in the advanced parameters to 1.0 or lower to see what unclipped output might be.

Please can you tell me any free software tool to calculate PV panel output power during the year ?

My adjustable pole mount routinely goes over rated capacity for sustained periods on cold clear winter days. The combination of snow on the ground and a cold clear day bumps up both the insolation and allows the panels to be more efficient due to the thermal coefficient. My general suspicion is that PV Watts and other databases don't factor this in very well.

I personally don't believe in planning to run electronics flat out at 100% rated capacity. I expect that in order to be competitive that the components are sized right to the edge of their capacity. My preference since I eventually will most likely paying for the replacement is to slightly oversize the inverter so it doesn't clip routinely. Its part of my theory that I take some of the substantial savings I realize doing my own installs and spending a bit more to get longevity.

Unfortunately I think that the solar installers tend to push the limits of undersizing inverters as a means of doing a cheaper install. Contrary to the hype most of the business models are based on the lowest first cost of the array and many incentives are based on installed PV wattage, thus the best return is lots of wattsof panels installed at the lowest cost with no real interest in actual long term production. I expect if the incentives were weighed to long range production (feed in tariffs) that the equipment configuration would change. Vermont's incentives are oriented to Feed in Tariffs and trackers are quite popular as they maximize long term production. It would be interesting to see if the installation in VT use undersized inverters.

That is what i'm confused about undersizing inverter OR oversizing inverter

even in PVsyst help they said that (PV array - to - inverter) ratio should be in between 1.25 & 1.3 >>>>> that means the array always should be oversized than inverter capacity which conflict with what we are saying that PV panel may goes over rated peak capacity specially in winter
link for PVsyst document http://files.pvsyst.com/help/inverter_array_sizing.htm

[QUOTE=peakbagger;n298934] The combination of snow on the ground and a cold clear day bumps up both the insolation and allows the panels to be more efficient due to the thermal coefficient. My general suspicion is that PV Watts and other databases don't factor this in very well.

FWIW, PVWatts does not account for irradiance enhancements from the surroundings. It does allow a downward adjustment for snow cover on an array.

A lot of time spent looking at such things led me to the opinion that performance enhancement due to snow is not anywhere near as much as might be thought, and in any event, quite variable and difficult to quantify in terms of POA enhancement. The approx. SWAG I used was 5-10% for new, fresh snow on 70-90deg. tilted south facing surfaces @ 43 deg. lat.

What PVSyst is saying does not conflict with good design practice. Even if the panels go over their STC value, the inverter will not go beyond its rated output. Any excess power that might have been available from the panels is not generated, because the inverter will adjust the operating voltage of the array to keep the incoming power at a level it can handle.

With regard to operating the inverter at its rating... I think peakbagger's theories are incomplete. Only a single power rating is published to be used across the entire environmental operating range of the inverter, which is typically around -40 deg C to +60 deg C. However, knowing what we do about how heat affects inverters, I think it would be safe to say that if that same inverter could be marketed for an application in which the upper temp limit could be restricted to 20 deg C, representing the winter operation of concern here, the power rating would probably be higher.

We also know that some inverters operate more efficiently when they are loaded at close to their rating. For example, a SolarEdge 3000 and most others start to fall off when they are loaded at less than 20% of their rating. If you oversize the array per standard design practice, you will be staying above 20% for more of the day.

With respect to modeling software, PVWatts is free and is a good starting point. As mentioned above, it ignores albedo so the model error will exist when snow is on the ground and the array has steep tilt. Stepping up to the next level of modeling, you can download NREL's System Advisor Model (SAM, also free), and tune it to match the expected conditions and losses of your system. It will also handle systems with sub-arrays and different orientations more gracefully than PVwatts. It includes albedo, so the effects that PVWatts is missing will be included to some extent. It also gives the choice betwee a couple different models to handle the diffuse fraction of irradiance. The models used by NREL are detailed in the technical documentation and are based on peer-reviewed publication in which they have been tested against real-world data in a variety of conditions. With some googling, you can find and read those, and see the conditions in which the different models are expected to perform better or worse.

The TMY (or epw, etc) files used by the NREL models are intended to represent typical weather conditions across a year. If you download those files and modify them by hand, you can see how the array might perform in atypical conditions... heavy snowfall, exceptionally warm or cold temps throughout the year, etc. If you have good data collection (like a Davis Weather Station), you could even build a weather file based on conditions recorded at your house, and fine tune the model parameters even further to represent your specific array to project how it might perform in the future.

Stepping up to the next level of modeling, you can download NREL's System Advisor Model (SAM, also free), and tune it to match the expected conditions and losses of your system. It will also handle systems with sub-arrays and different orientations more gracefully than PVwatts. It includes albedo, so the effects that PVWatts is missing will be included to some extent. It also gives the choice betwee a couple different models to handle the diffuse fraction of irradiance. The models used by NREL are detailed in the technical documentation and are based on peer-reviewed publication in which they have been tested against real-world data in a variety of conditions. With some googling, you can find and read those, and see the conditions in which the different models are expected to perform better or worse.

FWIW, I've found that the HDKR model represents the best match for my terrain. The Perez model has a slightly more optimistic output, probably due to the horizon brightening term which may be inappropriate for hilly or terrain surrounded by objects that obscure the horizon. PVanywhere uses most of the Perez methods in the diffuse fraction calc.

The TMY (or epw, etc) files used by the NREL models are intended to represent typical weather conditions across a year. If you download those files and modify them by hand, you can see how the array might perform in atypical conditions... heavy snowfall, exceptionally warm or cold temps throughout the year, etc. If you have good data collection (like a Davis Weather Station), you could even build a weather file based on conditions recorded at your house, and fine tune the model parameters even further to represent your specific array to project how it might perform in the future.

I've done a clear sky weather model using the HDKR method for irradiance and TMY temps/wind/etc. to estimate a clear year's output. For anyone interested, in zip 92026, the TMY3 data for Miramar gives an estimated annual output of about ~ 82 to 85 % of the output using a clear sky model for P.O.A irradiance and TMY3 Miramar data for the rest of the variables.

The Davis data collected at the array is pretty slick to confirm and in some cases point out shortcomings in modeling.
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Finally my conclusion is that PV panels exceed peak power rarely and don't require to oversize inverter capacity over array capacity >>>> is that right??

That's about it.

newbie here but have spent some time reading on this very informative, useful and helpful forum. SolarEdge has a good write-up about inverter oversizing (up to 135% is allowed without voiding the warranty). However, for abundant sunshine areas, I read in some posts here that it is safer to maintain the ratio at 125% or less to avoid clipping, check it out...

How to know the expenses on What Size Solar System You Need?

during winter temp. decrease so Voc increase and irradiation decrease so current decrease
2- during summry temp. increase so Voc decrease and irradiation increase so current increase
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