Feed wire size

[lockem]

I use #10 Cu for 3.6/3.8 kW inverters, where #12 would meet code. Otherwise I'll use #6 ACWU, or #8 Cu if the client prefers the cable run inside PVC conduit.
For DC I only use #10 Cu RPVU, because that's all my suppliers stock.

p.s. I ignore power factor since I've never measured anything less than 99% above 50% load.

In my system I measured a power factor of 0.83. In particular 20.8A * 247V yielded only 4300W. Sunpower updated the FW, perhaps to correct the power factor. I need to measure again, but on my way to do that my car broke down <sigh> so it will have to wait until next weekend.

The economics of oversize wiring depends on the value of the generated electricity, the incremental cost of the wire, constraints on wire size (code, physically fitting it), the current and special constraints such as wire being too large to utilize. The value of the generated electricity is the big wild card in that equation; mine is about $0.23/kWh. In my case 8ga is the largest wire size that will fit in my conduit and also the largest wire size that will fit in the wire clamps of my 30A circuit breakers, so 6ga was never a consideration.

Also as Bruce Roe ( bcroe ) pointed out in extreme cases you may be forced to select bigger than economic size wire just to make the system work properly, particularly if you have a really long run.

[nerdralph]

The calculations are easier if you notice that each step in AWG changes the resistance by 1.26x. So 12 AWG has 58.74% higher resistance than 10.

http://nerdralph.blogspot.com/2023/10/calculating-copper-wire-characteristics.html

[nerdralph]

I use #10 Cu for 3.6/3.8 kW inverters, where #12 would meet code. Otherwise I'll use #6 ACWU, or #8 Cu if the client prefers the cable run inside PVC conduit.
For DC I only use #10 Cu RPVU, because that's all my suppliers stock.

p.s. I ignore power factor since I've never measured anything less than 99% above 50% load.

[lockem]

Thanks for all the info. In Silicon Valley a heatpump only needs to be efficient down to 35F. I have seen it get down to 21F once, but nothing below 30F for the last 20 years.
In the Bridgeport California area lows are -20 to -10F, it is the coldest inhabited place in California. My 5 ton ground source heat pump never works very hard and is clearly over specified. Actual heat load at the typical 20F outside temp and 72F inside temp is about 12,000 BTU/hr (measured). This demands about (12/(3*3.4))*24 = 28kWh in energy to run the ground source heat pump, while at least on winter sunny days I typically get over 30kWh from my little array. That heat load increases to about 20,000 BTU/hr at -10F outside temp, which is still only 1/3 of the heat pump capacity.

Under NEM2, my excess power output from summer counts against my power deficit in the winter, so I end up getting an end of year check from the power company. I expect that once I am living full time in the house my consumption will exceed my production, but not by a large amount.

Sunpower panels are compact relative to their output and even though I have plenty of space, I only have a small area where the panels do not become an eye sore. Besides, ground mount support frames with a reasonable snow load capacity are expensive. Everything needs to handle up to 150PSF snow load. After all, Sierra Nevada is Spanish for snowy mountain or some such.

I don't think any trencher would chew through the local granite, andesite and mystery iron bearing rocks. They are hard and likely to damage a trencher. No one in the area uses trenchers, including public utility companies. Explosives or expanding chemical rock crackers are required if the rock cannot be moved via backhoe or excavator.

I have a grizzly to bulk separate large rocks from dirt. I have enough large rocks to build a fire barrier rock wall, and have plenty left over.

[bcroe]

Hail sometimes happens here, not on my list because it is generally not
life threatening. Only once did I see some orange size that damaged
cars, very rare.

Get the tools you need. A mile from the Rock River, there is a lot of that
layered yellow rock. My trencher works its way through, pieces come up
from gravel size, to hand size that may jam things and require removal.
Have encountered those hard egg shaped rocks that the trencher cannot
cut, dug them out and dumped on my rock pile. The pile started with the
solar array, now is bigger than my rooms. Have a mini back hoe too.

