Requesting assistance in off grid system design

leftcoastadv · August 2011

Hello,
Long time lurker, first time posting. Tons of very useful info on this forum and I have been doing a ton of reading.

The wife and I are in the middle of our bus to rv conversion and solar system planning is the next step for us.

We would like to run our lifestyle completely off grid with the occasional generator back up if/when needed.

One of the hard parts I am coming up against is trying to accurately determine how much sun we will really get. Since we will be full timing in the bus our biggest intention is to follow the warmer seasons. We currently live in Northern Arizona but plan to spend a fair amount of time in the PNW. With that being said, I have been using 4 hours as my yearly average peak sun hours.

I am calculating our usage right at about 1800 total watt hours per day. Some of the numbers I used are verified with my kill-a-watt meter and others are educated guesses. (1800 watts sounds like an awful lot to me and wife would agree so we maybe looking at our electrical usage with a bit more conservation in mind)

I can list my electrical needs if that would be helpful? (I may be off on my calculations)

I have room for plenty of batteries in the under belly storage along with about 6' x 36' of roof space for panels, etc.

The bus electrical is going to be a combination of DC and AC appliances with the largest single draw being 300 watts.

For the bigger items like A/C, micro, etc those will be run off the generator if needed.

Sorry for the long post but I do thank you in advance for any help that can be given.
Feel free to ask for more information as I am sure I have left out something important along the way.

Thanks again, Ryan

PS, money is an issue but we have chosen to dedicate a significant amount of money if needed to the solar fund as it is imperative that we get it right the first time.

niel · August 2011

Re: Requesting assistance in off grid system design

sounds like a good project you have going there. let's start with your 1800wh for your loads and divide that by your estimated 4hrs of full sun to give a per hour estimate of the power needed. 1800/4=450w. now pvs don't often put out there stc ratings under normal conditions and we derate them by 77%. this means dividing the 450w by 77% will give a better stc pv requirement. 450/.77=585w. now we aren't done here because that assumes the pvs are aimed directly at the sun and many with rvs put them flatly on the roof for easier transport so let's add another 35% (arbitrary and variable) due to the off angle. 1.35x 585=789w. are we there yet? no quite as it will not be sunny all of the time and this is highly dependent on where you are at. we can again arbitrarily say a number and let's go with 1 day in 4 being a wash meaning this ups the solar requirement another 25% if you wish to have enough capacity in the batteries to account for holding that extra for (and it's a good idea to have at least a day's backup that won't allow the batteries to drop below 50%). now you could press the generator (with a quality 3 stage battery charger if need be) into service rather than going that extra 25%, but you do need to track the state of charge in the batteries somewhat to prevent getting stuck without and to keep a good lifespan for the batteries so getting a battery monitor could help here.

this is not an exacting answer because the usage and generation of power is variable and often unpredictable at times. i would say to go with at least 600w of pv and an mppt controller of a high enough amp output rating to allow for possible future expansions as power consumption always seems to go up and as such you may need to add a pv or 2 later.

i'd like to kick this around a bit more, but i have to run out right now. this is enough to start the conversation though.

Cariboocoot · August 2011

Re: Requesting assistance in off grid system design

It's a bus. How much room have you got on the roof?? Fill it with PV.

As Niel said, you can't count on good insolation with RV installs. Coming up with 1800 Watt hours worth of battery isn't difficult; recharging them strictly from solar is.

Assuming a 12 VDC system (handy for interfacing with automotive/RV stuff) you need roughly 150 Amp hours to provide that much power, so that's at least 300 Amp hours of battery. (That's with no reserve for cloudy days.) That wants 30 Amps of charge current (peak) @ 14.4-14.8 Volts: 432 Watts minimum. Then you run into that "how much will I really get from the panels?" problem. Imperfect angle, traveling around, inconsistent sun ... It's a wild guess how much your derating will be. 600 Watts (approximately 50% more than the minimum) would be a starting point.

You also have to adjust as you go when picking equipment. Since there probably isn't any such thing as a 300 Amp hour battery bank (at least not easy and cheap) you might pick four "golf cart" T105's and get a 450 Amp hour bank - a bit of reserve power there, but needs more panel to recharge. You'd be looking for 850-900 Watts in that case.

Which brings us back to my facetious remark about filling the roof with PV: that much panel is going to take a up a space roughly 5' wide and 12.5' long (five 175 Watt panels, for instance - make it six and you get 1050 Watts, you'll "max out" a 60 Amp MPPT controller, and it will be 15' of panel length).

There is some concern with mounting panels, especially larger ones, on vehicles. They don't flex well. One company makes mounts specifically for the smaller Kyocera 135 Watt panels: http://www.solar-electric.com/rvflmoforkc.html

Sorry it isn't easy.

leftcoastadv · August 2011

Re: Requesting assistance in off grid system design

Thank you both for such a quick and info packed reply.

