Upgrading from 12 to 48 volt system?

albertaalberta Posts: 10Registered Users
I have a summer cabin completely off-grid. We use the cabin on weekends from April to June, then full-time in July and August, and then weekends in the fall. I am upgrading my 12 volt system to either a 24 or 48 v. system. I presently have 6 Kyocera 185 watt panels running through a Midnite combiner box to an MX60 controller, a Trace DR 2412 inverter and 10 Trojan T-105's, which have reached the end of their useful life. I use a Honda generator, (upgrading to a EU Honda 6500is to be a little quieter for the neighbors-have had a Honda EV6010 RV gen, which only has 110 volt output) for running larger loads, like pumping water from the lake up to a holding tank up the hill, and charging the batteries if the sun doesn't shine. We have propane for hot water and cooking, and have had a propane fridge. As the cabin is on a mountain, we have no septic field, and pump from a holding tank to a pit up the hill with a Honda 2" trash pump. We get about 5 hours of sunshine during the summer.
I have used the electrical system to run 12 volt fluorescent lights at night, charge cellphones and laptops, and run a saw for the occasional cut when we don't want to start the generator. We now want to add an electric fridge, as the propane one is not working that well, is small (9 c.f.) and has trouble meeting the needs of our extended family when we're all there in the summer. We would also like to move the septic pumping to a 220 volt submersible grinder pump, instead of the Honda pump, to simplify the cabin operation when I'm not there.

The new fridge will use about 1kw per day (18 c.f. top freezer). I expect the other loads will not exceed 3kw per day, so I expect the system will need to be sized for about 4 to 5 kw per day.

My initial thought is to go to a 48 volt system. Is there any advantage to running a 24 volt system over a 48 volt one?

I plan on adding 6 more Kyocera 185's while they are still in production to match the panels I already have. NAWS has a good price on them, and the shipping on 6 is only slightly greater than the shipping on 2. I plan on running 4 strings of 3 panels each. Spec's on the panels are Max power voltage 23.6, max current 7.84 Amps, open circuit voltage is 29.5 volts, short circuit current is 8.58 amps. Spec's on the MX 60 are max input 150 volts (125 v. or less is recommended), with recommended array size of 3200 watts at 48 volts, or 1600 watts at 24 volts. This would seem to dictate a 48 volt system if I keep the same controller. Then I would upgrade the inverter to a Magnasine 4448, so I could then run 220 power from the generator to the inverter, and run the 220 v. septic pump from the inverter (while running the generator).

I am still trying to figure out the optimal size of the battery bank. As it is only summer use, and we have a generator to back up the solar panels, my thought is to have about 2 days reserve, which would be 10,000 watt hours, or 208 amp hours at 48 volts, times 2 for a 50% d.o.d. This would mean a battery bank of 516 amp hours at 48 volts, if I am calculating correctly. A local dealer has 8 new Surrette S-600 (450 amp hr at 20 hr rate) for $375 each, which is not a bad price in Canada, but I would have to maintain them in my garage until spring, and that would only mean a 450 amp-hr bank. I figure my solar array would put out about 7500 to 8000 watts on a good day. If I made the new battery bank larger than 450 amp-hr, I don't know if the panels could fully charge the batteries, and it would likely mean supplementing the charge each day with the generator (during the bulk stage preferably). So I am up in the air about what size of battery bank to add and would appreciate any advice.

Sorry for the long post, but I thought I'd list as much information as possible to reduce the number of supplemental questions for anyone who might have the patience to read this long post and provide me some advice! I certainly have learned a lot from this forum!

Comments

  • mike95490mike95490 Posts: 7,598Solar Expert ✭✭✭✭
    Re: Upgrading from 12 to 48 volt system?

    I thnk once you decide to move from 12V, it's silly to stop at 24V, just go to 48V, and you have more options, and easier to series batteries, than to parallel them. And with the XW inverters, you have 220V for the pumps...
    But with all the new loads, you will have to upgrade the charging system, or run more generator time for recharging.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • CariboocootCariboocoot Posts: 17,615Banned ✭✭
    Re: Upgrading from 12 to 48 volt system?