Abiout the time my solar went in, my ancient AC went bad and I had an
air to air heat pump replacement installed for about $5k. This old school
unit was about 14 SEER, would only work down to around 20F, had a
very noisy fan and reversing valve, and the compressor vibrated the
whole house. I was not very successful trying to supplement heat with
it. After a couple failures I gave up, but was looking at new school hps.

You should carefully research a heat pump to fit your needs. I started
looking for low temp capability to below -10F, that for the car shop is rated
-25F. COP or SEER is pretty important. The small air to air mini splits
were extremely quiet, efficient, and fit my DIY skills. Basically one at
each corner of the house and one in the exposed wall of the basement
met my needs, enough capacity to cover the coldest temps, and with
independent operation one could not take down the whole system. I
avoid larger units serving multiple locations for that reasoning. Running
the old furnace fan evened out temps and cleaned the air.

The car shop has a 1.5 ton heat pump that can maintain about 65F
working temp at least 9 months of the year. Ceiling fans run all
winter to keep the heat down where I am. The very coldest months
the hp may only manage low 40s, but that is enough to keep my
snow blower melted clean after each use. Very little is done there
those days, but I stlll can if needed, more capacity would be a waste
of energy.

No brand loyalty here, I have different sizes, MITSUBISHI, FUJITSU,
SENVILLE.

My panels are varied brands and configurations, just made sure each
string had 720 cells so they would play together nicely. An advantage
of string inverters, it is not necessary to add an inverter every time a
panel is added. So any cheap panels on the market work. Bruce Roe

[lockem]

Bruce,

What is the brand name on those split units? I have another house that has air flow issues causing temperature balance issues. I was quoted about $5000 to install a SEER 10 unit.
Some mfgs refuse to ship to California with our out of control "everything causes cancer" labelling requirement and all. Did you know that wood saw dust causes cancer--just inhale an entire tree ground down to a fine powder and you are in trouble? How about stainless steel metal dust--this is a real thing, but mostly from factory processed foods?

Another hazard we don't have in California is large hail. I can reasonably expect my panels to last the length of the warranty, rather than until hail breaks them. 400W microinverter based Sunpower panels here are $1440 each, so pretty far from cheap.

A trencher will not work at my home. The "soil" is 25% rocks; often 6" or larger. I have a backhoe for digging. It can move rocks up to 6' round by 3' thick and pick up rocks up to 3' more or less spherical.

[bcroe]

In 2018 I ordered a mini split on line from
Budget Heating & Air Conditioning Inc. 813 885-7999
To the inside and outside units I added several things like a wall
mount, tubing and cable kit, electrical whip and disconnect box,
brought me to $1662, shipping to the door was $200. Still testing
on this new item then, was the SEER 36, or 33, or 30.5? It has
served me well for 5 winters. I decided I did not like the metal wall
mounts offered, for later units made my own from the same 6061
aluminum and 18-8 stainless hardware I used to mount panels.

Note, units tend to be rated in cooling capacity, which sends the
motor operational power outside. But in heating mode that energy
goes inside, so heating is more powerful than cooling. COP does
decrease with outside temp, but I have enough capacity to cover it.
3 of these smaller units got me thru a winter unaided, but I then
added more to cover severe cold or any failures.

An auto transformer could be much smaller than a full isolated
transformer, since it only handles the DIFFERENCE in voltage
I need. And losses would be proportionately smaller. That
thought was based on getting down to a level to avoid tripping
inverter voltage monitors. But the idea of using units at both
ends was never seriously considered. Can only DIY so much
in a year, I lived with high AC line losses for 7 years. Bruce Roe

[lockem]

My ground source loop is outside of the house. Putting it under the house seems a bit fool hardy to me. I used 1800W "heat cubes" as backup. They are cheap and only a few of them can keep my house warm.

I would not consider an autotransformer for this application due to the balance issues. The bigger problem with transformers is that you are going to have a loss of several percent per transformer, so it isn't going to "fix" a 4% loss.

In these more-or-less constant voltage systems it is easier to use the following to compute wire loss:

Loss = I^2*R
W = I*V*power_factor
I = W/(V*power_factor)
Loss = R*(W/V*power_factor)^2

So step up the voltage and scale down the loss in inverse SQUARE proportion. Double the voltage, 1/4 the loss. At the same time, the loss scales with the resistance -> wire size linearly.
Also note that squared power_factor in there. You may or may not have any ability to control it, but it is something to be wary of.

