Ground mount vs Roof mount?

[nerdralph]

Ground always first with overwhelming advantage except for loss of ground space.

I disagree there is an "overwhelming" advantage. Ground will often produce more than roof-mount, but BoS costs are much higher. And I've seen a few examples where PVWatts estimates were lower for ground mount than roof. Roofs, being higher up, get less indirect shading.

[davidcheok]

I disagree there is an "overwhelming" advantage. Ground will often produce more than roof-mount, but BoS costs are much higher. And I've seen a few examples where PVWatts estimates were lower for ground mount than roof. Roofs, being higher up, get less indirect shading.

Ground mounts will be easier to service/maintain/clean, they can use sun tracking, upgrading will be easier and cheaper and a lot more scaleable assuming you have the space. Yes, if the ground is surrounding by tall buildings/trees/mountains then of course that scenario it wont be suitable but that also applies to roof mounts.

[vnatale]

1.) Thank you. As a preliminary estimate, as a long-term annual average, every unshaded, installed STC kW ground mount of PV can be expected to generate something like 1,300 kWh/yr. of electricity. That's with a 45 degree tilt and a 180 degree array azimuth.
2.) So, once you get a design load in kWh/yr. for the project (not before) divide that design load by 1,300 kWh/yr. for a working estimate of system size in STC kW.
If cost effectiveness is one of the project goals, you'll need a reasonable estimate of the load the system is expected to meet to achieve that goal or balance it with other project/system goals.

In all the reading I've done regarding solar (since about 2016) I think this is the first time I have encountered the term "STC kW". I'm assuming that if I thought I'd needed an output of about 6,500 kWh per year then I divide that by 1,300 then I need a 5 kW system?

You came up with the 1,300 by using my zip code to determine the amount of sun my house would roughly be getting? With the array directly facing south and no shading?

[vnatale]

PV equipment for consumers tends toward the most elementary.
Not ground mount, not snow repelant, not variable tilt. Bruce Roe

I assume that you have variable tilt? If so, how often do you adjust it? Done on some sort of seasonal basis?

How much more complicated / expensive is adding variable tilt to a ground mount?

[vnatale]

You are going down the same energy saving trail as me. 100%
solar electric was achieved 9 years ago. Getting numbers on everything
reveals a lot. A meter connected for a month revealed that the 4
decade old electric range was using far less than 1% of my energy.
The microwave was an efficient cooker, but its clock was a vampire
load using more energy in a year than the short time cooking. The
cure was a circuit to electrically disconnect it from the outlet 7 min
after it finished cooking and the door was closed. It stayed at zero
energy until the next time the door was opened.

Finding and replacing older inefficient appliances really helps.
Chasing down vampire loads on some 60 circuits here turned up
half a hundred things that could be improved.

Your 100A service and transformer size are a limit, but possible
with the size equipment you are looking at. My situation is similar,
but the numbers are about 2 X yours at this originally all electric
home built in the 70s.

Sizing my max Solar output at 15KW, was based primarily on the
heat load over a year. I knew how many gallons of propane were
consumed in a year, and a gallon has the energy of 27KWh. But
electric resistance heat is near 100% efficient, and the furnace was
less than that, so I needed to generate about 25KWh from solar
to replace each gallon of propane. PV estimates indicated I would
be a bit short, but the plan was to buy the energy shortfall while
continuing to try and improve the efficiency of the house. My chart
says about 40 KWh have the energy of a gallon of oil, do not know
the efficiency of your furnace. Have you done this calculation?

The efficiency gain was done here several different ways. One was
to use (6) heat pumps to boost heating efficiency to anything from
100% to 400%, depending on how cold it was. There are a few
really cold days when the heat pumps drop to 100%, but overall
there was a big surplus. It was sufficient that I was then able to
maintain a shop well above freezing all year, and a quite comfy
working 65F most months.

The other improvement was to boost solar energy collected. Peak
power was limited to the same above, and also by my net metering
contract and inverter size. What was not limited was how many
hours I could collect solar energy. Normally peak power only lasted
several hours around solar noon. But I replaced a string facing south,
with a pair of strings facing east and west. First the east panels
deliver energy from the morning sun, then the west panels deliver
afternoon energy. With an elevation of near 57 degrees, the power
(limited by the same string inverter) is about the same but for twice
as long.

This scheme gives a big increase in energy collected without an
increase in peak power. It is also quite effecive in increasing output
under clouds of varied density. It reduces the effectivness of the
cheap PV panels, but greatly increases the utilization of all the rest
of the system. Even the ground mounts, which may cost more than
their panels, can be made double sided for considerable economy.

