What is the best solar panel?

[GoingElectric]

If space is a concern, the panel STC wattage matters little. What counts most is the panel's STC wattage per area and a low(er) power degradation as f(temperature)

Depending on the available space and its layout, physically larger panels by themselves may actually be a drawback.

The only real benefit to larger panels besides maybe a bit less $/STC watt for the panels is probably a bit less wiring for the array.

Yes, as you more clearly wrote, in space limited area need higher PV production density.

Then I see STC, a new term for me, so I look up
“Standard Test Conditions”

then see
“Photovoltaics for Utility Scale Applications Test Conditions” or PVUSA Test Conditions; more commonly “PTC.”
Under PTC, everything is heated up as if it were in the sun. The solar cells within the panel are raised to their “normal operating cell temperature” which is typically around 113°F (45°C). The ambient temperature is set to 68°F (20°C), and a 2.2 mph (~1 meter/second) breeze blows across the panel. Here is a summary of the two sets of test conditions:


PTC is a much better metric to look at, closer to real life conditions.

[J.P.M.]

Yes, as you more clearly wrote, in space limited area need higher PV production density.

Then I see STC, a new term for me, so I look up
“Standard Test Conditions”

then see
“Photovoltaics for Utility Scale Applications Test Conditions” or PVUSA Test Conditions; more commonly “PTC.”
Under PTC, everything is heated up as if it were in the sun. The solar cells within the panel are raised to their “normal operating cell temperature” which is typically around 113°F (45°C). The ambient temperature is set to 68°F (20°C), and a 2.2 mph (~1 meter/second) breeze blows across the panel. Here is a summary of the two sets of test conditions:


PTC is a much better metric to look at, closer to real life conditions.

I do not believe PTC to be a better measure of panel performance for several simple reasons.
- The first one being too many variables that can change under real world conditions.
- What's the panel orientation ?
- What if the wind vector changes in either magnitude or direction ? BTW, wind vectors are seldom steady and also have a vertical component. PTC makes no mention of how those variables may affect the output.
- You left out that the panel is tested 10m above the ground. Does that mean the panel has nothing under it ? If on a flat surface, what's the clearance under the panel ? That clearance will affect the NOCT more than being 10m above the ground, especially if the velocity vector's magnitude is fixed at 1 m/sec.
- The panel's tilt is not specified in the PTC method. A non horizontal panel's performance will be more affected by wind than will a horizontal panel.

- Another reason I don't think the PTC is a good thing to have around is it's not as universally applied to all panels.
- Having more than one test method only muddies the waters when trying to compare panels.
- Test methods should not be used to estimate operating conditions.

Any test method ought to be used to compare panel performance between and among panels but not site performance.

The STC method, or any test method is in that sense similar to vehicle mileage numbers posted by vehicle manufacturers.
The numbers will most likely never be achieved under operating conditions which are never the same from one site and setup to the next.
To say that the PTC method is closer to real life conditions is simplistic. The PTC method is probably more accurate at estimating performance when site conditions match the PTC test conditions.
Use PVWatts to model expected performance using site conditions. The method is reasonably fast and with reasonably accurate inputs will give results a lot closer to reality than using any test condition standard.

[GoingElectric]

- The first one being too many variables that can change under real world conditions.

All good reasons not to use PTC, always good to question a standard.
The one factor I think PTC is better at is higher operating temp. While 120F may be high (100F ideal?), IMHO is closer to real world than the 75F of STC.

[J.P.M.]

All good reasons not to use PTC, always good to question a standard.
The one factor I think PTC is better at is higher operating temp. While 120F may be high (100F ideal?), IMHO is closer to real world than the 75F of STC.

The cell and ambient temp. for the STC method is 25 C, not 75F.

Not a standard, but empirical correlations exist that get pretty close to estimating a panel's cell temp. relative to ambient air temperature using the P.O.A. irradiance and wind velocity. I've helped develop them in the past with some solar energy society folks and have one specific to my array that's good for estimating my array's cell temp. +/- a couple deg. C or less for winds < ~ 8 m/sec. Given an average cell temp. and a Global Horizontal Irradiance (G.H.I.) from a Davis weather station that gets converted to P.O.A. (Plane of Array) irradiance by conversion algorithms I and others have written, along with ambient air temp. and wind velocity from the Davis, my calc'd array power output is quite close (~ +/- 0.25% or so) to that of a clean array. I've used that method for a bunch of years to estimate how fast my array fouls under various conditions over the past 9 years or so. Long, boring story, but I'm pretty confident of my data and methodology.

If you know the P.O.A. and the wind speed you'll not have to screw around using panel test methods to estimate cell temps. and so panel efficiencies and so panel output. You'll be able to get panel output in ways similar to how PVWatts and for that matter most other PV panel modeling stuff models output in one form or another.

If you would like the temp. correlations, I'll post them once I get back home where all the data and the array are located.