What mini-split heat pump has an SEER of 36 and costs only $1400?

If a tsunami reaches my house at 7500' elevation, it is likely to not stop before it gets to your house.
Southern Il has a similar earthquake risk as my house:

https://hazards.fema.gov/nri/earthquake

Tornados are so rare in California that it is fair to say "they never happen". OK San Diego averages something like 2 per year, while the entire rest of the state manages to average not much higher.

[bcroe]

Thanks for all the interesting details. You had said 4% power loss cost you 800kwh/year, so I just computed 800/0.04 = 20,000kwh/year total output.
A ground source heat pump might have been an easier (more cost effective?) way to meet requirements than a huge solar array, but you have already sunk your money into the solar array.
When I mentioned transformers, I meant between your inverter and your main panel. Step up to 600V for the run, then back down to 240V at the panel. Transformers are not cheap and have some losses, so the decision is not trivial.

The one year that I added up everything, my system produced about 12,000kwh for the year. My home, located in the coldest inhabited place in California used 10,000kwh. My home is insulated similar to a house in Alaska and has a ground source heat pump. I have 5600 DC watt rating (assumes standard light incidence) and 5160 watt inverter capacity.

Regarding the insolation discussion with J.P.M., of course actual output of panels is dependent on a lot of factors that I glommed together with the assumption that a reasonably well oriented system will produce roughly average sun-hours*365 per year. Although solar energy varies with the cosine of the angle between the sun and the exposed surface, solar PV output may vary by more than that for several reasons including the variable amount of reflection of the light off the glass.

Earthquakes are not much of a risk in Bridgeport, Ca. Neither are floods. Snow avalanches,
volcanoes and wildfires are a risk, but there are risks anywhere you go.

Yes good to go thru the numbers.

Actually best estimate of AC transmission loss was 9V out of 260V
generated, or near 3.5% peak. I cut that to more like 0.9% with bigger
wire. 2.6% of 29,000kWh is 754kWh, just an estimate because line
voltage and output power both vary, changing the instantaneous loss.

Earlier the ground source heat pump was about the only solution. I
did look at them earlier, expensive and difficult on this terain. I did not
like the idea of using plastic tubing either in a horizontal land sink,
which might some day require a very difficult replacement, and a friend
after some decades is having leak problems with it in his concrete floor.

By 2017 I was reading the latest Mini Splits with a SEER of 36, low
temp capability far below 0F, and lending themselves to (economical)
DIY. I started out with 3 of these in 2018, at $1400 each for the basic
1 ton (heating) units. These saved so much energy, I eventually
bought 3 more, could now nicely warm (or cool) the car shop, run
my dehumidifier summers, and keep the air nearly dust free running
the furnace blower air thru the electronic air filter often. I did have to
replace that well worn propane furnace blower motor, a common part.

One goal here was an HVAC that could not be completely disabled by
a single component failure, and leave me in a desperate winter
situation. I also put in enough capacity that it would likely never be
necessary to turn on any auxilary heat on the coldest day. Again a
bit oversized by some standards, not very costly to me.

This 70s ranch is a poor candidate for saving energy, but I am not
going to tear down a functional brick house. Work to better insulate
it is slowly underway, but I was not going to wait to finish that
(probably never) before installing PV solar. As a consequence all
the equipment is scaled up quite a bit to cover the extra losses. If
the time comes the solar is to be shut down, more insulation will rise
to a much higher priority.

I did consider the transformer idea, but it had so many problems.
It would have to be a dual auto transformer at both ends, to keep
the inverter neutral voltage monitor happy. The more line loss I
wanted to save, the bigger the auto trans would have to be, adding
a lot of clumsy equipment at both ends. It would consume idle
current 24/7, which would probably cancel or exceed my line gains.
And it might add enough impedance to the apparent line that the
inverters think they have been islanded, and shut down. No.