    The thing you need to get a handle on is the number that governs all off-grid design: loads.
    Maximum Watts at any given time and daily Watt hours. Without those figures, even if tentative, you are guessing at what the system should be.

    I run a full size refrigerator, full size water pump, digester (septic sump) pump, microwave, and computer/satellite set-up plus lights et cetera on a 24 Volt system. It is perfectly adequate because the total Watts at any given time does not exceed 1500 (start-ups mostly) and most of the time it's under 200. This is on 700 Watts of panel and 320 Amp hours of battery. Both of those, btw, need to be increased. Right now I get by with a lot of load-shifting. If you wanted to be able to use anything/everything "on demand" extra capacity is nice.

    Although Xantrex XW's are fairly good units, they are expensive and if you don't need GT capability why pay for it? There are alternatives and all of them should be explored. Including 24 Volt systems (which can be less expensive than 48 Volt).

    Again, without the load requirements there's no way to determine what system design is best for you.
  • BB.BB. Posts: 27,329Super Moderators admin
    Re: Upgrading from 12 to 48 volt system?

    Once you know your loads--then you can look at the 12/24/48 volt hardware (inverters, chargers, etc.) and see which will work best for you (you may not find many low wattage 48 volt inverters--for example).

    Matching system to loads will, usually, give you better efficiencies too.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Dave AngeliniDave Angelini Posts: 3,623Solar Expert ✭✭✭✭
    Re: Upgrading from 12 to 48 volt system?
    mike90045 wrote: »
    I thnk once you decide to move from 12V, it's silly to stop at 24V, just go to 48V, and you have more options, and easier to series batteries, than to parallel them. And with the XW inverters, you have 220V for the pumps...
    But with all the new loads, you will have to upgrade the charging system, or run more generator time for recharging.

    Except in cases where you can take advantage of all the 24VDC loads that exist and do not at 48VDC. In some cases I will maintain that 24VDC is the most efficient. It is the total design and the loads that really drives this.
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
     http://members.sti.net/offgridsolar/
    E-mail [email protected]

  • albertaalberta Posts: 10Registered Users
    Re: Upgrading from 12 to 48 volt system?

    So I have gone through a load calculator at ABS Alaskan website. Daily usage is about 4700 watts (Watt*Hours? -Bill B.) with no air conditioning. That chart recommends increasing the wattage by 1.15% for inverter losses, although some of the inverters I have looked at say they are 93% efficient. Should a conversion factor for AC loads still be 15%, or do you use 7%?

    The ABS chart then uses an additional load correction factor for other inefficiencies in the system. In my case, this would mean 4700 watt hours per day, times 1.15% for inverter loss, or 5405 watt-hours, and then increased further for the 70% efficiency factor, to 7,721 watt hours per day.

    As I have a generator to fall back on, and since the system is used mainly in the summer, I think would be OK having 2 days reserve in the battery bank. Using a 50% dod, I would need a battery bank of 643 amp hr at 48 volts, or 1286 amp hrs at 24 volts.

    Do these calculations seem right?

    Inverter
    Peak usage appears to be about 3000 watts, being when the fridge is starting, a saw is starting, and some miscellaneous loads are running.

    I think I could use either a 24 volt or 48 volt system, with the inverter putting out at least 3000 watts? I prefer the Magnasine so I could run 220 in from the generator, and then 220 out to the panel for my 220 loads(water and sewage pump)

    Batteries
    In terms of batteries, I have boat access only to my cabin, and have to haul the batteries up a hill, so I would prefer limiting each battery to about 120 lbs, so that eliminates forklift-type batteries.

    Thanks for your advice to a newbie.
  • BB.BB. Posts: 27,329Super Moderators admin
    Re: Upgrading from 12 to 48 volt system?
    alberta wrote: »
    So I have gone through a load calculator at ABS Alaskan website. Daily usage is about 4700 watts (Watt*Hours? -Bill B.) with no air conditioning.
    Just to be clear--You are probably talking about Watt*Hours here... I.e., 100 watt light running for 5 hours would be 100w*5h=500WH.

