How much solar does one need?

Shared owner in an older houseboat at Halls Crossing. Current system is two 100 Ah 12v batteries charged by one 85W BP Solar panel with a MorningStar PS-15 controller. We can also charge with the engines or a generator as needed.

1) Best we can figure daily usage may go as high as 95 Ah. Does our current system have sufficient capacity?

2) Based on my limited math, one 85W panel seems too small. Adding a second, newer, 100W panel seems a minimum if we actually used 95 Ah in one day. Does this sound right?
2a) BP Solar no longer exists. Since the panels will be from different manufacturers, can they actually be connected to increase solar output?

3) The batteries probably need to be replaced as they no longer stay above 50% even during the day. How can we test them?

4) Batteries are inside cabin thus AGM. Looking at two 220 Ah 6v AGM. Any suggestions?

5) We are talking about extending but the load is still TBD. One idea is three 100 Ah 12v AGM. Related to 1), should we add a third panel in this case?
Another idea is to move the bank outside the install four 220 Ah 6v lead acid. Again, add a third and fourth panel?

Of course, any other questions or suggestions will be greatly appreciated!

Kurt

Comments

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: How much solar does one need?

    Welcome to the forum Kurt.

    Your one 85 Watt panel is by no means sufficient to recharge 200 Amp hours @ 12 Volts on its own. If you wanted to go "full solar" you would need more like 400 Watts of PV in good sun for that:

    20 Amps * 17.5 Vmp = 350 Watts on a PWM controller.

    If you are using 95 Amp hours daily you will find that 200 Amp hour battery bank is stressed as that usage represents 47.5% DOD - and that with new batteries (and assuming zero power from other sources). Switching to 220 Amp hour batteries will ease this a bit, but not much. Expect a short lifespan from batteries so deeply discharged.

    If you can't up the system Voltage due to needing 12 VDC then consider higher capacity 6 Volt cells. For example L16 size with a 390 Amp hour rating. This would reduce your DOD to around 25% daily. But you would need a significant increase in PV to charge those: at least 600 Watts on an MPPT charge controller.

    As it is your 85 Watt panel puts out about 4.5 Amps peak, which is only a 'maintenance' charge for 200 Amp hours. If you want to reduce generator run time you'll need a lot more solar panels.
  • BB.BB. Super Moderators, Administrators Posts: 32,639 admin
    Re: How much solar does one need?

    As Marc says--That battery bank is pretty small. If you are mostly using the boat on sunny days, and much of your loads are during daylight hours, you can make the small bank work.

    We size battery banks to support the loads. For full time off grid living, a battery bank that is ~4x daily loads works well (2 days of storage, 50% maximum discharge). For RV applications, sometimes you have to live with the smaller battery bank (and make sure you have worked hard to conserve power; LED lighting, small/energy efficient appliances, turn stuff off when not in use).

    For the solar array, we size them two different ways. One is based on the size of the battery bank and minimum charging current. The second is based on the amount of sun you have and your loads.

    First, based on size of battery bank. Generally, you want around 5% to 13% rate of charge. 5% can squeak by on a weekend/seasonal cabin. 10%+ for daily/full time use is better. With solar panels so cheap these days, more is usually better. Of course, you have limited roof space, so that is an issue too:
    • 200 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge= 188 Watt minimum
    • 200 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge= 377 Watt nominal
    • 200 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge= 490 Watt array "cost effective" maximum
    The second based on your power usage and location. Cedar City UT, flat to roof (no tilt) array, using PV Watts:
    Month    Solar Radiation (kWh/m 2/day)
    1      2.71     
    2      3.54     
    3      4.70     
    4      6.12     
    5      7.02     
    6      7.97     
    7      7.35     
    8      6.55     
    9      5.50     
    10      4.41     
    11      2.90     
    12      2.36     
    Year      5.10      
    

    If you use the boat between March and October, the minimum amount (long term average) of sun you get is 4.41 hours of sun (noon time equivalent) per day. Your break even array would be:
    • 95 AH * 12.5 volts = 1,188 WH per day
    • 1,188 WH per day * 1/0.61 DC system eff * 1/4.41 hours of sun per day = 442 Watt array minimum
    Based on my guesses--I would be suggesting a minimum of 442 Watt array, and upwards of 500 Watt array would be nice with your existing battery bank.

