want to design a 3.5 kw off grid system with cheap panels

Cariboocoot

Re: want to design a 3.5 kw off grid system with cheap panels

northerner wrote: »

Not to mention that even with a grid tie system there are losses as well. Does anyone know what those losses are (ie through the inverters, distribution lines, transformers, etc...)??? I know that a grid tie system is more efficient, but you can't assume it's 100 % either.

Array to GTI output is roughly the same as array through MPPT controller output: about 77% +/- a bit.

northerner

Re: want to design a 3.5 kw off grid system with cheap panels

Cariboocoot wrote: »

Array to GTI output is roughly the same as array through MPPT controller output: about 77% +/- a bit.

And I take it losses through the distribution network would be highly variable, dependent on where the load requirements are.

Cariboocoot

Re: want to design a 3.5 kw off grid system with cheap panels

northerner wrote: »

And I take it losses through the distribution network would be highly variable, dependent on where the load requirements are.

Yep. Some inverters will be a bit better (transformerless claim >95%) some wiring will be worse, et cetera. But it would have to be very badly done to be as low as a typical off-grid system's efficiency.

Txrancher

Re: want to design a 3.5 kw off grid system with cheap panels

Cariboocoot wrote: »

Don't.
Sometimes there's darn good reasons for all that legal mumbo-jumbo.
Add a non-permitted system and if anything goes wrong you are on the hook for all liability whether or not the system is to blame.

NEC rules about back-feeding limits (120% of bus rating) are there to prevent overloading the circuitry. A 125 Amp panel could only be fed about 30 Amps @ 240 Volts (7kW). The 200 Amp panel already has 6kW on it but could have 9.6kW max. Depending on the wiring some of the current rating may be allocated for the smaller load panel (seems like an odd way to wire from your description - it's the equivalent of a 325 Amp service attached to the meter?) Possibly you could add another 3kW here.

I find it hard to believe that NE Texas exempts itself from NEC. Practically unheard of in this age when every little government office is out to grab a buck wherever they can. :roll:

Please understand that we have now entered the dicey guessing game of advising on system configuration and addition. Without precise knowledge of what's there already we can only generalize. I know some of the forum members are in Texas and they may have more insight for you on this.

regarding the backfeed issue on the 125amp panel, I found this thread which makes it sound as to not be a problem for me if I lower the size of the main breaker in that panel.

http://forum.solar-electric.com/showthread.php?4759-NEC-requirements-for-a-back-fed-subpanel

solar_dave

Re: want to design a 3.5 kw off grid system with cheap panels

Txrancher wrote: »

correct, I already have the agreement with the utility for the 6.44kw leased system. I will have to look at the agreement and see what it says about over 10kw total, although adding 3.5kw more would be just under 10kw

Well I think you could add 3.5 Kw of panels via the 125 amp panel especially if there is no permitting agency involved. You can have 120% of the panel rated power sources on that bus bar. So 125 X 120% = 150 amps. Up to a 25 amp breaker could be added for solar back-feed. Your 3500 watt panel add should produce well below that even with a safety factor. Also you are under the Utility net metering of 10Kw system.

Cariboocoot

Re: want to design a 3.5 kw off grid system with cheap panels

Yes. Then the problem becomes: can you find that smaller breaker for that panel?

Another curious thing about this method is the limited capacity at night when the GTI outputs nothing. This is not a problem for most people as power use tends to go down then anyway. Still, out there somewhere is an installation where that presents a problem. :roll:

Another possible solution for you (if allowed): line side tap. Although this may not be available with the existing two service panels.

Txrancher

Re: want to design a 3.5 kw off grid system with cheap panels

Cariboocoot wrote: »

Yes. Then the problem becomes: can you find that smaller breaker for that panel?

Another curious thing about this method is the limited capacity at night when the GTI outputs nothing. This is not a problem for most people as power use tends to go down then anyway. Still, out there somewhere is an installation where that presents a problem. :roll:

Another possible solution for you (if allowed): line side tap. Although this may not be available with the existing two service panels.

I probably only use 20 amps max from that 125amp subpanel, its a ge subpamel, pretty sure I could get a smaller breaker for it just to keep it more legal.
better question is if anyone from elec co will ever come out and say, how, when,where,why do you have 2 grid tie systems ????? we only know about one system.......

decisions, decisions.....

oh I just realized that I might need to get them out here to shut off power to insert an AC disconnect between the subpanel and the meter, unless that can go between the inverter and the sub panel ??
just answered that myself looking at current system
AC disconn goes between inverter and panel, not between panel and meter

Cariboocoot

Re: want to design a 3.5 kw off grid system with cheap panels

Txrancher wrote: »

oh I just realized that I might need to get them out here to shut off power to insert an AC disconnect between the subpanel and the meter, unless that can go between the inverter and the sub panel ??
just answered that myself looking at current system
AC disconn goes between inverter and panel, not between panel and meter

Any service panel should have a main breaker on it. That would count as an AC disconnect in my book, but perhaps not in theirs. Did they require a separate "accessible" (outside) disconnect for that GT system?