Let me add a little more info the mix.... the bus will be parked more as a base camp vs everyday driving. The plan is to use the bus as a mobile home for months at a single location. 2-3 months here, travel 500 miles, 2-3 months there, rinse repeat. Not really sure if that will make much of a difference.

I hear what you are saying about larger panels and flexing not being a good idea.
The roof rack will be 36' in length and allocating 15' to panels is an ok number with me. I understand about RV's generally flat mounting their panels, would putting them on a tilt be a wise move at this point? The kyocera mount that was linked to earlier sounds like a good option. It is realistic to assume being on the roof (which is not going to be difficult at all with our setup, think more observation deck and less spongy rv roof) to move the panels every day might become a burden after awhile. But if that is a realistic sacrifice that needs to be made to maintain a good solar array then it can be done.

Would adding more generator be a little more realistic? Maybe a high efficiency generator coupled with a high end 3 stage charger running every x number of days? Is that doable?

I have done a little reading on Wind generation and although there seems to be a fair amount of negatives to it, if we happen to be located in a windy area for any length of time it might be a viable option. That can be a topic for a different discussion if need be.

Florescent lighting, fans and water pump are going to be 12 volt DC side with everything else running on the AC side.
Small lcd tv, two laptop computers, stereo, 5 cu ft chest freezer, 5 cu ft chest freezer converted to refrigerator (which we currently use in our house and love), camera and cell phone charger and possibly some small led lighting. That's it.
Everything we plan to run currently is or will be the most efficient we can find.
All other appliances are propane or diesel.

Thanks again for the help. Very much appreciated.

PS, just to give you an idea of where I'm at, I have roughly allocated $7-$10k for the solar setup.

edit* I believe I was wrong on one of my calculations... the start up of the freezer and fridge compressor is 8.8 amps. Which it pulls for about 10 seconds and slowly decreases to about 1.3 amp. To figure watts I took 8.8 x 120 volt = 1056 watts. Would that be correct? So my biggest start up would be 1056 watts but could also be 2100 if both the freezer and fridge cycle on at the same time. Am I on the right track?

niel · August 2011

Re: Requesting assistance in off grid system design

if it is of a semi-permanent nature you could take the pvs down for traveling and mount them when you get situated as this is not something one would want to do very often.

the angle on the pvs is a tough one because making an angle would mean the bus/rv would only be able to park one way to allow the pvs to face south. that is why many lay the pvs flat so as to take that complication out of the equation. another possibility is to remote ground mount the pvs and that gives more flexibility on where you park including being able to park the bus/rv in the shade to stay cooler while the pvs stay in the sun. these mounts can then be angled (make them adjustable) and face the sun much easier and with much better efficiency in your collecting. ground mounting has the drawback of making theft of the pvs much easier though. if you go with the roof mounting then i don't see the need to completely cover the roof in pvs as you don't require that much pv power and that would be a much more expensive endeavor.

putting an inverter generator there to take up the slack of your system or rare excessive uses of power is by far better to do and with a good 3 stage charger it can not only run the ac stuff, but charge up the batteries. sizing the generator would depend on the total power of your loads of course.

i would stay away from wind. wind is tough to do with a permanent location and portable would just not be worth the hassle for the little power reaped and the high maintenance required. turbines aren't as small as you might think making transporting it tough too. add to the fact that it needs to be around 30ft higher than any immediate surroundings makes mounting it a royal b...h.

leftcoastadv · August 2011

Re: Requesting assistance in off grid system design

Niel,
Thanks for the help. I think at this point I want to keep the panels on the roof. I hear ya on remote mounting them and being able to harvest more efficiently but moving them from the roof does sound like quite the experience.

I have scratched Wind off the list of possibilities.

As far as the generator goes, is it possible to work into this backwards? Could I maybe setup a battery bank to be charged via generator every x amount of days and just have the solar supplement when it can? Does that make sense? I think I maybe off base here but just want conformation.

Another idea I had.... Having two smaller battery banks, one connected to the generator only and the other one to solar. Basically I could run my fridge and freezer off a smaller solar/battery system and then for the extra (fun) items like tv, radio, etc, that could be run on a bigger battery bank that is charged with the generator every x number of days. That would allow me to only need to recharge with the genny if and when I actually use any power. Does that make any sense?
Not exactly KISS but it might be doable.
I could even run two chargers so anytime the genny is running I am charging both banks.

Just thinking out loud.

Please let me know where you think I'm way off base. Thanks again for the help.

Cariboocoot · August 2011

Re: Requesting assistance in off grid system design

leftcoastadv wrote: »

As far as the generator goes, is it possible to work into this backwards? Could I maybe setup a battery bank to be charged via generator every x amount of days and just have the solar supplement when it can? Does that make sense? I think I maybe off base here but just want conformation.

