Are STC conditions for installed panels even possible?

[offgrider]

Lets say the panels are at the perfect tilt for the time of year, and the sun is at the exact spot for the time of day, theres no clouds/fog/haze whatsoever. Would we expect to see 1000 watts of sunlight per square meter (STC) hitting the panel?

Im trying to size panels/microinverters, and its very interesting to do the math where you subtract PTC from STC on various panels, then compare the result actoss different panels. Chinese ones usually have a difference of over 100watts. I think I saw some Panasonic ones with 330wSTC and 311wPTC which seems an almost negligible difference when you consider +100w for others.

So for my project and PV/MI sizing, at about what point will a microinverter fail due to conditions where an intentionally-oversized panel actually comes across with the wattage and produces higher than PTC for some period of time?

 

[svetz]

Lets say the panels are at the perfect tilt for the time of year, and the sun is at the exact spot for the time of day, theres no clouds/fog/haze whatsoever. Would we expect to see 1000 watts of sunlight per square meter (STC) hitting the panel?

Yes, 1000 W/m^2 or higher is possible for a brief period around solar noon during the month of the sun's highest elevation for some latitudes. But it's not common at sea level. In orbit it's a fairly steady 1366 W/m^2. On the ground it depends on elevation, air mass, latitude, angle of the sun, etc.

Of course, on those days my microinverters clip. ?

 

[GXMnow]

My 300 watt panels will clip out my 240 watt for up to 2 hours on cool sunny days. So they can easily exceed 80% of STC, even with less than ideal panel angles and almost any time of year here in So Cal. My panels are Sil Fab brand made in Canada.

 

[fafrd]

My 300 watt panels will clip out my 240 watt for up to 2 hours on cool sunny days. So they can easily exceed 80% of STC, even with less than ideal panel angles and almost any time of year here in So Cal. My panels are Sil Fab brand made in Canada.

I’ve got 335W panels on dual-microinverters that can peak at 600W but only deliver 500W sustained.

I rarely ever saturate and when I do, it’s a nice rounded hill, not a clipped, flat-topped trapezoid (meaning there was not much of any rrmaining power above the clipped peak).

So my panels are rarely if ever getting past 250W (75% of rating).

 

[GXMnow]

I’ve got 335W panels on dual-microinverters that can peak at 600W but only deliver 500W sustained.

I rarely ever saturate and when I do, it’s a nice rounded hill, not a clipped, flat-topped trapezoid (meaning there was not much of any rrmaining power above the clipped peak).

So my panels are rarely if ever getting past 250W (75% of rating).

Part of that loss may also be the inverter efficiency. My Enphase iQ7's are rated at 97.5% which is quite amazing. So when mine clip at 240 watts, the solar panel has to be putting out at least 246.15 watts. So just 6 watts of loss, so I didn't bother figuring that in. But some of the units I have seen on Ali Express brag about 92% efficiency. To get the same 240 watts out, they would need 260.87 watts in. Almost 21 watts lost to heat. That is more than triple what is lost in the Enphase. Then add in any MPPT error as well. If it is off a little on calculating the MPPT point, you could be losing another few percent. This is why it is good to over panel a bit. But even adding up those losses, your 335 x 2 = 670 watts, never topping 500 watts does seem like there is a bit of extra loss somewhere. What latitude are you at, and what is your panel angle? Are they facing straight south, or turned a it? Mine are at a 20 degree tilt and facing 25 degrees east of south.

 

[fafrd]

Part of that loss may also be the inverter efficiency. My Enphase iQ7's are rated at 97.5% which is quite amazing. So when mine clip at 240 watts, the solar panel has to be putting out at least 246.15 watts. So just 6 watts of loss, so I didn't bother figuring that in. But some of the units I have seen on Ali Express brag about 92% efficiency. To get the same 240 watts out, they would need 260.87 watts in. Almost 21 watts lost to heat. That is more than triple what is lost in the Enphase. Then add in any MPPT error as well. If it is off a little on calculating the MPPT point, you could be losing another few percent. This is why it is good to over panel a bit. But even adding up those losses, your 335 x 2 = 670 watts, never topping 500 watts does seem like there is a bit of extra loss somewhere. What latitude are you at, and what is your panel angle? Are they facing straight south, or turned a it? Mine are at a 20 degree tilt and facing 25 degrees east of south.

My NEP microinverters are rated for 96.3% peak efficiency and CEC weighted efficiency of 95.5%.

So assuming when I max out it’s the weighted efficiency I should be using, 250W out = 262W in (78% of rating).

Also, the panels are now 5 years old, and guaranteed not to lose more than 3% the first year and 1% per year after that, so the panels should be rated at 93% of their new rating now, meaning 312W (meaning 262W out amounts to 84% of rating

My grid-tied array is facing 223-degrees SW (47 degrees west of south) and the panels are tilted 27 degrees.

And when I consider the Microinverter efficiency as well as the fact that I maxed-out more frequently in the first year of operation, it’s true upon reflection that my array peaks out at closer to 85% of current (adjusted) rating rather than 25%...