Beyond that the breaker that connects the GTI to the panel is also a disconnect. 3500 Watts @ 240 Volts is roughly 15 Amps so the 25 Amp breaker should be right and ought to be within the NEC rules for max current on a 125 Amp service (might have done the math wrong there).

Txrancher

Re: want to design a 3.5 kw off grid system with cheap panels

Cariboocoot wrote: »

Any service panel should have a main breaker on it. That would count as an AC disconnect in my book, but perhaps not in theirs. Did they require a separate "accessible" (outside) disconnect for that GT system?

Beyond that the breaker that connects the GTI to the panel is also a disconnect. 3500 Watts @ 240 Volts is roughly 15 Amps so the 25 Amp breaker should be right and ought to be within the NEC rules for max current on a 125 Amp service (might have done the math wrong there).

well I dont know if the AC disconn was required, but it is installed between the inverter and the main panel
also the 125amp panel I would install it in is actually a main panel not a sub, so I would probably put one in

Cariboocoot

Re: want to design a 3.5 kw off grid system with cheap panels

Txrancher wrote: »

well I dont know if the AC disconn was required, but it is installed between the inverter and the main panel
also the 125amp panel I would install it in is actually a main panel not a sub, so I would probably put one in

I know some jurisdictions get crazy about this but the breaker that connects the GTI is a disconnect and the breaker that disconnects the service panel is a disconnect so in my opinion adding yet another disconnect is redundant X3. Some require it to be outside so that the fire department can kill the power, not understanding that pulling the meter or otherwise cutting power to the home does the same thing with a standard GTI (no grid = no power). If it were required only for hybrid systems they might have a case, although even there the output to the grid goes down when the grid does.

I keep looking at your proposal and trying to find the fly in the ointment but I just don't see it anywhere. It all looks like it will work: 3500 Watts, 25 Amp breaker, 125 Amp service. It even looks like it would meet code and stay under the 10kW service limit.

Okay, where are all those people who can't wait but jump at the chance to prove the old man wrong?

Txrancher

Re: want to design a 3.5 kw off grid system with cheap panels

Cariboocoot wrote: »

I know some jurisdictions get crazy about this but the breaker that connects the GTI is a disconnect and the breaker that disconnects the service panel is a disconnect so in my opinion adding yet another disconnect is redundant X3. Some require it to be outside so that the fire department can kill the power, not understanding that pulling the meter or otherwise cutting power to the home does the same thing with a standard GTI (no grid = no power). If it were required only for hybrid systems they might have a case, although even there the output to the grid goes down when the grid does.

I keep looking at your proposal and trying to find the fly in the ointment but I just don't see it anywhere. It all looks like it will work: 3500 Watts, 25 Amp breaker, 125 Amp service. It even looks like it would meet code and stay under the 10kW service limit.

Okay, where are all those people who can't wait but jump at the chance to prove the old man wrong?

sounds good to me, we will see if anyone comes up with something.....
all this caused by panels at 50 cents/watt, burning a hole in my pocket

solar_dave

Re: want to design a 3.5 kw off grid system with cheap panels

Cariboocoot wrote: »

I know some jurisdictions get crazy about this but the breaker that connects the GTI is a disconnect and the breaker that disconnects the service panel is a disconnect so in my opinion adding yet another disconnect is redundant X3. Some require it to be outside so that the fire department can kill the power, not understanding that pulling the meter or otherwise cutting power to the home does the same thing with a standard GTI (no grid = no power). If it were required only for hybrid systems they might have a case, although even there the output to the grid goes down when the grid does.

I keep looking at your proposal and trying to find the fly in the ointment but I just don't see it anywhere. It all looks like it will work: 3500 Watts, 25 Amp breaker, 125 Amp service. It even looks like it would meet code and stay under the 10kW service limit.

Okay, where are all those people who can't wait but jump at the chance to prove the old man wrong?

Well here is another old man that says it looks OK too. No flys in that design except if there was an AHJ involved and they wanted a AC single disconnect for both solar back feeds like I think mine would be likely to ask for. That should be way over an above the NEC though I would think.