Yes, that could work. Many people start out with batteries/gen/inverter to have "quiet power" at night - recharged by the gen during the day. Add solar when it becomes feasible. A small amount at first can off-set the daily drain, rather than fully recharge the batteries. As long as the batteries do get fully recharged, they don't care from where it comes.

Another idea I had.... Having two smaller battery banks, one connected to the generator only and the other one to solar. Basically I could run my fridge and freezer off a smaller solar/battery system and then for the extra (fun) items like tv, radio, etc, that could be run on a bigger battery bank that is charged with the generator every x number of days. That would allow me to only need to recharge with the genny if and when I actually use any power. Does that make any sense?
Not exactly KISS but it might be doable.
I could even run two chargers so anytime the genny is running I am charging both banks.

That is also possible; a small, solar-recharged system for "essential power", and a separate power source for "occasional" items. You can use the generator exclusively for this if you like. As for recharging two banks off one gen, just be sure it has the capacity to handle both chargers, with power factor losses, and any loads. You'd be amazed how small a generator you can use if you're good at shutting things off!

Just thinking out loud.

Please let me know where you think I'm way off base. Thanks again for the help.

I think you're being very practical about it and remaining open to different solutions. Finding which one best suits your needs is the tricky bit.

Shasta1 · August 2011

Re: Requesting assistance in off grid system design

I suggest you read everything this man writes: http://handybobsolar.wordpress.com/

I found it very educational and his advice has proved to be very beneficial to my needs and system design. Print out each article and keep them someplace where you can read it each time you get to sit down for a few minutes......;) It may be the single most important read you'll find concerning RV Solar.

I found a very simple, yet effective method for orienting RV roof mounted panels for maximum efficiency in most scenarios. It also might save you up to several hundred dollars, based on mounting 4-5 panels, over what vendors will try to sell you.

I too live in AZ (southern) and plan to full-time in the PNW and Mexico, so I share some of your concerns.

This is a great forum with a wealth of expertise and info available, however, and with no disrespect intended, there are many fine points in reference to RV solar applications that are sorely lacking. And they can add up to lots of $ if you don't think the whole install through.

Read: http://handybobsolar.wordpress.com/ !!!!!!!!!!!!

I'll post pics and experiences as time permits over the next day or so.

P.S. NAW&S are wonderful people to deal with, but do not carry some of the smaller key items you might need for RV solar system (depending on your design).

leftcoastadv · August 2011

Re: Requesting assistance in off grid system design

Thanks for the link. I am headed to his site right now.....

leftcoastadv · August 2011

Re: Requesting assistance in off grid system design

Cariboocoot wrote: »

Yes, that could work. Many people start out with batteries/gen/inverter to have "quiet power" at night - recharged by the gen during the day. Add solar when it becomes feasible. A small amount at first can off-set the daily drain, rather than fully recharge the batteries. As long as the batteries do get fully recharged, they don't care from where it comes.

So the pv panels would be undersized for the battery bank? Could you then run the battery bank down say 60% and then the solar would bring it up to x % at which point the solar would fall off and the generator can finish charging? I'm reading that the battery bank does not need to be brought back up to 100% every day. What's your take on re-charging to 100%?
Would it be safe to run the bank up and down with just the solar and then maybe every x number days charge it to full? x being 3 days or maybe 6 days, etc. Pretty much the longer the better because of generator fuel costs.

At the point in which the solar system is not able to produce enough electricity to recharge the batteries, what happens to the extra power being generated? Does it just get bled off as heat somewhere?
If so, would it be possible to use that energy for some other task? Maybe wire an electric heating element that goes to the propane water heater. Just to supplement. Throwing that idea out there.

Cariboocoot wrote: »

That is also possible; a small, solar-recharged system for "essential power", and a separate power source for "occasional" items. You can use the generator exclusively for this if you like. As for recharging two banks off one gen, just be sure it has the capacity to handle both chargers, with power factor losses, and any loads. You'd be amazed how small a generator you can use if you're good at shutting things off!

Good to know it is possible.

Cariboocoot wrote: »

I think you're being very practical about it and remaining open to different solutions. Finding which one best suits your needs is the tricky bit.

Yup. Especially when it seems there are quite a few different ways to get it done. 8) Thanks again for the help.

Cariboocoot · August 2011

Re: Requesting assistance in off grid system design

leftcoastadv wrote: »

So maybe run the battery bank down say 60% and then the solar would bring up to x % at which point the solar would fall off and the generator can finish charging?

Semantics: I think you mean run the battery bank down to 60% SOC, not run it down 60% (which would be 40% SOC). You never want to go below 50% SOC.

You can use the generator and charger to do the Bulk charging stage; that's the part that uses the most power and where the gen would be most efficient. Running a 2kW gen to provide a few Amps of charge current during the Absorb stage is wasteful.

