21kW System: Fixed Roof Mount vs Ground Mount w/Adjustable Tilt - Wooded Property

[rendrag]

I'm planning a 21kW system and analyzing mounting options for my wooded property (aerial view attached). System specs:

• 36x Bifacial 585W panels (if ground mounted)
• 1x EG4 FlexBOSS21
• 3x EG4 WallMount Indoor Batteries
• 1x EG4 GridBOSS

I've run production calculations comparing:

1. Fixed roof mount at ~ 26° pitch
   • Annual production: 25,589 kWh
2. Ground mount with 5-period tilt adjustment (10° summer, 30° late summer/early fall, 50° fall/winter, 80° winter/early spring, 30° spring)
   • Annual production: 26,891 kWh
   • 4.8% increase (1,302 kWh annually)
   • Most significant gains in winter months (15-16% improvement Dec/Jan)
   • Minimal difference in spring/summer (0.5-5%)

Key considerations:

• NE Ohio area - in the snow belt with an average of 100" of snow / year
• Heavily wooded surroundings but cleared space around house
• ToU rates available
• Winter production is priority due to EVs
• Have space for ground mount in cleared area shown in aerial view

Main questions:

1. Is the 1,302 kWh annual gain worth the additional cost/complexity of ground mount? I'm also imagining that the ability shed snow with the steeper mount will possibly compound this gain.
2. With the tree coverage shown, would ground mounting in the cleared area actually achieve these calculated gains, or would roof mounting be more practical despite slightly lower production? If you notice, the tree line to the west would block out some later afternoon sun.
3. For those with adjustable ground mounts - what's your real-world experience with seasonal adjustments? Time commitment? Maintenance issues?

I'm leaning toward ground mount for winter production, but concerned about tree coverage and the ease of just having them on the roof.. Would appreciate insights from those who've made similar decisions.

 

[mciholas]

Are you willing to clean snow off the array? 26 degrees might not be enough to shed snow by itself. Can you reach the roof from ground level with a long handled brush? Snow can remain on an array for along time, the panels chill down at night due to radiation cooling and don't warm up as fast as on expects. The clear cold morning sun is good energy so you don't want to lose that if you can.

If the goal is maximum production in the winter (maximize the minimum month), then 26 degrees isn't very good for your latitude. At steep ground mount, say 60 degrees, would shed snow much better and get you more winter production. it is also easier to clean off snow from a ground mount if you need to, and service it in general.

Ground mount removes the need for MLPE RSD devices, saves money and reduces complexity. That's about $2-3K of stuff all told. Put that towards the ground mount structure can help offset the cost.

If the inverter is outside by the array, then its heat and noise are not in the house, but it is outside. If the inverter is inside, the heat in the winter is useful. You do have to watch the "1 meter" PV wire rule for RSD needs (see NEC 690.12 for that). I personally feel the equipment lasts longer inside so best to keep panels outside and everything else inside. Given your more northern climate, and the fact you will have ample sun in the summer, I'd go for the inverter heat in the house.

I don't think the tilting option is worth it. Will drive up the cost of the ground mount, make it weaker, and for that money, you could add more panels and get more overall production. Panels are among the cheapest part of the system now.

When you have to reroof the house, the ground mount will save a bunch over having to remove and reinstall a roof system.

Do you get any sort of net metering? That may influence the setup quite a bit. If you got 12 month net metering carry over (like FL gets), then max yearly production. If you get 1 month net metering with no carry over (like KS gets), then you need to go for the best match between your usage and your generation in a month. This usually means optimizing for Nov, Dec, Jan production and accepting the excess the rest of the year. My reading of the OH rules suggest it is like KS, you get to net meter in a month, no carry over. You pay for the net import each month, if any, or you get a small credit for the net export each month, if any. This may suggest you don't need that large a battery array unless this is for grid down backup reasons.

Good luck with your system!

Mike C.

 

[shadowmaker]

Those calculators don't account snow properly. With 26 degree roof mount and 2,5m snow your winter production is close to zero. Cleaning snow helps, but who would want to do that on purpose? I live ~3000km more north than you, but we get only like 0,5-1m snow yearly (air needs to go over mountains so it dries). My ~55 degree shed roof panels (15,4kWp) made ~30kWh, while my vertical bifacials (44,1kWp) made ~770kWh this December.

These pictures were taken ~10sec apart. It depends on your climate, but in here even 60 degree isn't enough for shedding snow.

If you go with ground mount you need decent ground clearance for snow. My verticals are 1m from ground (top is 5m high) so there's little chance to ever need to clear snow. For you 2,5m starting height might be impractical, but make sure you have easy access to clear snow under your setup with snow blower or similar. Also bifacials really like snow/ground mount.

Other things to consider is those trees around your property. There are some phone apps where you look through your camera and app adds sun position to it at any day/time around year you want. I haven't use any as my set is on open field but have seen people using those here. That might help you locate your ground mount better and see if some trees are going to be a problem.

My verticals can't be adjusted, but even in summer our optimal sun angle is just over 50 degree and day is 19 hours long, so I still get plenty of production. For you being so much more south, it might be better to have adjustable array or just heavily overpanel as panels are cheap.

 

[rendrag]

Those calculators don't account snow properly.

That's my biggest concern; we can get a lot at once here and I see other houses still have panels covered in a foot of snow.