Txrancher

Re: want to design a 3.5 kw off grid system with cheap panels

Well, Im back with an update. I got some different panels than we discussed here before.
Ended up getting 3.36kw thats 14 panels Sharp 240 watt ND-240QCJ,
panels never installed at 60 cents/watt = $2016

VOC 37.5v
VPM 29.3v
ISC 8.75a
IPM 8.19

even though it is much more expensive, pretty sure we are going with an off grid setup for this system, we already have a 6.44kw grid tie

of course we are now designing the system bass akwards.....but I will be able to use whatever power we can make

so the question would be how to construct the best bang for the buck system.

To allow further expansion, I assume it would be best to go 48v for the system, correct ?

using the 10% charge formula (if thats the thing to do) what would be the largest battery bank 14 of these panels could charge ? would 8 trojan L16 at 6 volts be too small or too large ? I think they are around 325AH
or what would you suggest for the batt bank ?

thanks

Cariboocoot

Re: want to design a 3.5 kw off grid system with cheap panels

Fourteen of those panels is going to be awkward. With the Vmp @ 29.3 you need at least three in series to get the array Voltage up to where it will charge a 48 Volt system. At that point you have four parallel strings of three, with two panels left over. Can you buy one more panel? Or two less?

Let's see what 12 * 240 Watts gets you: 2880 Watt array. Using an MPPT controller (must) on a 48 Volt system that should provide about 46 Amps peak current. That's enough for using L16 B's @ 390 Amp hours or two parallel strings of GC2's for 450 Amp hours (probably cheaper). If you go with the latter you get up to about 10kW hours of stored power, which is quite a lot for off-grid.

I think that's the way I would go. The golf cart batteries are usually cheapest per Watt hour, two parallel strings of panel and two of battery present maximum power potential with minimal wiring difficulties. You'll have to spend a bit to be sure you have fuses/breakers proper for 48 Volts, but long-term it would be worth it (as opposed to using 24 Volts where the current doubles on the DC side; you couldn't handle the array on one controller).

Eric L

Re: want to design a 3.5 kw off grid system with cheap panels

so the question would be how to construct the best bang for the buck system.

I have two other suggestions. One is that you consider getting an inverter that can use grid support so that it switches to grid power when the battery voltage falls below a certain threshold. Many inverters can do this. This is for two reasons. First, you avoid deeply discharging the batteries, thereby prolonging life, and in fact can find an economical "sweet spot" of D.O.D. vs. battery lifespan and set the grid-support voltage around that. Second, you can add more loads to the system because you won't have to worry about stressing the batteries during cloudy periods -- with grid support there is no need to design loads around, say, 2 or 3 days of battery reserve, as there is on a true off-grid system. This will let you more fully use the system when sun conditions are good, while letting the grid handle it when they aren't.

The other suggestion for "most bang for the buck" is to find ways to maximize opportunity loads. This can have an impact on the choice of charge controller, with some better equipped to support such loads than others.

Txrancher

Re: want to design a 3.5 kw off grid system with cheap panels

Cariboocoot wrote: »

Fourteen of those panels is going to be awkward. With the Vmp @ 29.3 you need at least three in series to get the array Voltage up to where it will charge a 48 Volt system. At that point you have two parallel strings of three, with two panels left over. Can you buy one more panel? Or two less?

Let's see what 12 * 240 Watts gets you: 2880 Watt array. Using an MPPT controller (must) on a 48 Volt system that should provide about 46 Amps peak current. That's enough for using L16 B's @ 390 Amp hours or two parallel strings of GC2's for 450 Amp hours (probably cheaper). If you go with the latter you get up to about 10kW hours of stored power, which is quite a lot for off-grid.

I think that's the way I would go. The golf cart batteries are usually cheapest per Watt hour, two parallel strings of panel and two of battery present maximum power potential with minimal wiring difficulties. You'll have to spend a bit to be sure you have fuses/breakers proper for 48 Volts, but long-term it would be worth it (as opposed to using 24 Volts where the current doubles on the DC side; you couldn't handle the array on one controller).

you said 2 parellel strings of 3 panels, that would only use 6 panels or did I mis understand something ?

in a nut shell, could you tell me what values for volts and amps we want to create when we decide on strings for a 48v system ?

thanks

Txrancher

Re: want to design a 3.5 kw off grid system with cheap panels

Eric L wrote: »

I have two other suggestions. One is that you consider getting an inverter that can use grid support so that it switches to grid power when the battery voltage falls below a certain threshold. Many inverters can do this. This is for two reasons. First, you avoid deeply discharging the batteries, thereby prolonging life, and in fact can find an economical "sweet spot" of D.O.D. vs. battery lifespan and set the grid-support voltage around that. Second, you can add more loads to the system because you won't have to worry about stressing the batteries during cloudy periods -- with grid support there is no need to design loads around, say, 2 or 3 days of battery reserve, as there is on a true off-grid system. This will let you more fully use the system when sun conditions are good, while letting the grid handle it when they aren't.