J.P.M. is quite technically correct, we sometimes kick around goals.
I should add snow avalanches, tsunami, sink holes, and volcanoes
to my list. About the only real threat here is the Byron Nuke plant,
just minutes away.
Bruce Roe

[lockem]

Thanks for all the interesting details. You had said 4% power loss cost you 800kwh/year, so I just computed 800/0.04 = 20,000kwh/year total output.
A ground source heat pump might have been an easier (more cost effective?) way to meet requirements than a huge solar array, but you have already sunk your money into the solar array.
When I mentioned transformers, I meant between your inverter and your main panel. Step up to 600V for the run, then back down to 240V at the panel. Transformers are not cheap and have some losses, so the decision is not trivial.

The one year that I added up everything, my system produced about 12,000kwh for the year. My home, located in the coldest inhabited place in California used 10,000kwh. My home is insulated similar to a house in Alaska and has a ground source heat pump. I have 5600 DC watt rating (assumes standard light incidence) and 5160 watt inverter capacity.

Regarding the insolation discussion with J.P.M., of course actual output of panels is dependent on a lot of factors that I glommed together with the assumption that a reasonably well oriented system will produce roughly average sun-hours*365 per year. Although solar energy varies with the cosine of the angle between the sun and the exposed surface, solar PV output may vary by more than that for several reasons including the variable amount of reflection of the light off the glass.

Earthquakes are not much of a risk in Bridgeport, Ca. Neither are floods. Snow avalanches, volcanoes and wildfires are a risk, but there are risks anywhere you go.

[bcroe]

Sun hours is the average amount of sunlight per day, for the entire year:

TOTAL_SUN_HOURS_IN_ONE_YEAR/365

The expected value in Illinois is 4.4hours per day. The expected value in the high desert in California is 5-5.5, although that fails to consider raised solar intensity due to reflections off snow covered cliffs and high altitude dry air. The highest in the country is 6. I believe that getting 7 hours at max output is a symptom of an undersized inverter. Based on your numbers, your system is 3 times the size of mine, but produces less than twice as much energy. Part of that is due to location, but some of it is also likely due to an undersized inverter.

To estimate return on investment in units of time compute

(cost of upgrade)/(value of electricity saved in a year)

It is a straightforward computation. It should be done by system installers when they quote the system.
In your case it seems unlikely that upgrading a 4/0 cables would be economic as it would cost at least several hundred dollars and likely increase electricity production by $20/year. At some point running higher voltage via transformers is going to be less expensive than heavier cables.
In my case I reduced my loss from 0.011%/foot to 0.007%/foot. Over your 600 foot run that would be an improvement from 6.6% to 4.2%. Since you were already at 4% loss, the level of wire upgrade that I was discussing was already present in your system before you made any changes.

I believe you are using micro inverters, so all your long runs are AC?
Running string inverters here gives 2 loss considerations, DC & AC.
Here the DC loops vary from around 500 ft for the closest panels, to
approaching 1000 ft for the farthest. My estimate is around 1% loss
at full power, at 400V a fair amount of drop can be tolerated. If I wanted
to get back the loss, just adding a panel or 2 would be a lot easier than
changing wire. My intent was to ground mount solar completely out of
view of my house or the neighbors. This is hardly the most cost effective
per kWh, but it does give a lot more options.

From the inverters there are a lot of AC restrictions. The 70s house was
originally an all electric Medalion build, so the service is 200A 240V
(48KVA) and here in the country I have my own transformer. Long ago
the previuos owner changed over to propane, my goal was to change it
back using 2013 technology. Propane bills gave me a pretty good idea
of how much solar generated energy would be needed, at 25.5 kWh
per gallon. Solar installers here would go up to 15KW of inverters,
though some would argue that is overload. I made sure the PoCo
feed came into the top of each breaker box, the PV inverter power
into the bottom, so it was physically impossible to overload a box
busbar. In addition the 4 gauge between the inverters in the shed
and the house was close to the limit at 60A. Given all this I estimated
I was close to heating and electrifying the house with PV solar energy.