If you go with ground mount you need decent ground clearance for snow

Anything over about 1m base height is starting to get prohibitive with local code requirements on wind rating of 130 mph and snow of 30 psf.

I always forget how far north much of Europe is relative to the USA.

Do you get any sort of net metering? That may influence the setup quite a bit. If you got 12 month net metering carry over (like FL gets), then max yearly production. If you get 1 month net metering with no carry over (like KS gets), then you need to go for the best match between your usage and your generation in a month. This usually means optimizing for Nov, Dec, Jan production and accepting the excess the rest of the year. My reading of the OH rules suggest it is like KS, you get to net meter in a month, no carry over. You pay for the net import each month, if any, or you get a small credit for the net export each month, if any. This may suggest you don't need that large a battery array unless this is for grid down backup reasons.


Mike C.

Yes - Ohio is actually more like FL from my reading of this bit of the Net Metering Rider from First Energy.

I think I'm fairly convinced that Ground is the best way to go with our roof at 6 years old, a planned addition to the house in 2-3 years, and the snow concern. Now for distance, it's about 60 feet to the entrance to the house ( crawlspace on the side of the panels, basement on the other side where service enters) and 125 total feet to the service entrance. Is that going to cause any major voltage drops?

 

[mciholas]

NE Ohio and Finland are at very different latitudes. A vertical array won't be very good year round.

A 60 or 70 degree array might be pretty good, but may require snow cleaning for maximal performance.

Mike C.

 

[SparkyJJO]

Is that going to cause any major voltage drops?

Use the right size wire. And a higher array voltage helps to offset that too.

Go ground. I have roof mount, don't get near as much snow as you, but 2 inches in my roof one day means the next sunny day I am getting a dozen watts maybe out of my 8400 watts of panels until they melt off. And 2 stories up I'm not climbing up there to clean em off!

 

[shadowmaker]

NE Ohio and Finland are at very different latitudes. A vertical array won't be very good year round.

For you being so much more south, it might be better to have adjustable array or just heavily overpanel as panels are cheap.

 

[mciholas]

Anything over about 1m base height is starting to get prohibitive with local code requirements on wind rating of 130 mph and snow of 30 psf.

That's probably not as severe a requirement as you think. It doesn't take much structure to reach those values.

Also, there may be code relaxation at higher pitch angles on the 30 psf number. It sort of doesn't make sense a 60 or 70 degree array would carry that much weight a those angles.

I always forget how far north much of Europe is relative to the USA.

Yes, they are really far north and enjoy the warmth of the North Atlantic Current which brings warm water to the coasts of Europe. If the current ever shuts down or reverses, Europe will freeze and Novia Scotia will become pleasantly warmer. There's models that suggest this is a real risk and it seems to be driven by the salinity of the water, which could go down with rising sea levels.

Yes - Ohio is actually more like FL from my reading of this bit of the Net Metering Rider from First Energy.

Well, not really. There's subtlety in the statement. I believe it means this:

Within a billing period, about a month, you get what appears to be 100% net metering. That is, you subtract what you exported from what you imported. Each KWH of export offsets a KWH of import at 100% value. If your export is less then your import, you pay retail price for the electricity.

If your export is more than your import, you have a surplus. But you don't get to count that in KWH that you can use to offset future import. Instead, that surplus is converted to a monetary credit BUT NOT AT THE RETAIL RATE. You get a credit at the rate in the GSR (Generation Service Rider) which you can bet will be quite a bit below retail prices. I don't know what those are, but commonly it is about 25% of retail in the numbers I have seen for other utilities.

In summary, you get 100% net metering in a month, but perhaps only 25% credit of any surplus beyond that. In FL, you get to carry that surplus at 100% for 12 months, which radically changes the full year performance. This seems more like KS, which you get 100% in a month, but only a small credit past that.

What this means practically is that you need to try and balance usage and generation in every month versus the entire year. It would be economically better to bring up Nov, Dec, Jan than to increase the full year output number. This is because you can't use May Jun Jul to fully pay for Nov Dec Jan.

Now for distance, it's about 60 feet to the entrance to the house ( crawlspace on the side of the panels, basement on the other side where service enters) and 125 total feet to the service entrance. Is that going to cause any major voltage drops?

No, those distances are very manageable. PV wires are usually high voltage (~350-400 volts is ideal) and not much current so it is easy to get the power to the house without much loss.

Mike C.

 

[mciholas]

If you want to go completely gadget on this, build an array with an electrically adjustable angle.

For each day, it adjusts to the best angle.

If snowing, it adjusts to vertical. Minimizes build up. You can also have it do a "dump" process after it snows.

If windy, it adjusts to flat. Minimize wind resistance.

Your trees do concern me, they will shadow the array outside of the mid day hours. You may need to get busy with a chainsaw to improve the performance.

Mike C.

 

[shadowmaker]

Is that going to cause any major voltage drops?

No. Just like others already said, you just need to determine correct wire size. You just need to know some specs (operating voltage, current and distance) of your system first and use online calculators to do that.

Both my arrays are ~100m (330ft) away from my "solar corner" so distance isn't problem, cost of wire might be. I have 800V system so current stays low which enables smaller and cheaper wires. After finding out correct wire size I doubled it just that if I ever decide to expand my arrays. At the moment both arrays use those extra wires, so voltage drop is around 1% only.