The other suggestion for "most bang for the buck" is to find ways to maximize opportunity loads. This can have an impact on the choice of charge controller, with some better equipped to support such loads than others.

That inverter type sounds exactly like what I need. any idea which ones have that capability?

thanks

Txrancher

Re: want to design a 3.5 kw off grid system with cheap panels

Cariboocoot wrote: »

Fourteen of those panels is going to be awkward. With the Vmp @ 29.3 you need at least three in series to get the array Voltage up to where it will charge a 48 Volt system. At that point you have two parallel strings of three, with two panels left over. Can you buy one more panel? Or two less?

Let's see what 12 * 240 Watts gets you: 2880 Watt array. Using an MPPT controller (must) on a 48 Volt system that should provide about 46 Amps peak current. That's enough for using L16 B's @ 390 Amp hours or two parallel strings of GC2's for 450 Amp hours (probably cheaper). If you go with the latter you get up to about 10kW hours of stored power, which is quite a lot for off-grid.

I think that's the way I would go. The golf cart batteries are usually cheapest per Watt hour, two parallel strings of panel and two of battery present maximum power potential with minimal wiring difficulties. You'll have to spend a bit to be sure you have fuses/breakers proper for 48 Volts, but long-term it would be worth it (as opposed to using 24 Volts where the current doubles on the DC side; you couldn't handle the array on one controller).

The 10kw of stored power is the full amount correct ? if I discharge to 50% max then it would be 5kw usable ?
and those 12 panels could fully recharge a 50% discharge in 1 day/5 hrs of sun ?

thanks

Photowhit

Re: want to design a 3.5 kw off grid system with cheap panels

Txrancher wrote: »

you said 2 parellel strings of 3 panels, that would only use 6 panels or did I mis understand something ?

The problem is that charge controllers only reduce voltage and 2 panels don't create enough voltage to charge 48 volt bank, so strings of 3 would be a minimum. Since you have 14 panels the next option would be a string of 7, which won't practically fit on any charge controller.

Eric L

Re: want to design a 3.5 kw off grid system with cheap panels

That inverter type sounds exactly like what I need. any idea which ones have that capability?

Many of the Outback models with the Mate controller, the Magnum MS series with the ME-ARC controller, and the Schneider/Xantrex XW inverters can all do it; you should talk to a dealer or research though to see what's best for your situation.

The 10kw of stored power is the full amount correct ?

No, at least not for the 450 amp hour bank. 450 ah x 48 volts = 21,600 watt-hours. So about 10 KWh usable to 50% D.O.D, but you probably wouldn't want to routinely discharge them that deeply. I have a grid-supported system similar to what you are thinking about, and I discharge only to about 75-80% D.O.D.

BB.

Re: want to design a 3.5 kw off grid system with cheap panels

Eric L wrote: »

... and I discharge only to about 75-80% D.O.D.

Probably a typo--not DOD (depth of discharge) but SOC (State of Charge)... 75% SOC means you only have used 25% of the battery's capacity (two days = 50% of battery capacity).

-Bill

Eric L

Re: want to design a 3.5 kw off grid system with cheap panels

Doh! That's right; thanks.

vtmaps

Re: want to design a 3.5 kw off grid system with cheap panels

Txrancher wrote: »

you said 2 parellel strings of 3 panels, that would only use 6 panels or did I mis understand something ?

He meant four strings of three. In the next line he wrote: Let's see what 12 * 240 Watts gets you: 2880 Watt array.

--vtMaps

Cariboocoot

Re: want to design a 3.5 kw off grid system with cheap panels

Txrancher wrote: »

you said 2 parellel strings of 3 panels, that would only use 6 panels or did I mis understand something ?

Me switching thoughts mid-sentence. Fixed it.

in a nut shell, could you tell me what values for volts and amps we want to create when we decide on strings for a 48v system ?

thanks

Amps is not particularly relevant for an array, but Voltage (Vmp) has to meet the minimum for charging the bank and not exceed the maximum for the input of the controller (Voc * temp coefficient). So we have a desired array Vmp minimum of 17.5 for a 12 Volt system, with multiples for 24 and 48: 35, 70 respectively. The reason being the 48 Volt system will charge @ roughly 60 Volts. Two of the panels of the type mentioned will only just reach that under STC ratings. Add in V-drop from the panels being hot and further loss through wiring and the Voltage available will no longer be high enough to achieve the Absorb setting for a 48 Volt bank.

