Fewer panels if ground mount?

[beowulf]

I am struggling to find sufficient roof area for 34 310 Q cells given set back requirements or for Panasonic 330s. We have a lot of land for a ground mount with a couple of areas acceptable esthetic wise (one area close to a meter and one area about 150 feet away from a meter). But ground mounting is more expensive, but I understand it is more efficient given the ability to set at the optimum tilt and perhaps adjust for seasons. My question - is ground mounting so much more efficient than a roof mount that I could get by with fewer panels- e.g., 30-32 panels instead of 34 on the roof, which could make up some of the additional expense for ground mounting?

Not a fair question, I know, but I am trying to educate myself as much as possible for those times I talk to the contractors making proposals. Any help is appreciated.

[J.P.M.]

For the same orientation and shading, because a ground mount will probably be in a slightly cooler environment, and also because there is usually more air circulation behind a ground mount because they are not flush with the ground, that arrangement may be a bit more efficient. However, roof mounts, because they are off the ground, are often in a slightly windier environment which tends to make them run cooler, particularly if there is a more generous space between the array and the roof deck. What are often steeper array elevation angles possible with a ground mount (or at least more flexibility in orientation) may also help or hinder array cooling by the wind.

Generally, but with no guarantees, ground mounts often run a bit cooler, by maybe a couple deg. C which might translate into a % or two improvement in annual output per installed STC kW, but with no guarantees.

Run PVWatts open rack for ground mount and then as a roof mount at the same orientation for a dart throw comparison. Then run the open rack at the optimum orientation.

[Sunking]

JPM covered the thermal aspects, but there other factors that come into play. As you noted seasonal tilt can be adjusted, but what has not been mentioned is cleaning panels. Initially no problem on a roof, but after time dirt, grime, bird droppings, pollen, tree sap, particulates start building up on the panels. You can only do very limited cleaning on roof mounts, ground no problem. Just a hose and squeegee.

[bcroe]

I like a ground mount which requires very little of my attention for clearing snow. That involves panel
spacing, orientation, ground clearance, and easy tilting, all of which will increase the price. See the
sticky Snow Tolerant PV Mounting

A somewhat lessor advantage is ability to get a bit more energy with a seasonal tilt change.
Bruce Roe

[jflorey2]

I am struggling to find sufficient roof area for 34 310 Q cells given set back requirements or for Panasonic 330s. We have a lot of land for a ground mount with a couple of areas acceptable esthetic wise (one area close to a meter and one area about 150 feet away from a meter). But ground mounting is more expensive, but I understand it is more efficient given the ability to set at the optimum tilt and perhaps adjust for seasons. My question - is ground mounting so much more efficient than a roof mount that I could get by with fewer panels- e.g., 30-32 panels instead of 34 on the roof, which could make up some of the additional expense for ground mounting?

Answer - probably. It will make a small difference, but not a lot. You might see 5% more power.

20 years ago this was a big deal. Going from 34 to 32 panels might have saved you $3000 - and that could pay for a ground mount. Nowadays it saves you $400, which doesn't pay for as much.

[beowulf]

Thanks to everyone who responded - the information has been helpful as I continue to plan and configure a system. I have now received a proposal for a ground mount with Panasonic panels which is only about $1,000 more than for a roof mount. As a roof mount is tight area wise I will probably go with a ground mount at the fixed optimal tilt. I have submitted comments and questions back to the contractor (who, per all reviews is reputable) and waiting for responses. One issue is that the proposal did not include SolarEdge power optimizers (which other proposals included). It might be they were inadvertently left out, or perhaps because the ground mount will be in full sun all the time, it was decided they were not needed.

A side note, as I researched advantages of power optimizers, I found the Panasonic has just very recently offered a new panel with SolarEdge optimizers built in. Those are not the panels specified in my proposal and I have not been able to find the cost for those new "S series" panels.

Likely there are other discussions in here somewhere about whether power optimizers are recommended for a full sun circumstance and I will search for those. In the meantime, any advice on that situation welcome.

Thanks again to all.

[jflorey2]

One issue is that the proposal did not include SolarEdge power optimizers (which other proposals included). It might be they were inadvertently left out, or perhaps because the ground mount will be in full sun all the time, it was decided they were not needed

Note that you can't just get the optimizers - you have to get the whole Solaredge system.