A read through the battery FAQ's may help you understand what you're dealing with: http://www.windsun.com/Batteries/Battery_FAQ.htm

I'm reading that the battery bank does not need to be brought back up to 100% every day. What's your take on re-charging to 100%?

I personally like 100% charge daily, because I run a pretty marginal system - no room for error. That's because I'm a cheapskate.

You're usually all right if you can keep the SOC above 70% and get a full charge at least once a week. This is with "standard" batteries like FLA's; let's not have everyone jumping in here suggesting some of the more exotic technologies, okay? :roll:

Would it be safe to run the bank up and down with just the solar and then maybe every x number days charge it to full? x being 3 days or maybe 6 days, etc. Pretty much the longer the better based strictly on generator fuel costs.

If you can do it all on solar, do it all on solar. If you can't, Bulk charging with the gen and letting the solar "finish" is the most economical way to go. If the solar still can't provide the full charge at least once a week you need to run the gen more. Letting the batteries go with whatever the loads take out and whatever the solar can put in is going to shorten the battery life considerably.

At the point in which the solar system is not able to produce enough electricity to recharge the batteries, what happens to the extra power being generated? Does it just get bled off as heat somewhere?

As stated, that doesn't seem to make sense. Are you wondering about the potential power the panels are capable of once the batteries do reach charged state (Float)? It doesn't go anywhere or produce any heat because it is not realized.

If so, would it be possible to use that energy for some other task? Maybe wire an electric heating element that goes to the propane water heater. Just to supplement. Throwing that idea out there.

It is quite common to "harvest" the "extra" power available from the panels during Float stage (or even Absorb if the system is large enough) by deliberately using more power then (load shifting) or having some form of automatic diversion such as a small electric element in an auxiliary water tank. Some people shift it to an additional back-up battery bank. There are options.

leftcoastadv · August 2011

Re: Requesting assistance in off grid system design

Excellent! Thank you for that response.
I found that FAQ earlier today. I still have about half to read through.

I think I'm on the right track, I just had it completely backwards.

To keep it most economical bulk charge with the generator running. Float with the panels. Makes sense. .

Originally Posted by leftcoastadv:
If so, would it be possible to use that energy for some other task? Maybe wire an electric heating element that goes to the propane water heater. Just to supplement. Throwing that idea out there.

Originally Posted by Cariboocoot:
It is quite common to "harvest" the "extra" power available from the panels during Float stage (or even Absorb if the system is large enough) by deliberately using more power then (load shifting) or having some form of automatic diversion such as a small electric element in an auxiliary water tank. Some people shift it to an additional back-up battery bank. There are options.

That answered my overall question. I will save this for down the road. 8)

leftcoastadv · August 2011

Re: Requesting assistance in off grid system design

Ok I've read his entire site. This guy seems pretty geuniue. Sounds like he knows what he is talking about.
I will take his info along with a few "Sailing/Crusing" sites I have found and see if I can make this mobile solar off grid system work. Thanks again for the link.

Shasta1 wrote: »

I suggest you read everything this man writes: http://handybobsolar.wordpress.com/

I found it very educational and his advice has proved to be very beneficial to my needs and system design. Print out each article and keep them someplace where you can read it each time you get to sit down for a few minutes......;) It may be the single most important read you'll find concerning RV Solar.

I found a very simple, yet effective method for orienting RV roof mounted panels for maximum efficiency in most scenarios. It also might save you up to several hundred dollars, based on mounting 4-5 panels, over what vendors will try to sell you.

I too live in AZ (southern) and plan to full-time in the PNW and Mexico, so I share some of your concerns.

This is a great forum with a wealth of expertise and info available, however, and with no disrespect intended, there are many fine points in reference to RV solar applications that are sorely lacking. And they can add up to lots of $ if you don't think the whole install through.

Read: http://handybobsolar.wordpress.com/ !!!!!!!!!!!!

I'll post pics and experiences as time permits over the next day or so.

P.S. NAW&S are wonderful people to deal with, but do not carry some of the smaller key items you might need for RV solar system (depending on your design).

leftcoastadv · August 2011

Re: Requesting assistance in off grid system design

The best location I have to store some batteries is in the belly storage area. With that being said, since the frame of the bus runs right through the storage bays I end up having a 2.5 foot by 3 foot area that is only 10 inches tall. On the passenger and driver side of the frame I basically have a box that is 2.5 foot by 2.5 foot and 2.5 feet tall. Does that make sense? It is shaped like a barbell.
The entire box is made of metal.
I was originally thinking I could fit a few under 10 inch tall 6 or 12 volt FLA's in the skinny part of the storage bay (makes for good weight balance but otherwise a useless area) but I'm a little concerned about the hydrogen off gassing and possible damage to the metal box.
From the batteries I have looked at on NAWS site, at best I would have maybe an inch of head room to spare.
Is this a cause for concern? Would passive or even forced ventilation via a set of fans that operate during charging only take care of any off gassing issues?