I am finding, my mini heat pumps are excellant for drying clothes
hung in front of them, receivng a lot of use in the winter when outdoor
lines are unusble due to the weather. So much so, I am considering a
couple of short lines to be easily hung near each mini. I do have an
unvented heat pump clothes dryer, but it tends to run too cool for the
job. Its virtue is not sending a lot of heat outside, the heat pump seems
redundant in that all heat remains in the house and and largely in the
machine. For summer of course clothes dry outside. Bruce Roe

A lot to respond to here!

I could put a meter on the microwave for a few days to see both how much it uses per minute when cooking plus how much it is using when it is just idle. I do have it on a strip that I could easily turn on and off as needed. I tried doing that but kept forgetting to shut it off.

Was that circuit to automatically disconnect it your own design?

I have a small house - about 900 square feet. When I bought it nearly 41 years ago it had a 60 amp service which was enough. It was half fuses and half circuit breakers. 10 years ago I had it replaced with a 100 amp service. The panel box is huge. I had been only using about 7 circuits until the three newly installed mini-splits each with their own heat pumps each took a circuit. I still have possibly 20 open slots left.

I have an oil furnace. Last efficiency reading I have on it was 2017 when it was at 85%, which is the same as what it had been in 2010.

Since it is usually only me in the house I tend to hibernate in my bedroom during heating season. That lets me keep the house temperature at 60 degrees while keeping my bedroom (through supplemental heat - space heaters) at around 75 degrees.

Doing that leaves me using about 175 gallons of oil year. I only had my 275 gallon tank filled four times in six years.

When you asked: "Have you done this calculation?" I do not know what the formula is to do the calculation.

When you are describing your use of strings ... not clear if you are referring to a roof mount or a ground mount?

Not following making the ground mounts being made double sided? My goal is to orient it south. I'd get nothing from having anything on the other (north) side.

[vnatale]

You're welcome. Keep learing.Your biggest enemy will be your lack of education.

After page 85 and until page 250 the book went into a rather large diversion into a lot of uses for solar aside from a whole residential solar installation. A lot of it was somewhat interesting but of marginal value in understanding a whole residential solar installation. Finally on page 250 there was one chapter on whole residential solar installation followed by another chapter on how to deal with contractors. Those were the most valuable chapters in the book.

On page 275 there was nearly a full page box titled "Common Terminology". It started off with: "You’ll need to understand some basic industry terminology in order to work with solar PV systems" It then listed these 8 terms: Load, Volt, AC, DC, Ampere, Resistance, Watt, Watt-Hour.

I may have no solar experience but I have had decades and decades of exposure to all those electrical terms and was quite familiar with all of them.

Brings to mind a discussion I had with an electrician one day who was clearly not understanding all the terms.

I asked him why he was configuring some lights to use a certain voltage or amps. He told me he was doing so because then they'd use less electricity. He did not understand when I tried to explain to him that raising one was going to lower the other because the light was still going to draw the same amount of watts no matter how much he raised or lowered either the amps or voltage. Watt used, of course, is how the light usage ultimately gets billed.

[vnatale]

Probably helpful for DIY, but being off grid is a lot of ongoing work and maintenance that's usually more effort than a part time job. It's a different ballgame than grid tied.

As soon as I finished reading the Dummies book last night I started reading that one. However, I think it may be the most poorly written book I have ever read in my life. It possibly could have been written in a different language then poorly translated to English. But it did go into a lot more substantial depth than the Dummies book in all the details of the components of a solar installation.

[vnatale]

I disagree there is an "overwhelming" advantage. Ground will often produce more than roof-mount, but BoS costs are much higher. And I've seen a few examples where PVWatts estimates were lower for ground mount than roof. Roofs, being higher up, get less indirect shading.

Another unknown acronym for the uninitiated! "BoS"?

[vnatale]

Ground mounts will be easier to service/maintain/clean, they can use sun tracking upgrading will be easier and cheaper and a lot more scaleable assuming you have the space. Yes, if the ground is surrounding by tall buildings/trees/mountains then of course that scenario it wont be suitable but that also applies to roof mounts.

How common is sun tracking?

This seems to be the only vendor offering something in my area and the company owner has already stated in an email to me that I am a fair distance away from them: Tracker mounted backyard solar panels VT NH MA | Solaflect Energy

Vinny

[J.P.M.]

How common is sun tracking?
Vinny

These days, not very, and probably impractical from a reality standpoint.
Given the problems and required maintenance associated with tracking systems as well as a general violation of the KISS principle as well as the decrease in panel prices over the years, the no brainer part is that while a tracking system may increase the specific output of a system (in kWh/yr. per installed STC kW), the same output results can be obtained by simply upping the fixed array size. Also, a tracking system's costs/installed STC kW may well put it beyond cost effectiveness for the application.

STC == Standard Test Conditions. See the Dummies book.