    That is a goodly amount of power... I like to talk about a 100 kWH per month/3.3 kWH per day as just about enough power for a very energy efficient off grid cabin/small home to live a pretty normal life (with propane and other fuels/heat sources).

    Your 4.7 kWH per day (or ~141 kWH per month) is not too far off of that "ideal" number.
    That chart recommends increasing the wattage by 1.15% for inverter losses, although some of the inverters I have looked at say they are 93% efficient. Should a conversion factor for AC loads still be 15%, or do you use 7%?
    I like to use 85% as the inverter efficiency number. A bit on the conservative side and accounts for times when you may be running small loads on a large inverter (usually inverters are much less efficient when lightly loaded) and the few times you may be using very heavy loads (again, efficiency tends to fall off a bit under heavy loads).

    It is a "good enough" to start number--but if you have an "unbalanced" setup (say a 3kW inverter that carries mostly 100 watts or so and only occasional heavy loads like a well pump--lots of "tare" losses as the inverter idles).

    You can address some of those issues with, perhaps two inverters. Once small one for your computer, few lights, battery/cell chargers that you leave on most of the time. And a second larger inverter that only turns on when you need to support the water pump, washing machine, etc.

    There are some nice inverters out there that remote On/Off switches and/or "standby modes" where they use much less power (turn on for a cycle every couple of seconds looking for a >6 watt AC load that was just turned on).
    The ABS chart then uses an additional load correction factor for other inefficiencies in the system. In my case, this would mean 4700 watt hours per day, times 1.15% for inverter loss, or 5405 watt-hours, and then increased further for the 70% efficiency factor, to 7,721 watt hours per day.

    For a first cut rule of thumb--I start with ~0.52 derating from solar panel to charge controller to battery bank to AC inverter. And you have to tell us how many hours of day of "full sun" you get/want to plan for.... Say 4 hours, then the equation for the solar array would look like:
    • 4,700 WH per day * 1/0.52 system derating * 1/4 hours of sun minimum per day = 2,260 Watt minimum solar array
    Of course, if you use much of your power during the day, you will have less battery losses. In sub freezing temperatures with a MPPT charge controller and snow on the ground--you can get some pretty high panel efficiencies, etc.

    Using PV Watts for Calgary Canada, a 2,260 Watt fixed array would produce around (and 0.52 derating, enter 2.26 kW in array size):
    "Station Identification"
    "City:","Calgary"
    "State:","AL"
    "Lat (deg N):", 51.12
    "Long (deg W):", 114.02
    "Elev (m): ", 1077
    "Weather Data:","CWEC"

    "PV System Specifications"
    "DC Rating:"," 2.3 kW"
    "DC to AC Derate Factor:"," 0.520"
    "AC Rating:"," 1.2 kW"
    "Array Type: Fixed Tilt"
    "Array Tilt:"," 51.1"
    "Array Azimuth:","180.0"

    "Energy Specifications"
    "Cost of Electricity:"," 0.1 CanB/kWh"

    "Results"
    "Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value (CanB)"
    1, 3.72, 142, 0.12
    2, 4.39, 148, 0.13
    3, 5.24, 193, 0.17
    4, 5.38, 182, 0.16
    5, 5.49, 185, 0.16
    6, 5.59, 176, 0.15
    7, 6.24, 201, 0.17
    8, 5.90, 193, 0.17
    9, 5.26, 168, 0.14
    10, 5.09, 178, 0.15
    11, 3.41, 123, 0.11
    12, 2.79, 106, 0.09
    "Year", 4.88, 1995, 1.72

    So a 2.26kW array would meet your demand for ~9 months of the year--and use the genset the other three as needed.
    As I have a generator to fall back on, and since the system is used mainly in the summer, I think would be OK having 2 days reserve in the battery bank. Using a 50% dod, I would need a battery bank of 643 amp hr at 48 volts, or 1286 amp hrs at 24 volts.
    Hmm... My simple calculations. Assuming 0.85 efficient inverter, 2 days of "no sun" and 50% maximum discharge for a 48 volt battery bank:
    • 4,700 WH * 1/0.85 inverter eff * 1/48 volt batt * 2 days no sun * 1/0.50 max discharge = 461 AH @ 48 volt minimum battery bank
    Do these calculations seem right?
    That is still a pretty good sized battery bank.