Against Cariboocoot's advice, I started looking into AGM's.

They sounded like a great solution but the "making sure you recharge them to 100% after every discharge" worried me a bit. I read a handful of stories from cruiser/sailors that used Lifeline AGM's and watched them go down the drain in under 3 years. That is a steep price to pay when it might be hard to guarantee a 100% recharge.

I might be in a bit of a jam with my possible battery storage location. Any ideas?

Shasta1 · August 2011

Re: Requesting assistance in off grid system design

Here's some pics of the simple mounting that allows panels to be easily tilted in all four directions. I'd love to give proper credit for this idea, but can't find the link.

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Attachment not found. Attachment not found.

There is no "wiggle" or movement of any kind of the hinge regardless of the position selected. This was one of my biggest concerns when I first saw this idea. The panels have no play whatsoever.

The pic with the wing-nut is the method I finally decided on as far as a fastener. I spent hours one day trying to make it loosen by rocking the strut back and forth, and was unable to make it fail. After several trips, I'm really happy with the results!

The hinges were less than $2 each at Lowe's, and the piece that is attached to the panel corners is cut and shaped from a 3' x 1.5" piece of aluminum stock from Lowe's. It cost approx $8 and made enough pieces for two panels. They are attached with self-tapping screws.

The struts for tilting the panels are made from thin-wall conduit, flattened on both ends and hole drilled. Two 10' sticks supplied enough material for all four of my panels, and can be used to set the panels at 30 degrees in any direction of tilt.

The struts will be stored on the panels with clips when not in use, and spare hardware is attached in the extra holes of the hinges.

Shasta1 · August 2011

Re: Requesting assistance in off grid system design

On the subject of excess power generated by the panels;

I made up a couple of 12v recepts (fused) that feed directly from the panels and before the charge controller. If my battery bank is charged, I can use the solar power for the "Fantastic fans", and for charging the laptops, phones, radios, etc.

When parked at home, I feed one room of our house from the RV inverter and power my music/computer stuff......it's free electricity, might as well put it to use.

Cariboocoot · August 2011

Re: Requesting assistance in off grid system design

Torch + welder = battery accommodation.

You're right that metal is far from ideal for a battery box. Mainly because of the real possibility of shorts, but also a bit of corrosion potential. You could coat the interior with some of that liquid truck bed liner. But it sounds like you've got some space limitations to solve. You don't want tight wiring space where you can't easily get the connections on/off.

AGM's, btw, are okay with low charge. They stand up to it better than FLA's. What they're not okay with is too much Voltage; pop the valve, vent the gas, kill the battery. You can't put the water back in an AGM.

bmet · August 2011

Re: Requesting assistance in off grid system design

Is that standard RTV keeping the hinge glued down? Don't want it to shake loose due to vibration.

Shasta1 wrote: »

Here's some pics of the simple mounting that allows panels to be easily tilted in all four directions. I'd love to give proper credit for this idea, but can't find the link.

Attachment not found. Attachment not found.

Attachment not found. Attachment not found.

There is no "wiggle" or movement of any kind of the hinge regardless of the position selected. This was one of my biggest concerns when I first saw this idea. The panels have no play whatsoever.

The pic with the wing-nut is the method I finally decided on as far as a fastener. I spent hours one day trying to make it loosen by rocking the strut back and forth, and was unable to make it fail. After several trips, I'm really happy with the results!

The hinges were less than $2 each at Lowe's, and the piece that is attached to the panel corners is cut and shaped from a 3' x 1.5" piece of aluminum stock from Lowe's. It cost approx $8 and made enough pieces for two panels. They are attached with self-tapping screws.

The struts for tilting the panels are made from thin-wall conduit, flattened on both ends and hole drilled. Two 10' sticks supplied enough material for all four of my panels, and can be used to set the panels at 30 degrees in any direction of tilt.

The struts will be stored on the panels with clips when not in use, and spare hardware is attached in the extra holes of the hinges.

leftcoastadv · August 2011

Re: Requesting assistance in off grid system design

Cariboocoot wrote: »

Torch + welder = battery accommodation.

You're right that metal is far from ideal for a battery box. Mainly because of the real possibility of shorts, but also a bit of corrosion potential. You could coat the interior with some of that liquid truck bed liner. But it sounds like you've got some space limitations to solve. You don't want tight wiring space where you can't easily get the connections on/off.

AGM's, btw, are okay with low charge. They stand up to it better than FLA's. What they're not okay with is too much Voltage; pop the valve, vent the gas, kill the battery. You can't put the water back in an AGM.

Oh believe me, I've welded enough stuff on this bus to last two lifetimes.