There have been regular energy upgrades since startup in 2013, based
on learning, technology, and the time very economical DIY work takes.
In 2020 while hiding from Covid, I was doing the upgrade of the buried
Inverter to house feed from 4ga copper to 4/0 aluminum. I cannot
change the inverters, as my PoCo contract allows connection of 15kW,
even the serial numbers are recorded. But there is no limit on hours,
my DC:AC is around 2.

As your figures indicate, generating PV energy here under the frequent
clouds and snow will be less efficient, more costly than in the SW desert.
I chose to live with some cold months we can deal with, avoiding such
things as earthquakes, mud slides, floods, droughts, hurricanes, forest
fires, smoke, extreme real estate prices, overcrowding, and regulations.
In 2012 my relations with the fossil fuel suppliers were poor, and PV
technology was looking good. Word was you cannot heat your house
year around with solar here, but I decided to try.

Energy varies with the weather, but generally I collect around 29,000 kWh
a year (Net metering, 1:1). Divide by 365.25 days per year and 15kW,
that comes to around 5.3 hours a day. If you divide by the .95 efficiency
of the inverters for the input DC its more like 5.6, since my inverters are
just into clipping under good sun its even more. Record one day inverter
output is 158kWh, looks like a 10.5 hour day.

So how to do that, trackers work very well in the SW desert. But a tracker
can do nothing about our clouds. But adding more of those now cheap
panels work. Panels are set up facing east, and my expensive ground
mount can be made more cost effective by adding another set of those
now cheap panels facing west. I believe the wind loading is essentially
the same. Since the 2 sides will never peak at the same time, there is no
need for more inverters. I get more cost effective use of the plant
including DC wires, inverters, and AC wires, by keeping them working
more hours. My long production sunny days are helping make up for
the cloudy days.

But what about cloudy days? The light is dispersed, all the panels work,
output is roughly double the SW desert design if used here. So even
then, the rest of the facility is twice as effective at the cost of some cheap
panels. I have seen 4kW output in a rain storm, light clouds will run it
near 100% capacity.

Does it cost more than the SW desert setup, yes we said that up front.
Does it work, 11 years here with the furnace off say yes.

I will show a curve of AC output under good sun. The inverters are
clipping at max, but just barely. Removal of just one of 6 strings
connected to an inverter will drop it out of clipping. Some juggling
of E, W, and S orientations does this, without any increase of the
rest of the plant, something not practical with mcro inverters. In
summer the rising and setting sun is so far north, the 8 hours in
clipping does not increase, There is a plan on paper to solve this,
it turns out these methods are also the most snow repellant.

That is not the end, things like 6 air to air mini split heat pumps,
non vented clothes drier, and many other details make it all work.
Cost effective, pretty hard to figure with no starting or ending point,
huge savings from a lot of repurposed stuff and DIY labor, no detailed
cost records over 20 years here, with ongoing upgrades. It is well
within my budget and quite successful, my calculations end there.
Bruce Roe

NScurve.jpg

[lockem]

Your definition of "sunhours" is different than the usual use of the term and an example of the confusion its use can cause.

About the closest to being a useful definition for the term is to define a "sunhour" or "sun-hour" as equal to insolation (energy), not irradiance (power) of 1,000 W/m^2 of energy received on a horizontal surface without regard to how long it takes for that energy to be received. So, for example, if at some location 1,000 W of solar energy is received on a horizontal surface over the course of one day, that location has received 1 "sunhour" for that day. If it takes 1 hour instead of 1 day to receive 1,000W of solar energy, the surface has still received only 1 "sunhour".

The term is of some use when comparing geographical sites one to another for solar potential.
However, it's of little use in the actual design of solar energy equipment.

I've been knocking around solar energy for almost half a century and to my experience most knowledgeable solar designers I've known don't use the term.
In informed circles the usual term (if used at all) is kWh/m^2 per day or W/m^2 with the surface orientation understood to be horizontal.

To my experience most of the confusion comes about when un- or semi-informed individuals think or assume that if a site or location "gets", say, 5 "sunhours" of sunlight that means a solar energy device will receive that much insolation. Not correct.