    And, I do not like paralleled strings of batteries. I would prefer larger AH rated cells (6 volt, or even 4 volt or 2 volt cells) in series (I would suggest no more than 2-3 parallel strings of batteries).

    Paralleled batteries have more cells to check for water, more wiring/current sharing problems, and a shorted cell can take down an entire battery bank.
    Inverter
    Peak usage appears to be about 3000 watts, being when the fridge is starting, a saw is starting, and some miscellaneous loads are running.
    You may want to split this into two inverters--A smaller (ideally) TSW inverter sized to run your home appliances (1,500-2,000 watt or so), and a second, possible MSW (less expensive) to run the hand tools.
    I think I could use either a 24 volt or 48 volt system, with the inverter putting out at least 3000 watts? I prefer the Magnasine so I could run 220 in from the generator, and then 220 out to the panel for my 220 loads(water and sewage pump)
    You are close to the 24 / 48 volt decision point... If you can use 24 volt DC appliances (truck appliances, 24 volt DC water pumps, etc. with relatively short 24 VDC wiring runs)--You might stick with 24 volt.

    If you have only AC equipment and/or longer wiring runs for your well pump, etc... Then running from 48 volts is probably better (keep battery strings down, and reduce DC current/wire gauge). Assuming you can find a 48 volt inverter that meets your needs and price point.
    Batteries
    In terms of batteries, I have boat access only to my cabin, and have to haul the batteries up a hill, so I would prefer limiting each battery to about 120 lbs, so that eliminates forklift-type batteries.
    Some battery vendors are offering some large AH batteries in 4 volt and 2 volt cells. That will allow you to have high AH cells to string together in series and keep the weight down somewhat.
    • Trojan has some 120 lb 6 volt and 2 volt batteries in higher AH ratings.
    • Crown has some 2 volt 500 AH batteries that only weigh ~76 lbs.
    What brand/model of battery to get--First size out your system / battery bank (with a few options), then see what is available in your area.

    There are lots of battery vendors and lots of opinions about battery vendors.

    I don't have an off-grid system, so I am not the guy to really recommend specific ones.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Dave AngeliniDave Angelini Posts: 3,623Solar Expert ✭✭✭✭
    Re: Upgrading from 12 to 48 volt system?

    If you seldom use the system in winter, then you do not need that much battery reserve. When I design for winter it is not so much the reserve capacity that is important but more the capacity to charge in poor weather. Even in places that require a generator, the ability to finish the charge with an array is an asset to the owner.
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
     http://members.sti.net/offgridsolar/
    E-mail [email protected]

  • albertaalberta Posts: 10Registered Users
    Re: Upgrading from 12 to 48 volt system?

    Thanks for all the information.

    We don't use the system from October to April, so winter sun is not an issue. And we are only there on weekends for April, May, June, Sept, & Oct.

    In July and Aug when the cabin gets used, we get about 5 hours of full sun. It is located near Salmon Arm, British Columbia, which will have about the same amount of sun as Calgary. So the calculations you kindly did would be about the same as our cabin.

    I have a dealer here who has 8 Surrette S-600 batteries (20 hr. rate of 450 AH) at a relatively good price -$375 each- but I would have to keep them in my garage and charge them occasionally over winter until I can take them out to the cabin in the spring. So for $3000 I could have a 450 AH bank at 48 volts. Is that advisable to buy them now and keep them over winter?

    Thanks for all your information. It certainly is a learning experience!
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