[Old_Man]

For the benefit of the metric-deprived, 75F is 24C, one degree Celsius shy of the goal, here.

This has been an interesting thread. Based on what I've read here, the solar, "industry," such as it is, is as corrupt as the music business and we KNOW what Hunter S. Thompson said about that, a statement I can personally verify. I'm putting solar on my truck but a friend was looking into real estate and installing it until he found out how much HASSLE it'd be. So he bought a house with it already installed. He doesn't have to go through what can only be called the insanity of trying to buy solar on your house, installed by some booger-picking kids with a gnat's knowledge about what they're doing, coupled with a commensurate attention span.

I don't know if Bob's system is *hit-tied, but I will suggest he go off grid. Power companies? No f-n' way I want any piece of that. Off grid or nothing would be for me.

[jflorey2]

The one factor I think PTC is better at is higher operating temp. While 120F may be high (100F ideal?), IMHO is closer to real world than the 75F of STC.

Agreed - but - it really doesn't matter, since you can calculate the panel's performance at any given temperature as long as you know the tempcos and the performance at _any_ temperature. 25C is as good as 45C. The critical issue is that all panels be tested the same way, and that's what STC is intended for - since it's relatively easy to flash every single panel at 25C.

[littleharbor2]

Agreed - but - it really doesn't matter, since you can calculate the panel's performance at any given temperature as long as you know the tempcos and the performance at _any_ temperature. 25C is as good as 45C. The critical issue is that all panels be tested the same way, and that's what STC is intended for - since it's relatively easy to flash every single panel at 25C.

True, STC has nothing to do with real world performance. It is merely a level playing field all panels are tested on for comparison's sake. Of course most arrays won't produce at their STC numbers although some folks at high and cool elevations CAN see higher than STC numbers from their arrays, proving that STC numbers aren't entirely misleading.

[GDunk]

I live in Massachusetts where net metering is mandated. We have a 3-story building with a flat roof, so no obstructions. Proposals now from Panasonic, Waaree, SunPower, Qcells. Any recommendation? Inverters are Enphsse, SunPower, Solar Edge HD Wave. How much over current electric use should this be sized for? Currently I have proposals from 103%-109%.

[J.P.M.]

True, STC has nothing to do with real world performance. It is merely a level playing field all panels are tested on for comparison's sake. Of course most arrays won't produce at their STC numbers although some folks at high and cool elevations CAN see higher than STC numbers from their arrays, proving that STC numbers aren't entirely misleading.

Any panel testing standard has nothing to do with real world performance under day/day conditions.
Uniform testing standards' best use is for system design which is also why it's best to have one system for panel rating.
STC has fewer complications. In that sense, PVUSA violates the KISS principle.
Everyone on the same page with the least complicated testing system seems best to me even if some folks need to read a data sheet and actually learn what they're comparing.
Having different standard testing methods (I think perhaps that's an oxymoron) leads to the situation like that now going on between EV range rating differences between U.S. and European versions of how it's done which can lead to easily avoidable and unnecessary confusion which usually means more bureaucracy as well as more money for those with equipment to (oversell)sell to more confused users.

[Old_Man]

Then I see STC, a new term for me, so I look up
“Standard Test Conditions”

To this old pilot, "STC" will always mean "Supplemental Type Certificate."

[jflorey2]

Any panel testing standard has nothing to do with real world performance under day/day conditions.
Uniform testing standards' best use is for system design which is also why it's best to have one system for panel rating.

It is also very useful for comparing panels. Imagine the unholy mess it would be if everyone used a different insolation/temperature/backpanel illumination to test their panels and provide a power rating.

[J.P.M.]

It is also very useful for comparing panels. Imagine the unholy mess it would be if everyone used a different insolation/temperature/backpanel illumination to test their panels and provide a power rating.

Indeed ! And, agreed.
Never said, implied or meant for anyone to infer otherwise.

BTW, if talking about panel testing standards, the commonly accepted term when dealing with power or power flux (W/m^2) is irradiance. Insolation has a time dimension as in kWh and is used when discussing or measuring energy. Panel testing deals with power, not energy.

[GDunk]

We are reviewing proposals that are sized 3-5% over current t energy use. Yet these panels denigrate efficiency over time. In 10, 20, 25 years where will we be? Shouldn’t they be sized appropriately to take this into account?

[azdave]

No. The goal is not to make sure that in 25 years you are still producing the same as your actual needs were 25 year earlier besides, many utilities will not allow you to install more capacity than your historical use from the last 3-5 years. My contract is that way so I could not add a few more panels to make up for future system losses or consumption increases. Just be happy you can match you current needs reasonably well and then hope the utilities don't change the rules after you've signed a 20-year agreement.

[J.P.M.]

We are reviewing proposals that are sized 3-5% over current t energy use. Yet these panels denigrate efficiency over time. In 10, 20, 25 years where will we be? Shouldn’t they be sized appropriately to take this into account?

Among and along with other considerations including your situation and preferences, yes.