Solar Setup for Off Grid home and Electric Kiln

[Shazbot]

Hi all,

I'm already off-grid, but using a very underpowered system currently:

• Grape Solar PV: 8x185w
• Mighty Max ML100-12 (DS-IGEL): 4x12v100AH
• Backup generator that I use on the regular.

I'm looking to upgrade to something that supports home electricity usage as well as an electric kiln.

• Home electricity usage targets 2kw.
• Electric kiln:
  • 13kw rating - 70% duty cycle as it turns on and off as part of the heating process so 9.1kw actual
  • Kiln duration targeting 10 hours.

I'm at the point where I feel I've done a decent enough research where I'd appreciate other eyes:

• Signature Solar E000 + 2 additional Battery Racks
  • 12kw PV
  • 81.9kWh Battery
  • 13k Inverter

Current goals/questions:

• Is the PV, Battery, or Inverter too little, too much, or all wrong?
  • I aimed at the worst month in southern California and it'd leave me with surplus of 5.9kWh of buffer.
    • Requirements: 2kw home + 9.1kw kiln = 11.1kw
    • Starting the kiln at 10am PT, assuming batteries are fully charged at this point (2-4 days to charge)
    • Requirements for the 10 hour firing = 11.1*10 = 111kWh
    • PV Generates 35kWH (per Average Hourly Profile on GlobalSolarAtlas, 10a to 8p in December)
    • And 76kWH is taken from the 81.9kWh Battery, leaving 5.9 kWh buffer.
  • Walking through this... I see that the 13k Inverter is maxed with a 13kw rating kiln... I think I can adjust the kiln target to being closer to 11kw rating (7.7kw actual) and still meet my needs.
  • I realize this isn't the cheapest route, and a gas kiln would reduce cost significantly, but there are quality of life considerations that will make an Electric Kiln still desirable in addition to a gas kiln.
• Can I get a NEMA 6-50p plug connected to the Inverter? Related, can the Inverter support a well pump?
• How far from the Inverter can I put the Electrical panel without "significant" power degradation or could I have the panel close to the Inverter and have the supported AC lines extended? I have 5 acres of multi-tiered boonies, so distance flexibility is good.
• Any questions I should be asking that I'm not? Does any of my info or assumptions seem off?

Thanks for reading and any and all input.

 

[sunshine_eggo]

Looks like you have it pretty much covered.

You're pulling > 90% out of the battery pretty regularly. I would be looking to keep that under 80%.

Inverters simply have places to connect wires, so you could certainly install a NEMA 6-50p outlet.

Based on 240V and 50A draw, your distance between inverter and unit could be 180 ft before you exceed the 3% voltage drop limit.

Voltage Drop Calculator

This free voltage drop calculator estimates the voltage drop of an electrical circuit based on the wire size, distance, and anticipated load current.

www.calculator.net

Concur that your 13kW inverter may be undersized for a 13kW load, especially if you have other loads running.

An inverter of that size should be able to handle a well pump. My neighbor had a 700' deep 3hp Grundfos, and he was able to run it with a 6kW Sigineer inverter. That inverter had a 18kW surge rating.

 

[Shazbot]

Thanks a bunch @sunshine_eggo ! I super appreciate the feedback after peering into the hypothetical of an unfamiliar area.
Reducing the kiln pull will keep the battery drain in an healthy zone as well as keep within inverter range.

Oh yeah, Inverters=>Panels=>Outlet ofc.. that makes sense. For some reason I was looking at outlets on the inverter.

The voltage drop is super important to planning, thank you for that. I was thinking of a central system for the property, but it seems that might not be feasible if my AC lines will go beyond 150 ft? I see the chart here which covers it well. https://www.solar-electric.com/learning-center/wire-loss-tables.html/

Would I get more distance... but perhaps less efficiency by putting the battery bank further from the solar array if I had to choose between the two? It seems like there’s a limit there too, looking around, and that the general rule of thumb is to have the panels be close to the rest of the equipment, including outlets…. But how do big fields of solar panels compensate for voltage loss?

 

[sunshine_eggo]

The battery bank needs to be snuggling up to the MPPT/inverter in an almost inappropriate way. You're going to be pulling over 300A @ 48V from the battery at peak power. That's 4/0 gauge, and you don't want that to be any longer than it has to unless you have fat stacks of Benjamins.

High voltage, low current = happy long cable runs, hence the 180ft calculation on the 240VAC and 50A.

Low voltage, high current = shortest runs possible.

Panels are high voltage and low current compared to battery voltage, so you can typically get away with reasonably long cable runs depending on MPPT limits.

PV ------long run --------MPPT/Inverter
MPPT/Inverter - super short run with 4/0 cable - battery
Inverter output ----------- long run (180' @ 50A/240VAC) ------------- AC loads

I don't bother with tables. I use the voltage drop calculator. Anything under 3% meets code. If you need to run longer, get thicker wire or use multiple wires.