    Of course, if you have the room (and money), you could go with a larger array... Or even replace your present battery bank with a 400 AH size (which could justify a bit larger array). 4x 6 volt ~200 AH golf cart batteries would be nice (two batteries in series, then two parallel strings) for this application.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • MegaBiteMegaBite Registered Users Posts: 8
    Re: How much solar does one need?
    BB. wrote: »
    If you are mostly using the boat on sunny days, and much of your loads are during daylight hours, you can make the small bank work.

    Wondered about that. How does loads during daylight hours help with a small bank?

    Kurt
  • BB.BB. Super Moderators, Administrators Posts: 32,639 admin
    Re: How much solar does one need?

    You are not cycling the battery. The solar panels supply current to both the battery bank and the loads at the same time from ~9am to 3pm or so.

    Of course, if you have large daytime loads, you will probably have to "up-size" the solar array to supply both the loads and the battery bank recharging too.

    There is nothing magic about solar... It is very much like your car's battery+alternator+loads system.

    The battery bank really regulates the system to ~12-15 volts. The solar charge controller does its best to estimate the state of charge of the battery bank and keep it full (during the day). If there are loads at that time, the solar charger will let additional current from the array to supply those loads and not discharge the battery bank.

    Most of the time, we size solar power systems for charging during the day and discharging at night (when people are home using power/cooking/studying/using the computer+TV, etc. Also, we size as if the power is supplied during the day in stormy weather (heavy clouds will block almost 95% of the sun, and even a light overcast can easily block 50% of the sun's energy).

    The battery bank will carry through for a couple of days of bad weather--And by then, the sun hopefully appears, or you fire up the genset to keep the loads running/get the battery bank back >~75% state of charge to reduce sulfation.

    Taking a lead acid battery bank below ~50% state of charge consistently is pretty hard on batteries too... They will not cycle/last as long.

    But for portable/RV use--Many times the weight/size of the battery bank is an issue... So you have a 1/2 size battery bank that lasts 2-3 years instead of a "full size" bank that can last 4-6 years. In either case, it is usually close to a wash in price anyway. A 1/2 size battery bank for 3 years cost the same as a full size bank that lasts 6 years.

    And if you have a shared house boat--If somebody/guests/etc. kill the battery bank (leave everything on overnight) and kill the battery bank--The smaller bank is certainly cheaper to replace. Another reason to keep battery banks small--And if practical, use the "cheap" golf cart batteries for these types of applications (shared responsibility, guests/kids visiting, etc.).

    Many battery banks do not die of natural causes--The are frequently "murdered" in one form or another.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: How much solar does one need?
    MegaBite wrote: »
    Wondered about that. How does loads during daylight hours help with a small bank?

    Kurt

    Well, you have to have enough PV.

    It's what we call "making use of opportunity loads" or a variation thereof.

    Normally as a battery charges it requires less energy the higher its SOC becomes. As such you reach a point where the panels' power potential is much higher than what is actually being used to charge batteries and run loads at that moment. This is why battery systems are so inefficient: if all the power requirements are stored in and taken from the batteries then you have the charge/discharge loss on every Watt hour.

    Now if the batteries are charged or being charged at a level that only requires say 10% of the PV output, then that's 90% waste of solar power. Unless you can turn a load on at that time. In essence the load is run "directly" from the PV: no charge/discharge losses for that particular load. This has the effect of increasing the over-all system efficiency by increasing the Watt hour harvest.

    As an example, my water pump is turned on midday when I know there's enough sun to run it without interrupting the charging of the batteries. Water is stored in a large pressure tank, and can then be used any time without the pump running; no draw on the batteries.

