On picking a PV module: the Effect of Altitude

Sage,

First I would post the question here:Solar Electric Power Discussion Forum by Northern Arizona Wind & Sun - Powered by vBulletin As these guys have forgotten more about PV that most of us will ever know.

Second, Most panels will never reach STC performance except in very cold very clear conditions, and even then only for short times. The exception is edge of cloud events. My rule of thumb, for what it is worth is I count on ~80% as full output. My 400 watts of off grid, peaks occasionally over 400 watts, but most daily peaks are ~325-350 watts. In any case, most inverters will just "ignore" power that spikes above rated, assuming these spikes are short lived, like edge of cloud events.
I can't say for sure about Enphase, Drees is the resident Enphase inverter expert.

Good luck with your project,

Icarus

 

Any reason why you don't mention what your climate / state / latitude is? Also important ... the amount of available space to do your layout? They have just as much bearing as your altitude.

EDIT:
Oh yea ... and remember that a panel rating is when it's ideal ... perpindicular to the sun. At best, that'll happen for just a few minutes of a few days in the summer. Produciton after that ... it's down hill, baby.

 

I don't know for sure - all I know is that altitude will result in additional output - 5% seems reasonable but am curious as to how you got that number?

As icarus mentioned it's fairly rare that your panels output more than 80% of their STC rated value once you take into account real-life conditions and DC-AC conversion losses.

Enphase typically recommends a 230w panel - but they have listed panels that are compatible with ratings of up to 250w - so obviously they don't have any concerns with high power panels. The M190 inverter will limit power at 199w AC so it gives you a bit more room than 190w. Not sure what the M210 inverter really maxes out at.

How much power you lose will depend a lot on your mounting angle and local weather - I doubt you'd lose more than a percent of total power even with 240w panels.

Enphase does have an article calculating estimated power loss with large panels - see their "Module Rightsizing" whitepaper where they compare a 216w and a 235w panel in various locations.

As long as you use a module that is on their compatibility list, I doubt you'd have any issue with warranty. If the module isn't on their list, you can contact support and ask them about it.

What module are you looking at?

 

Sage,

One other way to look at this is this way. Even if your panels are subject to 45 more watts per meter under ideal circumstances, that translates at 15% PV efficiency to ~6 watts more on the panel per meter sq over and above STC. Not enough to worry about.

Icarus

 

The Enphase inverters are all right around 95% efficient and very flat in their efficiency curves - one of the best I've seen. California requires manufactures to submit their product to an independent lab for testing before allowing them to be used for rebates and make the results public. See: Equipment Section - Go Solar California

Add in a bit for wiring losses, but since your DC wires are so short for microinverters, it's pretty much negligible.

While most seem to assume that module aging will reduce power output to the tune of ~0.5%/year or so, people who have tested actual 30 year old modules seem to find that the modules don't seem to lose capacity anywhere near that fast. Thin-films are a whole different story, though.

 

My 25 year old Siemens/Solarex panels still routinely put out over 80% of rated stc, which is above the bench mark for a new panel. STC conditions rarely exist in the real world. WE get a bonus at -40 however, although even at ~-30 a panel with just a tiny piece showing though the snow will melt off in a matter of an hour or two if there is little wind, so they do absorb heat rather quickly,

T

 

I actually think the 77% is a pretty good estimate. Remember, most panels put out a consistent ~80% of STC, so in reality, 3% for wiring and inverter loses is not too bad.

Be glad you are not off grid. I advice my clients to figure (at best) on ~50%, and that only over a 4 hour average per day. The problem with battery based systems is they start being way less efficient, and only get worse as you look into it a bit further. For example, batteries need ~120 of ah of charge for every 100 ah of draw. Add into that, as batteries come close to charged, the charge controller begins to dial down the output, leaving lots of potential power on the table.

In our off grid situation we manage power fairly carefully so that we use large draw items when the batteries are nearly full, and we have excess power.

Icarus

 

While those are reasonable defaults, Enphase also addresses them in their "PV Watts Calculation Values" application note.

Their suggested settings are 0.819 - a bit higher than the default 0.77.

Realize that this PVwatts derating factor doesn't have anything to do with temperature deratings - In other words, your maximum power will vary even more depending on temperature.

PVwatts takes the derating factor, then applies a correction based on temperature which it gathers from history weather data. It applies a 0.5% for each degree above 20C that the module experiences.

For example, Calfornia uses PTC ratings to calculate the maximum typical output that a system will see multiplied by inverter efficiency.

For example, your 240W Sharp NU-U240F1 panel is rated at 216.3W under STC conditions * 0.95 = 205.5W.

So under PTC conditions (20*C air temp, 1 M/s wind), I would expect the inverter to be reducing output by 6W or so from theoretical when there is 1000 W/m^2 and the sun is nearly perpendicular to the panels.

 

1. My roof never operates at STC conditions (20*C panel temps, 1000W / sq/m). PTC conditions more accurately represent typical conditions - my 180W panels (ET Solar ET-M572180) have a PTC rating of 157W.
2. My panels aren't perpendicular to the sun. They are mounted on my nearly flat roof (4* angled west). Today the sun is about 60* overhead, which means I lose about 14% (sin(60) = 0.86).
3. Inverter is 95% efficient.
4. Dirt. Haven't cleaned them in a few months and it hasn't rained, so I'm sure there's a fine layer of dirt on the panels. Probably worth 5% or so.

Add them all up:

157W * 0.86 * 0.95 * 0.95 = 122W.

Looks like today the panels maxed out at 130W - perhaps one of my reduction factors is a bit overzealous? Today was very close to PTC conditions.

So far my system is outperforming PVwatts by about 10% using a .82 correction factor over the past 6 months so the system is operating as expected.

I've only seen the panels hit STC ratings once (over 5 minutes). It was a cool, clear, windy day that was partly cloudy back in May. The clouds parted briefly and the system produced over 3200W for 5 minutes. It was glorious until the panels warmed up and then another cloud came by.