If you say agm's are ok for low charging then I guess that means they are still on the table.
I was drooling over these Concorde PVX-2580L (8D), figuring I could slide them into the skinny part of the battery box, leaving the terminals in the much more roomy part of the box. That would give full access to the terminals. I have other battery box options but using this otherwise crappy spot for batteries would be excellent.
Just for grins, I could theoretically fit 6 of these in the center area of the box. 1548 amp hours... wow that’s a small power plant. Not to mention roughly $3600 just in batteries. Ouch

A quick simple break down would be: 1548 amps / 2 =774 useable x 12 volts = 9288 watts / 1800 daily use = 5.16 days. Rounded to 5 days avg run time before absolutely needing to recharge.
Is that a fair (simple) break down?

I am trying to figure out how to size the charger and generator. Using the above numbers, 1548 amp hours would take a minimum (10%) 150 amps to recharge efficiently?
Would you then take 150 amps / 12 volts = 12.5 amps x 120 volt = 1500 watts? I might have just made up that math equation in my head.

:cool:
If I am off can you point me in the right direction? Maybe there is a good document online?
Thanks again for the help.

leftcoastadv · August 2011

Re: Requesting assistance in off grid system design

Shasta1 wrote: »

On the subject of excess power generated by the panels;

I made up a couple of 12v recepts (fused) that feed directly from the panels and before the charge controller. If my battery bank is charged, I can use the solar power for the "Fantastic fans", and for charging the laptops, phones, radios, etc.

When parked at home, I feed one room of our house from the RV inverter and power my music/computer stuff......it's free electricity, might as well put it to use.

I like that idea. Maybe we can come back to that discussion once I get some more of the basic stuff nailed down.

Thanks for the pictures and explanation on the panel mounts. I think I really like that design. At least now I know it is possible.

mike95490 · August 2011

Re: Requesting assistance in off grid system design

A quick simple break down would be: 1548 amps / 2 =774 useable x 12 volts = 9288 watts / 1800 daily use = 5.16 days. Rounded to 5 days avg run time before absolutely needing to recharge.
Is that a fair (simple) break down?

Fair, simple, AND a battery killer.

Once you get below 80%-75% of full (25% used) sulphation begins to set in, and damage the battery. So if you have enough capacity that after 5 days you are only 20% down, Great.

Cariboocoot · August 2011

Re: Requesting assistance in off grid system design

Stop.
Do not pass "GO!", do not collect 1548 Amp hours of battery.
Trying to get six batteries to share current equally, even with bus bars, is a nightmare. When you need that kind of power you should up the system Voltage; three strings of two each making a 24 Volt system is much easier to handle (although still not the best idea).
Then there's trying to get 150 Amps of charge current, whether from PV or generator it is difficult task. On PV, you end up buying some expensive charge controllers to handle the current; it's at least two, even of the good ones. From a generator - likewise you end up getting big chargers. Most inverter chargers won't do that rate. Not the 12 Volt units anyway. The charge requirement would be: 150 Amps * 14.2 Volts (charging Voltage) = 2130 Watts - after derating for power factor of chargers and efficiency of generator. Probably a 3kW gen at least (no loads on).

You're better off using a smaller battery bank and recharging it more frequently. Smaller gen, less Amps, lighter wiring, fewer current sharing problems, less power loss to heat, not as many connections, t cetera.

For 9kW hours it's worthwhile to go to 48 Volts: 188 Amp hours. 18 Amps @ 57 Volts charging. Way easier to handle. And if you need 12 VDC for something you can run it from the inverter via a power supply; less efficient than straight from a battery but the over-all system will be more efficient so it wouldn't matter.

leftcoastadv · August 2011

Re: Requesting assistance in off grid system design

Ok good to know! I want my battery investment to last as long as possible so if that means I don't allow it to get anywhere near 50% then so be it. Thanks for that bit of education.

mike90045 wrote: »

Fair, simple, AND a battery killer.

Once you get below 80%-75% of full (25% used) sulphation begins to set in, and damage the battery. So if you have enough capacity that after 5 days you are only 20% down, Great.

leftcoastadv · August 2011

Re: Requesting assistance in off grid system design

I'm STOPPED!

My next question was going to be at what point do you go to 24 or even 48 volt but you pretty much answered that.

I've read what you said a few times now.... it seems to be sinking in.

Cariboocoot wrote: »

Stop.
Do not pass "GO!", do not collect 1548 Amp hours of battery.
Trying to get six batteries to share current equally, even with bus bars, is a nightmare. When you need that kind of power you should up the system Voltage; three strings of two each making a 24 Volt system is much easier to handle (although still not the best idea).
Then there's trying to get 150 Amps of charge current, whether from PV or generator it is difficult task. On PV, you end up buying some expensive charge controllers to handle the current; it's at least two, even of the good ones. From a generator - likewise you end up getting big chargers. Most inverter chargers won't do that rate. Not the 12 Volt units anyway. The charge requirement would be: 150 Amps * 14.2 Volts (charging Voltage) = 2130 Watts - after derating for power factor of chargers and efficiency of generator. Probably a 3kW gen at least (no loads on).