    But such an arrangement requires that it is practical to use the loads when the PV is producing and not all of the array's output is needed for recharging.
  • MegaBiteMegaBite Registered Users Posts: 8
    Re: How much solar does one need?
    BB. wrote: »
    Many times the weight/size of the battery bank is an issue... So you have a 1/2 size battery bank that lasts 2-3 years instead of a "full size" bank that can last 4-6 years. In either case, it is usually close to a wash in price anyway. A 1/2 size battery bank for 3 years cost the same as a full size bank that lasts 6 years.

    -Bill
    Size is a problem - L16 will not fit in the current space.

    It is possible to move the bank outside the cabin. Could we get longer life going with larger or lead acid batteries?
  • BB.BB. Super Moderators, Administrators Posts: 32,639 admin
    Re: How much solar does one need?

    I moved your thread to the RV/Marine forum--We just created so people can look for RV/Boat/etc. questions here.

    Based on some numbers I have seen--In theory, if you cycle a battery bank from 100% to 80% or even to 20% state of charge, and recharge promptly, the "AH Throughput" is roughly the same (20% cycling to 80% cycling). I.e., if you cycle a battery 20% it will "last" 4x longer than cycling 80% depth of discharge. But, notice that you are taking 4x the current for 1/4 the life. It is a "wash" in terms of cost of batteries.

    In practice, probably I would only suggest cycling Fork Lift type batteries below 50% state of charge on a normal basis. Standard storage batteries don't seem to do as well if you take them really deep discharge (below 50%).

    But Fork Lift batteries have their own charms... They tend to use more distilled water, and have a higher self discharge rate (especially as they get old).

    More or less--Recommend your loads to be a minimum of 80% state of charge (20% depth of discharge). And 50% is about as deep as I would plan (for long battery life).

    And, it is probably just as likely the batteries will be killed by over discharging/forgetting to check electrolyte levels/not filling with distilled water/over charging/forgetting to charge/a bad cable/connection isolates one battery/etc.

    If you have a backup genset or motor around the lake and the motor can keep the battery bank charged (if this an outboard--They usually do not have much in the way of alternator output current)--Just accept the fact that you will be using the genset more.

    When you get very large battery banks, the solar array becomes large, the boat's alternator may not be able to properly recharge the bank, need more shore power/larger AC battery charger (when tied up at the dock), etc.

    How well has the present setup/battery bank lasted so far?

    Can you fit two 6 volt @ ~200 AH batteries in the spot? You will not have more stored power, but the Golf Cart batteries can be pretty cheap and you will have fewer battery caps to check (6 instead of 12) and fewer battery connections.

    But if the present battery bank is giving you good service--It is hard to recommend major reconfiguration/re-wiring. The batteries will have have similar performance--And, would anyone notice if you repaced the battery bank every 4 years vs every 3 years?

    If the present battery bank is lasting only 1 season, then figuring out why the batteries are dieing and what you can do to fix the issues would be worth some time and money to address.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: How much solar does one need?
    MegaBite wrote: »
    Size is a problem - L16 will not fit in the current space.

    It is possible to move the bank outside the cabin. Could we get longer life going with larger or lead acid batteries?

    All we're talking about here are lead-acid. L16 is a larger size than a GC2 and will have more Amp hour capacity. A higher capacity will be cycled to less depth of discharge for a given load, resulting in a longer lifespan (if comparing properly recharged banks).

    If space is a limitation then you can only use what will fit. For that reason some RVers are going to LiFePo batteries, which have higher energy density. But they also have higher cost and are a bit more complex to manage. There are threads about these in the Advanced section.

    At the moment that may not be necessary for you. The two biggest problems you have now are the really small amount of PV and the aged RV/Marine batteries. Just changing those two items, even if still at near 50% DOD, may be enough for your circumstances.
  • AuricTechAuricTech Solar Expert Posts: 140 ✭✭
    Re: How much solar does one need?
    MegaBite wrote: »
    4) Batteries are inside cabin thus AGM. Looking at two 220 Ah 6v AGM. Any suggestions?