But, and the point is, the term "sunhours" is not much use in system design or analysis.
It's a B.S. term that leads to confusion and often to costly errors and/or less than optimal or cost-effective design.

As far as ROI goes, while that may be your definition, but there's a lot more to it, starting with the idea that ROI takes a longer takes a longer look at the economics than just one year.

On Bruce's system, and with all possible respect to Bruce, his system is unusual in many respects, starting with its size, orientations and goals. And again, with all respect, it's most likely not cost effective but I recall reading on several occasions over the years that cost effectiveness was not one of his goals.
If it was I'm pretty sure there would be more systems like his, but I believe his system is unique. And more power to him for what he's done even if I and others may think it's eccentric.
FWIW, if he's having fun with it, I'm a fan but I doubt many informed people would call it cost effective or practical.

I did not make up my definition of sun hours and did not give a complete formula. Plenty of information is available similar to this:

https://unboundsolar.com/solar-information/sun-hours-us-map

Yes, it is average over a year insolation per day. It is useful for estimating the annual energy output of a system, and thus useful for determining if a system is performing as expected. My point is that it is not the number of hours of production on the best day of the year.

At my location I get about 1080 Watts/m^2 perpendicular to the direction of the sun, except in the spring when snow reflections increase that to about 1200 Watts/m^2.

ROI is a financial term. Properly it should include cost of money, the changing value of electricity generated (or cost avoided) over time and should be the total yield over the life expectancy of the investment. I simplified that to the time to get back the investment with no consideration of varying electricity cost and money cost (which more-or-less cancel each other out), which is much easier to compute and easier to understand.

My point is and remains: spend a little extra on wiring and get a little extra in power--it will pay for itself quickly.

Regarding lots of solar experience, please have a look at my request for information in the Sunpower performance thread. It appears to me that Sunpower has "upgraded" the FW recently in its microinverters to cap max current and as a consequence if one has a bad power factor it degrades the system performance.

[J.P.M.]

Sun hours is the average amount of sunlight per day, for the entire year:

TOTAL_SUN_HOURS_IN_ONE_YEAR/365

The expected value in Illinois is 4.4hours per day. The expected value in the high desert in California is 5-5.5, although that fails to consider raised solar intensity due to reflections off snow covered cliffs and high altitude dry air. The highest in the country is 6. I believe that getting 7 hours at max output is a symptom of an undersized inverter. Based on your numbers, your system is 3 times the size of mine, but produces less than twice as much energy. Part of that is due to location, but some of it is also likely due to an undersized inverter.

To estimate return on investment in units of time compute

(cost of upgrade)/(value of electricity saved in a year)

It is a straightforward computation. It should be done by system installers when they quote the system.

In your case it seems unlikely that upgrading a 4/0 cables would be economic as it would cost at least several hundred dollars and likely increase electricity production by $20/year. At some point running higher voltage via transformers is going to be less expensive than heavier cables.

In my case I reduced my loss from 0.011%/foot to 0.007%/foot. Over your 600 foot run that would be an improvement from 6.6% to 4.2%. Since you were already at 4% loss, the level of wire upgrade that I was discussing was already present in your system before you made any changes.

Your definition of "sunhours" is different than the usual use of the term and an example of the confusion its use can cause.

About the closest to being a useful definition for the term is to define a "sunhour" or "sun-hour" as equal to insolation (energy), not irradiance (power) of 1,000 W/m^2 of energy received on a horizontal surface without regard to how long it takes for that energy to be received. So, for example, if at some location 1,000 W of solar energy is received on a horizontal surface over the course of one day, that location has received 1 "sunhour" for that day. If it takes 1 hour instead of 1 day to receive 1,000W of solar energy, the surface has still received only 1 "sunhour".

The term is of some use when comparing geographical sites one to another for solar potential.
However, it's of little use in the actual design of solar energy equipment.

I've been knocking around solar energy for almost half a century and to my experience most knowledgeable solar designers I've known don't use the term.
In informed circles the usual term (if used at all) is kWh/m^2 per day or W/m^2 with the surface orientation understood to be horizontal.