You have three choices:
1) Simple string inverter. One big inverter, simple wiring to panels. Generally simplest/most reliable if there's no shading.
2) Microinverters. One inverter per panel*, all connected to a 240VAC bus. Great for dealing with shading issues.
3) Solaredge. One optimizer per panel, all connected to a big central inverter. Best (or worst) of both worlds, depending on your perspective. Good with shading; more complex.

I would tend to avoid "built in" optimizer/microinverter panels. They can be harder to replace than ordinary panels in the event of a problem.

[peakbagger]

You didnt say where you are located. If snow is an issue be careful. Many ground mounts do not account for snow depth. Yes they are far easier to clean off after a storm unless the bottom edge is too close to ground and a lump of snow forms. I drive by big commercial PV arrays in Mass on occasion where they are clear 3/4 of the way with the bottom edge buried in snow. My pole mount originally had good clearance but I replaced the panels with larger ones and the new clearance when tilted for winter is lower. I need to run my snowblower in front of it after most storms so the snow doesn't build up enough to block the lower edge. The big advantage is on a cold sunny day with snow on the ground the panels put out over the rated wattage due to the extra sun bounced off the snow combined with cold temps. The other FYI is more then a few ground mounts have been damaged by rocks thrown by lawnmowers. Make sure you cut the lawn so the chute is pointed away from the array. Same applies to snowblowers.

[beowulf]

Thanks again for input. It is all very helpful to my learning curve - becoming less steep.

A bit more information / comments:

The inverter will be SolarEdge. - that is what was specified in the proposals - I assume I can have a SE Inverter without optimizers???.
It makes sense not to combine a panel and optimizer - had not thought about that.
There will be absolutely no shade.
There will be snow at times but not a lot. We have been here 32 years and just an inch or two 2-4 times a year in recent years, and in past years we have had perhaps 9-10 inches maybe 4-5 times total, and usually gone in a few days. I do have to sweep out the satellite dish once in a while for snow Location is at 2400 feet in Sierra Nevada Foothills.
Ground mounted panels will not be any area requiring mowing, but will be adequately fenced for cattle, weed fabric and gravel underneath.
Proposal is for bottom edge of array to be at 4 feet and top edge at 7 feet.

[bcroe]

I have now received a proposal for a ground mount with Panasonic panels which is only about $1,000 more than for a roof mount. As a roof mount is tight area wise I will probably go with a ground mount at the fixed optimal tilt.

To keep snow from piling up in front of your array, mount them well above the ground, these clear
40 inches. In addition, to keep some snow from sliding down to the front at all, set the panels landscape
and leave a 6 to 8 inch gap between the bottom row and others. This will allow much of the snow to
fall behind the array instead, while greatly reducing the distance and labor to clear snow. A lot of snow
will fall through the gap by itself.

Being able to set the array vertical for snow months will reduce labor even more, but at considerable
complication to the mount. Bruce Roe

24FEast.JPG

[jflorey2]

The inverter will be SolarEdge. - that is what was specified in the proposals - I assume I can have a SE Inverter without optimizers???.

No. Solaredge systems don't work without the optimizers.

There will be absolutely no shade.

OK. So regular string inverters (SMA, Fronius etc) will work fine if you want to go that way.

There will be snow at times but not a lot. We have been here 32 years and just an inch or two 2-4 times a year in recent years, and in past years we have had perhaps 9-10 inches maybe 4-5 times total, and usually gone in a few days. I do have to sweep out the satellite dish once in a while for snow Location is at 2400 feet in Sierra Nevada Foothills.

That's pretty ideal. You'll get the gain from the reflection then. I assume you will use a fixed ground mount (i.e. no adjustments.)

[beowulf]

Again, my appreciation for the help - great forum for the uninformed.

If a SolarEdge inverter will not work without SE optimizers, that answers my question (which I already emailed to the contractor) that the optimizers are included in the proposal as a SE inverter is specified - so assume the optimizers are included but were just not separately specified.

Yes, plan is to use a fixed ground mount. I found a site on-line re how to calculate the optimal angle for a fixed system (involved latitude multiplied times a factor and then adding 3) - I assume contractor will have same formula or use some other way to determine best angle. Panels will face true south. My calculations - if I recall correctly - came out for about a 31 degree tilt.

I am awaiting clarification re a few of the contract terms and then will likely pull the trigger.

[J.P.M.]