    Two 220Ah 6V batteries, wired in serial, would give you a 220Ah 12V battery bank, which is not much of an improvement over your current 200Ah 12V battery bank.

    If you want to stay with AGM batteries, Northern Arizona Wind & Sun (our gracious host) offers quite a few Universal Battery AGM batteries, as well as Concorde Sun Xtender AGM batteries. Both product lines include 12V batteries of higher capacity than the batteries you now possess. Perhaps a pair of their larger batteries might fit your needs. As Marc and Bill have both pointed out, you'll want a significantly larger photovoltaic panel array to keep even your presently-sized battery bank charged.
    5) We are talking about extending but the load is still TBD. One idea is three 100 Ah 12v AGM.

    You'll find that most of the longtime participants on this forum recommend against having three batteries wired in parallel, due to problems with evenly charging such a battery bank. As such, if you choose to stay with AGM batteries, you would be better off getting two larger batteries.
  • CDN_VTCDN_VT Solar Expert Posts: 492 ✭✭✭
    Re: How much solar does one need?

    Never enough , for the least price .
    Dreams & wishes !
  • MegaBiteMegaBite Registered Users Posts: 8
    Re: How much solar does one need?

    If we get more panels and a new MPPT controller, can we connect the existing panel into the new controller?

    For discussion sake, one panel is rated for 17.6V, one or more other panels are rated for 18.9V.
  • EnduranceEndurance Solar Expert Posts: 40
    Re: How much solar does one need?

    Hi MegaBite, it's good to see you made it here.

    The short answer to your question is that you can combine your old panel in an array with new panels and a MPPT controller. There are some downsides, though, and you might not want to do it.

    If you're reading figures like 17.6V or 18.9V, you are probably looking at nominal 12v panels. Just like a 2x4 isn't 2" by 4", nominal 12v panels don't put out 12 volts. If you look at a spec sheet for a panel, you will see volts open circuit (usually abbreviated VOC), and another figure for volts at maximum power (usually abbreviated VMP). The 17.6 volt and 18.9 volt figures you're looking at are probably something like one of those. Here is a previous thread that talks about the difference: http://forum.solar-electric.com/showthread.php?19414-Voc-vmp-imp-isc-explained

    Within reason, you can get away with combining panels with some differences. If you have panels with the same nominal voltage and similar output, you would be able to combine them. If you combine them in series, you add the voltage of the panels; if you combine them in parallel, the voltage stays the same but you get more amps. Putting the panels in parallel does a better job of lessening the ills of a panel that isn't putting out as much as its neighbors. But in your case, you would want to combine your panels in series to get your voltage up. If you have nominal 12v panels and a 12v battery bank, you are kind of wasting an MPPT controller since you won't have the voltage differential to allow the MPPT controller to work its magic.

    If you combine two 12v panels in series, you have a 24v array. Three in series gives you 36v; four in series gives you 48v and so on. Although solar controllers can take high voltages like 150 or even higher, you won't want to exceed 48v (nominal) on a boat. The American Boat and Yacht Council (ABYC) gets grumpy with wiring over 50 volts. You would have to take some measures that make your wiring job harder than it has to be. Besides, too big a differential between your solar array and your batteries is not ideal for your controller. That's not a big deal, though, because you can combine panels in series and parallel combinations to get where you want to be. You could have two strings of four 12v nominal panels and have eight panels at 48v for example.

    Having said all that, solar panels themselves are getting pretty cheap compared to other parts of your solar system. You might do better to buy the panels you want and then sell your old panel and Morningstar controller as a package. My bet is that someone at Powell that is in the same boat (sorry about the pun) that you were in at the time you bought your first panel would snap it up. I'm not aware of classifieds here at NAWS, but the classifieds section at Waynes Words would match your target audience to a tee.
  • westbranchwestbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Re: How much solar does one need?