 

[RussNM]

Shazbot, how many firings do you have on your current set of kiln elements? To what cone are you glaze firing?

 

[Shazbot]

Hi Russ, I’m trying to think back to all the bent kiln sitter cones I had lying around, I’d guess 300-400 or so?

I’ve been firing cone 6, and am probably going to stay in that range for some time. I’m in the market for a larger kiln, more than I’m aiming for a higher cone.

If I go to cone 10, it’ll likely be sometime in the future so I can get nice copper red reductions.

 

[Shazbot]

Thanks for the guidance, @sunshine_eggo.

I’ve been digging into the wiring math a bit and it’s pretty interesting. Using combiner boxes (if needed) and wiring patterns, one can modify the volts/amperage and therefore modify volt drop impact.

So, trying to put the math together to understand the voltage drop from panels to batteries…

If I’m understanding correctly, I can wire the panes in series in order to increase voltage to ~1500, then run 10awg for over 1000ft before getting too much loss (1.85%).

This gives my solar panel location a lot of flexibility, and then I’d want the battery/inverter within 180 ft of the electricity output. It makes more sense to me as I write this out, presuming I don’t miss anything ofc.

In application, I think I’ll have a certain amount of PV go to the inverter’s battery bypass to cover battery outage.

I think I need to take a look at laying this out in the schematic form I see some folks using.

 

[sunshine_eggo]

If I’m understanding correctly, I can wire the panes in series in order to increase voltage to ~1500, then run 10awg for over 1000ft before getting too much loss (1.85%).

Conceivably, but most panels are not rated for more than 600-1000V, and hardware to handle that voltage gets VERY expensive, but you have the concept correct. Higher voltage, lower current = less wiring losses and enables thinner wires for a given distance.

This gives my solar panel location a lot of flexibility, and then I’d want the battery/inverter within 180 ft of the electricity output. It makes more sense to me as I write this out, presuming I don’t miss anything ofc.

The 180' is based on 6awg wiring - minimum needed for 50A. I you went to 4 awg or thicker, you could certainly go farther.

In application, I think I’ll have a certain amount of PV go to the inverter’s battery bypass to cover battery outage.

This makes absolutely no sense to me. Can you please elaborate?

 

[Shazbot]

Conceivably, but most panels are not rated for more than 600-1000V, and hardware to handle that voltage gets VERY expensive, but you have the concept correct. Higher voltage, lower current = less wiring losses and enables thinner wires for a given distance.



The 180' is based on 6awg wiring - minimum needed for 50A. I you went to 4 awg or thicker, you could certainly go farther.



This makes absolutely no sense to me. Can you please elaborate?

Good to know about panel voltage limits, it sounds like splitting into 2 lines to make it 750v would be more reasonable. The panels have a max voltage of 1000v, so that checks out.

Regarding the confusing statement I made; I may have misunderstood an inverter feature, thinking it would take a direct line from PV because it noted: “500VOC/8000W max input for PV”. Reading again, I’m guessing it’s a limit for a grid tie system.

 

[sunshine_eggo]

My interpretation of those numbers is:

500Voc maximum array voltage - this must include about 15% margin for cold temperature variation, i.e., array Voc should not exceed 425V. So your plan to hang 750V strings would destroy the MPPT.

8000W is the maximum power array that can attached to the MPPT.

 

[RussNM]

Wow thats a lot of firings...From what I understand the elements wear out and it takes longer and longer to reach the cone youre shooting for. with new elements the time to fire is greatly shortened making solar a possibility. I think Euclids has elements for almost every kiln. You might check the specs on the elements especially resistance and if the resistance on your elements is off by 10% or more its time to replace them..... Myself Im a c10 wood fire guy.. and reduction glazes are pretty cool especially the hyper finicky reds.

 

[Shazbot]

My interpretation of those numbers is:

500Voc maximum array voltage - this must include about 15% margin for cold temperature variation, i.e., array Voc should not exceed 425V. So your plan to hang 750V strings would destroy the MPPT.

8000W is the maximum power array that can attached to the MPPT.

Good call, I was aiming at watt limits (1/2 the array is 6500W) and not volts.

Half the array’s panels (15 of 30) would be 5500w and keeping with the 425 volt target would be (5500/425) 12.9 amps… actual would be 8 of 15 labels 50v panels in series (400v), and the rest parallel, making it 13.75 amps.

Running that 200 ft with 10AWG would be 1.71% voltage drop… which hopefully won’t destroy the MPPT as it has 18 amp limit per MPPT and it has 2.

 

[Shazbot]

Wow thats a lot of firings...From what I understand the elements wear out and it takes longer and longer to reach the cone youre shooting for. with new elements the time to fire is greatly shortened making solar a possibility. I think Euclids has elements for almost every kiln. You might check the specs on the elements especially resistance and if the resistance on your elements is off by 10% or more its time to replace them..... Myself Im a c10 wood fire guy.. and reduction glazes are pretty cool especially the hyper finicky reds.