[vnatale]

These days, not very, and probably impractical from a reality standpoint.
Given the problems and required maintenance associated with tracking systems as well as a general violation of the KISS principle as well as the decrease in panel prices over the years, the no brainer part is that while a tracking system may increase the specific output of a system (in kWh/yr. per installed STC kW), the same output results can be obtained by simply upping the fixed array size. Also, a tracking system's costs/installed STC kW may well put it beyond cost effectiveness for the application.

STC == Standard Test Conditions. See the Dummies book.

Thanks for the quick response. I will officially take that off my list of desired options.

By the way ... I just did a search of the Dummies book and no match was found for either "STC" or "Standard Test Conditions".

[J.P.M.]

Another unknown acronym for the uninitiated! "BoS"?

BoS== Balance of Sale.

[bcroe]

I could put a meter on the microwave for a few days to see both how much it uses per minute when cooking plus how much it is using when it is just idle. I do have it on a strip that I could easily turn on and off as needed. I tried doing that but kept forgetting to shut it off.

Was that circuit to automatically disconnect it your own design?

I have a small house - about 900 square feet. When I bought it nearly 41 years ago it had a 60 amp service which was enough. It was half fuses and half circuit breakers. 10 years ago I had it replaced with a 100 amp service. The panel box is huge. I had been only using about 7 circuits until the three newly installed mini-splits each with their own heat pumps each took a circuit. I still have possibly 20 open slots left.

I have an oil furnace. Last efficiency reading I have on it was 2017 when it was at 85%, which is the same as what it had been in 2010.

Since it is usually only me in the house I tend to hibernate in my bedroom during heating season. That lets me keep the house temperature at 60 degrees while keeping my bedroom (through supplemental heat - space heaters) at around 75 degrees.

Doing that leaves me using about 175 gallons of oil year. I only had my 275 gallon tank filled four times in six years.

When you asked: "Have you done this calculation?" I do not know what the formula is to do the calculation.

When you are describing your use of strings ... not clear if you are referring to a roof mount or a ground mount?

Not following making the ground mounts being made double sided? My goal is to orient it south. I'd get nothing from having anything on the other (north) side.

The microwave oven was one exercise in cutting down my
electrical vampire loads, which were some 100 devices and
300W, or 2,630 KWh per year. A KILL-A WATT meter will easily
make measurements of plug in stuff. I did design a small circuit
added to the microwave, others might go the replacement Energy
Star route. Things like the furnace, central vac, seurity devices,
doorbell, chargers, wireless phone, computer devices, smoke
detectors, door openers, TV remote monitor and distribution
system, and more have small transformers running 24/7, burning
several watts each. I trimed around 80% of that over time.

The 100A upgrade makes your solar possible. 200A would be
more comfortable, but would would involve the feed wiring and
possibly the PoCo pole transformer, besides the box.

That appears to be a very energy efficient house. Say you use
200 gallons of oil a year, 200 gallons X 0.85 eff X 40 KWh per
gallon = 6800 KWh. If you change to electric resistance heat
with 100% efficiency, you would need that production plus
your electric energy use, from your solar. Heat pumps with
efficiency from 100 to 300 % efficiency will reduce your need.
A mini split heat pump for your 75 F room will be more efficient
than a simple space heater. This if your net metering contract
allows energy collected in the summer to be used one for one
in the winter.

Doing all of what I discussed would be very difficult on a roof
mount. Double sided is for an east-west arrangement. Can work
on a roof, but optimum elevation is likely a lot steeper than your
roof.

The reasons for a 2 sided array here are to get near twice as many
productive hours out if a solar day, and to double the output under
clouds. It sacrifices individual panel efficiency, while maximizing
use of your mounts, electrical equipment, and net metering contract.
It works here by wiring an east facing string in parallel with a west
facing string (individual fuses). Since they do not produce peak
power at the same time, the inverter sees them as just a single string.

Here is a pic of my first 2 sided array, and a graph of more optimized
dual string output, 8 hours of peak production. Bruce Roe

3Direction.JPGNScurJn17.jpg

[J.P.M.]

Thanks for the quick response. I will officially take that off my list of desired options.

By the way ... I just did a search of the Dummies book and no match was found for either "STC" or "Standard Test Conditions".

From the Dummies index, see p. 264 of the 2d. ed., 1st para., 1st word.

Also, see any solar panel data sheet for details and parameters of std. test conditions. I specify STC as identifying the panel power rating as different from other ways of rating a panel which do little more than muddy the water and confuse people and are therefore B.S. The STC method keeps everyone on the same page and is the standard by which all panels are rated and their output measured.

[vnatale]

BoS== Balance of Sale.

From here: What does BOS stand for? — Page #4 (abbreviations.com)

It seems like it could mean close to a 100 different things! Good thing I asked.