You're better off using a smaller battery bank and recharging it more frequently. Smaller gen, less Amps, lighter wiring, fewer current sharing problems, less power loss to heat, not as many connections, t cetera.

For 9kW hours it's worthwhile to go to 48 Volts: 188 Amp hours. 18 Amps @ 57 Volts charging. Way easier to handle. And if you need 12 VDC for something you can run it from the inverter via a power supply; less efficient than straight from a battery but the over-all system will be more efficient so it wouldn't matter.

I was just using that 1548 number as a max I could possibly fit in the bus but what ever slight inkling I had was vaporized by your post. Fair enough. I was getting a little to big for my britches.:D

In all reality I was thinking more along the 650-750 amp hours. Is that still a little to big? Maybe a bit to much to run on 12v comfortably?

If you have some time, could you maybe tell me where I need to be?
I don't want this system to be running on the ragged edge but instead to be nice and efficient with plenty of longevity.
Like I mentioned way back, I have basically 6 x 15 foot of roof area for tilt able panels and room enough for 8d agm's.
I would like to run a trimetric 2025a or similar so I can keep a close eye on things. (I like data)
With the kind of money we are looking at investing, I would like to make it run as best as possible.

By the way, one of those Honda or Yamaha small inverter generators looks like the hot ticket. They just sip on the fuel and most people seem pretty happy with their level of noise.

Sorry for the 21 questions.
Thanks again for the help.

Cariboocoot · August 2011

Re: Requesting assistance in off grid system design

I simply have to write this down somewhere. In the past week I've re-calculated the 12V/24V threshold three times for people - and I keep forgetting it.

There's two things to consider: one is the maximum amount of Watts to be drawn at any one time. We can call this the Inverter Limit. If the loads are going to be habitually over 2kW it's time to go up to 24 Volts. Most of the time this is not a big consideration unless you're trying to run a typical household with kids.

The reason for this is the amount of Amps drawn to create that 2kW: at maximum output with minimum Voltage input (usually about 10.5 Volts on a 12 Volt system - too low for my tastes but that's where the manufacturers set the LVD usually) you're looking at the neighborhood of 200 Amps of current to supply that Wattage. To handle that current you're into some pretty big wire sizes (like 3/0 aka 000), big fuses, and a lot of power loss to heat generation.

The second thing is the one that comes up most often: stored power. It used to be the biggest charge controller you could get maxed out at 60 Amps. Now thanks to MidNite Solar and Outback that limit has been pushed upwards to 90 Amps. But handling high current is still a problem: bigger wire sizes, bigger fuses, more power lost to heat. So from the point of view of the controller you're limited to about 900 Amp hours of battery: 5.4 kW hours. I'm still not comfortable with that size. Frankly, trying to fit 2 AWG to a charge controller isn't very easy. :roll: The other factor here is the sizes of batteries available. Yes there are 2 Volt cells you can put together and come up with 1000 Amp hours @ 12 Volts. They're expensive and need that full current to keep going. They're also bloody heavy! You can parallel smaller batteries, but that gets problematic in he current handling department too.

So, do you put four 6 Volt 225 Amp hour batteries in series/parallel for 450 Amp hours at 12 Volts? Or do you string them all in series for 225 Amp hours @ 24 Volts? At that point is six of one/half dozen of the other, so to speak: both are 2700 Watt hours of power. But that's about the point when 24 Volts starts to look more attractive.

And so it comes back to load requirements. Your original estimate of 1800 Watt hours daily is not unreasonable, although it requires at least 300 Amp hours of battery. 600 Would be better, as it would keep your daily usage to 25% of the capacity, giving you longer battery life and an "extra day's worth" if the sun fails to shine. Then you have to pick from what batteries are available and meet your budget. So long as they have the minimum required Amp hours they'll do. Providing you keep 'em charged.

If you go with AGM's the battery monitor is almost a must, as you can't dip a hydrometer into one to see what the SG is.

leftcoastadv · August 2011

Re: Requesting assistance in off grid system design

Thank you for that explination. I am going to let it sink in for a bit and I will come back later.

leftcoastadv · August 2011

Re: Requesting assistance in off grid system design

Ok......

after reading this thread again and again, along with your last post I think I might have another game plan. I think I'm onto revision 8.2.

Hear me out on this one...