Again, my appreciation for the help - great forum for the uninformed.

If a SolarEdge inverter will not work without SE optimizers, that answers my question (which I already emailed to the contractor) that the optimizers are included in the proposal as a SE inverter is specified - so assume the optimizers are included but were just not separately specified.

Yes, plan is to use a fixed ground mount. I found a site on-line re how to calculate the optimal angle for a fixed system (involved latitude multiplied times a factor and then adding 3) - I assume contractor will have same formula or use some other way to determine best angle. Panels will face true south. My calculations - if I recall correctly - came out for about a 31 degree tilt.

I am awaiting clarification re a few of the contract terms and then will likely pull the trigger.

FWIW, I would have guessed an higher array tilt than 31 deg. for your latitude. Rules of thumb are nice (for measuring thumbs), but one optimal tilt and azimuth is pretty much a cake walk to SWAG. You can do better than an empirical one liner. Use PV Watts. I'd suggest you start with a tilt = local latitude less maybe 5 degrees or so and an azimuth of maybe 185 -190 deg. and vary tilt and azimuth in 5 deg. increments. Remember, a vendor may not be able to work that close on angles, and it's not super critical anyway - this ain't rocket science.

You can zero in on a theoretical long term optimum that's probably about as good as you'll get by doing maybe 10-15 runs that might take about 2 min. ea. after a 5 min. set up. You'll zero in on an orientation rather quickly and learn a lot about output as (orientation) while doing so. Don't be surprised if you find that annual output is somewhat insensitive to orientation if you stay within maybe +/- 5-10 deg. of optimum on tilt and maybe +/- 10-15 deg. on azimuth. Worst that will happen is you'll confirm that 31 deg., but note, due south is not necessarily or usually the absolute optimum.

After that, if you want to consider seasonal adjustments use the optimal azimuth you settle on and then vary the tilt you found by 15 deg. or so +/- for winter and summer operation. Then, in practice, change tilts as a seasonal thing. 3 tilts making for 4 adjustments/yr. will get you about 95- 98 % of the output of, say, monthly adjustments. Adjusting tilts is interesting the first couple of times. It probably gets old for most after that. For when to change tilts, you can use the PVWatts hourly output option and fool, with the spreadsheets and use running 30 day totals to shift when two tilts give the same running 30 day average output. That's one way of several and probably accurate enough for long term averages, although, just like the weather, any year or season can and probably will be different.

[beowulf]

JPM - thanks. I can't find all my notes re calculations I made, but the particular site did it this way: based on latitude of 37 (rounded up) it calculated the optimal Summer tilt to be about 13.2, and the optimal Winter tilt to be about 51, then it added those and divided by 2 to get to 31. Not sure that is how to do it - but will let the contractor figure it all out. I had not thought about the azimuth.

[J.P.M.]

JPM - thanks. I can't find all my notes re calculations I made, but the particular site did it this way: based on latitude of 37 (rounded up) it calculated the optimal Summer tilt to be about 13.2, and the optimal Winter tilt to be about 51, then it added those and divided by 2 to get to 31. Not sure that is how to do it - but will let the contractor figure it all out. I had not thought about the azimuth.

You're most welcome. I was figuring on your latitude as higher. A 37 deg. lat., a 31 deg. tilt may be pretty close. What's your zip ?

On relying on a vendor for sizing: As you wish, but if you're looking for max. annual output, rather than relying on a vendor to give you an orientation is the result of something that is easier/more profitable for that vendor with not much regard to optimizing your investment, PVWatts will get you closer. How much ? I'm guessing maybe not much, but it's always nice to verify. Besides, I can think of few better ways to self educate than an hour or two with PVWatts - except maybe a couple more hours spent reading "Solar Power Your Home for Dummies" before talking to vendors. You'll then know answers before you ask them of vendors leading to a real education on just how much they do know from their answers to your questions.

Or, related to max. annual output but not the same, if you're on a T.O.U. net metering tariff (or soon will be), and you want to minimize your annual bill using current rates and some dart throws about future rates and tariffs as well as your current and future use and use patterns as f(time by the hour), you can do that as well, but it's a bit more involved than can be explained in a reply to a post. For reasons dealing with the way T.O.U. tariffs and what are hourly rates as f(T.O.U), weird as it may sound, maximizing an array's electrical output under a T.O.U. tariff will not usually result in the lowest annual electric bill.