To my experience most of the confusion comes about when un- or semi-informed individuals think or assume that if a site or location "gets", say, 5 "sunhours" of sunlight that means a solar energy device will receive that much insolation. Not correct.

But, and the point is, the term "sunhours" is not much use in system design or analysis.
It's a B.S. term that leads to confusion and often to costly errors and/or less than optimal or cost-effective design.

As far as ROI goes, while that may be your definition, but there's a lot more to it, starting with the idea that ROI takes a longer takes a longer look at the economics than just one year.

On Bruce's system, and with all possible respect to Bruce, his system is unusual in many respects, starting with its size, orientations and goals. And again, with all respect, it's most likely not cost effective but I recall reading on several occasions over the years that cost effectiveness was not one of his goals.
If it was I'm pretty sure there would be more systems like his, but I believe his system is unique. And more power to him for what he's done even if I and others may think it's eccentric.
FWIW, if he's having fun with it, I'm a fan but I doubt many informed people would call it cost effective or practical.

[lockem]

Sun hours is the average amount of sunlight per day, for the entire year:

TOTAL_SUN_HOURS_IN_ONE_YEAR/365

The expected value in Illinois is 4.4hours per day. The expected value in the high desert in California is 5-5.5, although that fails to consider raised solar intensity due to reflections off snow covered cliffs and high altitude dry air. The highest in the country is 6. I believe that getting 7 hours at max output is a symptom of an undersized inverter. Based on your numbers, your system is 3 times the size of mine, but produces less than twice as much energy. Part of that is due to location, but some of it is also likely due to an undersized inverter.

To estimate return on investment in units of time compute

(cost of upgrade)/(value of electricity saved in a year)

It is a straightforward computation. It should be done by system installers when they quote the system.

In your case it seems unlikely that upgrading a 4/0 cables would be economic as it would cost at least several hundred dollars and likely increase electricity production by $20/year. At some point running higher voltage via transformers is going to be less expensive than heavier cables.

In my case I reduced my loss from 0.011%/foot to 0.007%/foot. Over your 600 foot run that would be an improvement from 6.6% to 4.2%. Since you were already at 4% loss, the level of wire upgrade that I was discussing was already present in your system before you made any changes.

[bcroe]

More owners should do the analysis you have done. My project starting
requirements were:
Reduce AC transmission losses to 1/4
Use hardware no larger that 200A breaker boxes and 2in conduit
Not spend thousands on wire.

To answer your questions...

The obvious benefit of my upgrade was some 800 kWh extra annual
energy, without any other change. This helped me run more
equipment on my generation, such as frequent air circulation for
evening out heat and fully utilizing my electronic air filter, and
keeping another building comfortable year around. It will eventually
help compensate for reduced aging panel output. Note, under good
sun my inverters run at capacity for 8 hours straight, not 5.

Here in IL I am not under NEM1 or NEM2.

With reduced voltages, I no longer have to worry about voltage monitor
tripouts. Those could be a real problem if my current (11 year old)
inverters (15kW) were replaced with a different model.

The major expense for the project was $650 delivered for 300 ft of
direct burial 4/0 aluminum triplex. It hardly seemed worth trying to
save a fraction with smaller wire, if all the labor was going to be the
same. It also helped justify my 2 year earlier purchase of a trencher,
its long term goal to allow easy trenching on many projects (5 so far)
without all that strain on my back. It also reduced the width of digging
to only 3 in, way less dirt to deal with than with a shovel.

Going for even bigger wire could only save a bit of my 1% loss, but
would violate my requirement of being physically able to fit into 2in
conduit inside buildings.

The project also added a ground wire between buildings, bringing
the 70s code installation up to date, and fitting into some future ideas.
And it let me bury a backup Cat5 cable to my radio tower, the original
cable that supplies my critical internet connection is older and not
buried deeply enough. It put to work a massive (4 position) 100A
breaker that had earlier been decommisioned in anticipation.

I do not know if there can be a cost-benefit analysis of these factors,
I do not care. The idea of the PV science project was to free me of
getting the finger from the fossil fuel cos, at a cost I could afford.
Eliminating electric energy bills is another benefit. Bruce Roe