Yeah, that’s over many years and I’ve seen extra delays and the elements need replacing.

I’ve been looking at some L&L kilns for my size upgrade needs.

Wood fire?! Super cool I have no experience with that but I’m excited as my ceramics teacher & friend plans to have her students build a wood kiln for me next year. Always interested in new techniques!

Currently I’m just trying to get back to what I know to continue growing from there

 

[MurphyGuy]

That's a lot of money to spend just to fire up a kiln.. Unless you run that kiln 24/7 and 7 days a week, could you consider a propane fired kiln?

Really, you're about to spend enough money to put a decent down payment on a nice house to support the operation of that kiln. While propane is expensive, that kiln would have to have a lot of hours on it before the propane costs started to approach the solar/battery route.

 

[sunshine_eggo]

Good call, I was aiming at watt limits (1/2 the array is 6500W) and not volts.

Half the array’s panels (15 of 30) would be 5500w and keeping with the 425 volt target would be (5500/425) 12.9 amps… actual would be 8 of 15 labels 50v panels in series (400v), and the rest parallel, making it 13.75 amps.

Not sure I completely follow you. You can't parallel 8 with 7. Parallel strings must be the same voltage.

Running that 200 ft with 10AWG would be 1.71% voltage drop… which hopefully won’t destroy the MPPT as it has 18 amp limit per MPPT and it has 2.

Use Vmp for voltage and Isc for current.

Have you identified specific PV panels?

 

[Shazbot]

Not sure I completely follow you. You can't parallel 8 with 7. Parallel strings must be the same voltage.



Use Vmp for voltage and Isc for current.

Have you identified specific PV panels?

I was thinking I could take 15 of the 30 panels I’ll get (400w boviets - 49.15v 10.45a - I’m not locked into these panels but it’s in a package deal) and wire 8 of them in series to get 400v and the rest in parallel if possible, making 31.5a.

Re: “Use Vmp for voltage and Isc for current.” - I’m not clear on what you’re referring to.

 

[sunshine_eggo]

I was thinking I could take 15 of the 30 panels I’ll get (400w boviets - 49.15v 10.45a - I’m not locked into these panels but it’s in a package deal) and wire 8 of them in series to get 400v and the rest in parallel if possible, making 31.5a.

This sounds a lot like you're trying to parallel an 8S string with a 7S string. You can't do this.

Re: “Use Vmp for voltage and Isc for current.” - I’m not clear on what you’re referring to.

When calculating wiring loses, you use the series Vmp and the Isc current of the panels.

Max voltage to the controller is Voc, but panels won't operate at this voltage. When you draw current from them, the voltage drops to Vmp.

In your case, the voltage you use for wiring losses is 8 * Vmp. For a single string, the current is the Isc value.

 

[Shazbot]

This sounds a lot like you're trying to parallel an 8S string with a 7S string. You can't do this.



When calculating wiring loses, you use the series Vmp and the Isc current of the panels.

Max voltage to the controller is Voc, but panels won't operate at this voltage. When you draw current from them, the voltage drops to Vmp.

In your case, the voltage you use for wiring losses is 8 * Vmp. For a single string, the current is the Isc value.

I was trying to split the 15 and append the others, like maybe 7S2P and append the other??? and while I’m not 100% sure why that won’t work, I’ve modified the plan to have 32 panels total so I can have 2 8S for each of the 2 parallel inverters. I confirmed it can accept 500v per terminal.

Ill be running the wire 150ft on 10awg. I could probably get away with 12awg but I might find I need a few more feet once the panel and inverter location are locked.

 

[sunshine_eggo]

What does "append" mean? Append: "add (something) as an attachment or supplement."

If you do anything with that panel, you won't have a 7S2P array. You'll have something different.

If you try to put it into series with the entire array, you will halve the output of the entire array by forcing the entire thing to operate at the lower current of the single panel. You would then have a 7S2P+1S that behaves exactly like an 8S array - only 8 panels.

If you try to put it in parallel with the 7S2P array, it will serve as a short circuit to allow the 7S2P array to dump all of its current through the panel and never send anything to the MPPT.

If you put it on either of the 7S strings, you then have 8S in parallel with 7S. It's likely the 7S will serve as a short circuit path for the 8S array, again sending nothing to the MPPT.

If you have multiple MPPT, and the voltages work out, you could put 8S on one MPPT and 7S on the other MPPT.

 

[400bird]

The 70% duty cycle of the kiln seems like something you could improve to decrease energy usage and get some more buffer for overcast days.

I realize the kiln needs to be very hot, over 2000°f if my memory of high school ceramics serves me, but if you can keep more of that heat in the kiln, the duty cycle should drop along with energy usage.

Maybe there's some other reason against this, but I'd suspect a few layers of rockwool or other high temp insulation would be a cheap investment to decrease energy usage.