    There is a for sale thread. http://forum.solar-electric.com/showthread.php?16275-For-Sale-Thread&highlight=for+sale
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • MegaBiteMegaBite Registered Users Posts: 8
    Re: How much solar does one need?

    Hi Endurance, thanks for the referral to this forum. This is one of the best forums I have ever visited thanks to timely replies and understandable answers.
    ...nominal 12v panels don't put out 12 volts.
    As I understand, that is why we want a MPPT controller. Since 85W / 17.8v = 4.9A, with an MMPT controller we can get closer to 85W / 14.4v = 5.9A.

    Current panel is probably BP485 (85W, VMP=17.8v). Despite the recommendations here, group is looking to add only one 100W panel, VMP=18.9v.

    Could add another panel to the current controller. But, thinking about improving efficiency to make up for lack of sufficient PV.
    Good idea about selling the current panel & controller. A new, larger, panel would also make up a little bit of our needs.
    If you have nominal 12v panels and a 12v battery bank, you are kind of wasting an MPPT controller since you won't have the voltage differential to allow the MPPT controller to work its magic.

    Please explain.
    And, how do we charge a 12v battery bank with 24 or 36 or 48 volts?
    If you combine two 12v panels in series, you have a 24v array

    Confused about the math here. Yes, nominal 12V, but 17.8+18.9=36.7...
  • BB.BB. Super Moderators, Administrators Posts: 32,639 admin
    Re: How much solar does one need?

    If you have "12 volt panels" (Vmp~17.5 to 18.6 volts or so) charging a 12 volt battery bank, that is about an optimum match.

    On hot sunny days, solar panels get quite hot (upwards of 80C or 176F is usually the top rated cell temperature). And when the cell temperatures get very hot, the panels can lose upwards of 20% of their output voltage:

    18 Volts Vmp * 80% = 14.4 volts actual [email protected]

    So you have 14.4 volts at the panel and 14.4 charging the battery (no wiring losses, no voltage drop across the controller, etc.).

    So--With a "12 volt panel" charging a 12 volt battery, there is no voltage differential that an MPPT charge controller can exploit.

    In real life, when panels are in below freezing temperatures on cool/clear/sunny winter days, then [email protected] can rise above the rated value, and an MPPT controller can obtain something like (on average) a best case of ~10-15% increase in additional power from the array.

    So--For most people with a smaller system and 12 volt panels, an MPPT charge controller does not really help much.

    However, MPPT controllers can help in several cases. Obviously one is very hot days with a cool battery bank needing equalization in the summer. You can put a Vmp-array>>18 volts and bring the battery charging voltage up to and exceed 16 volts--Which can be helpful for equalization and many industrial type storage batteries that require higher charging voltages.

    The second, "12 volt" panels tend to be rare for wattage ratings >>140 Watts, and 12 volt panels tend to cost more $$$/Watt than the "Grid Tied Panels" do at this time (GT panels tend to have Vmp~30 volts to 36 volts).

    The savings in costs for GT panels for larger systems, and because of higher Vmp-array allows us to use smaller diameter (AWG) copper wire, it can "pay" to use the more expensive MPPT charge controller.

    In the end, below ~400 Watt systems, PWM controllers and 12 volt panels can make sense.

    And over 800 Watt systems, should probably use "high voltage" Vmp-array + MPPT charge controllers. It makes for less expensive, easier to design/build, and more capable off grid power system.

    Using an MPPT charge controller for a smaller system to get ~10% additional energy in deep winter (in cold climates) is usually not enough of a justification for the additional costs by itself.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • MegaBiteMegaBite Registered Users Posts: 8
    Re: How much solar does one need?

    Thanks Bill.

    And a great big thanks to everyone who responded.

    Kurt
  • EnduranceEndurance Solar Expert Posts: 40
    Re: How much solar does one need?
    MegaBite wrote: »
    how do we charge a 12v battery bank with 24 or 36 or 48 volts?