This 17.5, 35, 70 rating is not perfect, of course, but it's close enough. Obviously there is less Voltage drop on a 48 Volt system than a 12 or 24 so the wiring losses are less.

The current then becomes a function of the Watts / Volts and is necessary for determining wire size and series string fusing (if required).

Txrancher

Re: want to design a 3.5 kw off grid system with cheap panels

Cariboocoot wrote: »

Me switching thoughts mid-sentence. Fixed it.



Amps is not particularly relevant for an array, but Voltage (Vmp) has to meet the minimum for charging the bank and not exceed the maximum for the input of the controller (Voc * temp coefficient). So we have a desired array Vmp minimum of 17.5 for a 12 Volt system, with multiples for 24 and 48: 35, 70 respectively. The reason being the 48 Volt system will charge @ roughly 60 Volts. Two of the panels of the type mentioned will only just reach that under STC ratings. Add in V-drop from the panels being hot and further loss through wiring and the Voltage available will no longer be high enough to achieve the Absorb setting for a 48 Volt bank.

This 17.5, 35, 70 rating is not perfect, of course, but it's close enough. Obviously there is less Voltage drop on a 48 Volt system than a 12 or 24 so the wiring losses are less.

The current then becomes a function of the Watts / Volts and is necessary for determining wire size and series string fusing (if required).

If I were to get 1 more panel, thats 15 total, would I be able to have 3 strings of 5 panels each ? or would that drive the voltage too high ? Vmp is 29.3 on these
is there any disadvantage or wiring issues with having more strings vs less ?

thanks

Photowhit

Re: want to design a 3.5 kw off grid system with cheap panels

Txrancher wrote: »

If I were to get 1 more panel, thats 15 total, would I be able to have 3 strings of 5 panels each ? or would that drive the voltage too high ? Vmp is 29.3 on these
is there any disadvantage or wiring issues with having more strings vs less ?

You could if you went with a Classic 200 or 250, I think the VOC would be too high for the Classic 150. Their names indicate the max VOC while producing power, they have a cushion above that where they will not produce power, the cushion equal to the nominal battery bank voltage.

a couple less fuse or breakers to buy, higher voltage often means you can use smaller gauge wiring.

vtmaps

Re: want to design a 3.5 kw off grid system with cheap panels

Txrancher wrote: »

If I were to get 1 more panel, thats 15 total, would I be able to have 3 strings of 5 panels each ? or would that drive the voltage too high ? Vmp is 29.3 on these
is there any disadvantage or wiring issues with having more strings vs less ?

With higher voltage input to your controller (assuming the controller can handle the voltage) there are advantages in the wiring as Photowhit mentioned. However there is a cost: Your controller will run less efficiently and the loss of efficiency translates into heat which shortens the life of the controller.

read:
http://forum.solar-electric.com/showthread.php?p=145663#post145663
http://forum.solar-electric.com/showthread.php?15907

--vtMaps

Txrancher

Re: want to design a 3.5 kw off grid system with cheap panels

vtmaps wrote: »

With higher voltage input to your controller (assuming the controller can handle the voltage) there are advantages in the wiring as Photowhit mentioned. However there is a cost: Your controller will run less efficiently and the loss of efficiency translates into heat which shortens the life of the controller.

read:
http://forum.solar-electric.com/showthread.php?p=145663#post145663
http://forum.solar-electric.com/showthread.php?15907

--vtMaps

Thanks for the links, I will read up on them.

so with 15 panels you could have 5 strings of 3 panels, or 3 strings of 5 panels, but just make sure the string doesnt make more volts than the controller can handle.
for some reason, the midnight classic 150/200/250 all seem to be about the same price.


.

Photowhit

Re: want to design a 3.5 kw off grid system with cheap panels

Txrancher wrote: »

Thanks for the links, I will read up on them.

so with 15 panels you could have 5 strings of 3 panels, or 3 strings of 5 panels, but just make sure the string doesnt make more volts than the controller can handle.
for some reason, the midnight classic 150/200/250 all seem to be about the same price.

They are made with different string voltages in mind, If you look at the amp output, the 150 can actually handle more Amp out put than the higher voltage input models. Since your looking at a 48 volt system I think the target for best conversion is roughly 2x the input voltage so somewhere around 100 volts would be your best input voltage, and likely you can do that with a classic 150...