3 of these: Concorde PVX-2120L (4D), 212 amp-hour, 12 volts = 636 amp hour. (If I understood you correctly, it might be best to go to 24 volt and I could switch to something like this: SunXtender PVX-2240T 224 AH, 6 Volt )

5 of these: Kyocera KD185GX 185 Watt = 925 advertised watts (I understand that’s not even close to actual)

The Charge Controller I am still learning about. From my limited knowledge, I think I want an MPPT and temperature compensation controller at the very least. Maybe one of these: Morningstar TriStar 60 Amp MPPT
I have not figured out the math yet on sizing the charge controllers. Based on a previous post it was mentioned that 1025 watts in panels was going to max out a 60 amp controller so I’m kind of guessing 925 is going to be just barley under that. edit* I just took 925 pv watts / 12 volts = 77 amps. 60 amp controller might not work, if the panels ever produced 100% as advertised.

Battery charger is also something I am learning about. If I followed the math correctly it would go something like this: 636 amp hour at c/8 = 80 amps
Maybe one of these: Iota DLS-90 Battery Charger

80 amps x 14.2 volt = 1136 watts, so maybe round that to 2k-2.5k for generator sizing. And that is without any other loads.
The battery charging faq's from NAWS indicates that the Concorde batteries can take some ridiculous amount of amps for charging but it goes on to say that c/4 or less is best. I am all for bulk charging the batteries as quick as possible if it really is that simple.
What do you think? Is c/8 a safe bet or can I maybe increase it a bit without sacrificing something else?

I think I'm getting a little closer to getting this solar puzzle figured out

Cariboocoot · August 2011

Re: Requesting assistance in off grid system design

The math is all related. In it's simplest form (no loss accounting):

You need 'X' (1800) Watt hours per day to supply your loads. Divide by system Voltage (12) and you get Amp hours needed (150). Since you don't want to discharge a battery more than 50% you multiply that by at least two to get minimum Amp hour capacity of the battery (300). That's the load side.

To recharge that battery, regardless of source, you want to be able to achieve a peak charge rate of 10% (5% to 13% technically, but 10% is a good target). So you take that number (30) and multiply it by the charging Voltage (14.2-14.8 depending on the battery recommendation) and get the array size (426) which needs additional factoring for averaged output (usually 77% = 553 Watts minimum).

The charge controller capacity is determined by the peak charge rate target: if you need 'Y' Amps it has to be able to handle that much at least (in this case 30 Amps or better). The MPPT advantage usually doesn't come in to play unless the array size is fairly large. We use 400 Watts around here as an arbitrary point of reference, but there are smaller MPPT controllers which can be useful with some systems where efficiency is very important. Sometimes the smaller systems are more demanding than the larger ones!

I'm not a big fan of more than two parallel battery connections, but it can be done. Bus bars and separate fuses per string. 636 Amp hours of battery would want more current than a 60 Amp charge controller can handle. Fortunately there is the FM80 and the MidNite Classic series.

What can you do with 925 Watts of array? Have a fit trying to work out a good configuration, for one thing. Odd numbers of panels don't work out to even numbers of equal strings. You don't want to wire them all in parallel because you'll A). have to put five fuses in and

. be running more current than Voltage to the charge controller. Best to keep the Voltage high and the current low. The Kyocera 185's Voc is 29.5. You could run them all in series with a MidNite Classic 200, but not with any other charge controller.

Otherwise: 925 Watts @ 77% efficiency is roughly 712 Watts, divide by charging Voltage of 14.2 (for AGM's) and you get a peak potential current of 50 Amps. A tad low, but it would do it. The other thing to check is replacing the used Watt hours. If you figure the same efficiency and a default of 4 hours of "equivalent good sun" you get 2848 Watt hours. If you use the highly-reliable Icarus Formula (which skips right to the end) you get a 925 Watt array producing 1850 Watt hours AC "out the door". Spot on.

As for a stand-alone battery charger to use with those batteries, I'd pick the DLS-75. It's closest to the charge current requirements. The AGM's can take more current than flooded cells, but it's nice to keep everything close to the middle-of-the-road target.

Keep in mind there are a number of variables which will effect your over-all system performance. You might never see peak charge current. You might never need it. And since your going to be moving about, you won't be able to adjust parameters to get the ideal system function for your site - because your site will change!

Confused more? I can go on like this ad infinitum, ad naseum until nobody understands what I'm rabbiting on about.

mike95490 · August 2011

Re: Requesting assistance in off grid system design

And the IOTA chargers are NOT power factor corrected, so you need 60% more generator to charge with @ max rate.

leftcoastadv · August 2011

Re: Requesting assistance in off grid system design

Oh bother!

Ok let me grab my scratch paper and calculator and jump into this. ;)8)

leftcoastadv · August 2011

Re: Requesting assistance in off grid system design

mike90045 wrote: »

And the IOTA chargers are NOT power factor corrected, so you need 60% more generator to charge with @ max rate.

There ya go throwing a new word at me.... Power Factor!

Requesting assistance in off grid system design

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