    The answer to that question underscores the beauty of an MPPT controller for a boat at Lake Powell. At the risk of offending some here by oversimplifying things, an MPPT controller starts with the basic function of a PWM controller of not allowing excess current into your battery and adds the additional function of a voltage converter. Let's say that your batteries are in a state of charge that they need 13.1 volts to charge them. An MPPT controller will take whatever voltage the solar panels are giving it in excess of 13.1 volts and turn it into exactly 13.1 volts. It doesn't doesn't matter if your panels are giving 13.1 volts or 30.1 volts. The output will be 13.1 volts.

    The way that helps you in the real world gets really clear when you make some logical extensions to BB's example. Let's say that your 85W panel or a combination of your 85W panel in parallel with a new 100W panel are capable of giving you 18 volts. As BB said, your panels can get hot and drop their output by 20%, which drops you to 14.4 volts. Since you're on a houseboat, it probably makes sense to mount your panels flat on your shade covering on your top deck rather than orient the panels to the sun like you'd do if you weren't on a houseboat. That is likely to give you another 20% drop in output. That takes 14.4 volts down to 11.5 volts. We could deduct a little more efficiency loss for wiring, your controller, and maybe even a little dust on your panels left over from the last windstorm, but we don't have to get to that level of figuring because even 11.5 volts won't do a thing for 12 volt batteries unless your batteries are seriously discharged. Even though it's a typical sunny but hot day at Powell, you don't get any charging until the next day when your panel can cool.

    Now let's take that same scenario with an MPPT controller and either your 85W plus a 100W panel wired in series to give you a maximum of 36 volts or, even better, a new 250 to 300 watt panel that has a maximum output in the 36 volt neighborhood. The panel gets hot and drops output by 20%. You're now at 28.8 volts. Your panel is flat and not facing directly at the sun, so you lose another 20%, taking you to about 23 volts. Your MPPT controller takes that 23 volts and converts it to exactly what your batteries need, whether that be 13.1 volts, 14,4 volts, or whatever. Granted, the hot panel and having it face straight up is reducing your output, but you're still charging rather than waiting for the next day and cool panels. That is the beauty of an MPPT controller on a boat where you mount a panel flat, especially when you're on a lake that has high ambient temperatures.

    The crazy thing is that a 100 watt panel that gives you about 18 maximum volts will probably cost in the $250 range. A larger higher voltage panel with 250 watts will cost remarkably close to that. The $150 or so that you will get from resale of your old panel and controller will make a pretty big dent in any difference if you've already decided to replace your PWM controller with an MPPT controller.
  • AuricTechAuricTech Solar Expert Posts: 140 ✭✭
    Re: How much solar does one need?

    I'm pretty sure that it's current, not Voltage, that drops due to suboptimal panel angle.

    ETA:
    Endurance wrote: »
    The crazy thing is that a 100 watt panel that gives you about 18 maximum volts will probably cost in the $250 range.

    The Renogy 100W monocrystalline panels have a Vmp of 18.9V, and they presently sell for $149.99 (shipping included).

    That being said, I'm not arguing against the potential benefits of MPPT charge controllers in this application.
  • EnduranceEndurance Solar Expert Posts: 40
    Re: How much solar does one need?

    One thing I should have added, but didn't, is that an MPPT controller only converts voltage down. It can't take 11.5 volts and turn it into anything more than that. That's why BB was saying to up your solar panel voltage if you get an MPPT controller so that your controller has some voltage differential to exploit.

    You do want to be careful to not make your solar panel voltage too high, though. It is work for an MPPT controller to convert voltage and that work creates heat. Heat is the enemy of efficiency and we have a lot of heat at Lake Powell. Something like 36 volt maximum (probably nominal 24 volt) panels would be a sweet spot.
  • MegaBiteMegaBite Registered Users Posts: 8
    Re: How much solar does one need?
    AuricTech wrote: »
    I'm pretty sure that it's current, not Voltage, that drops due to suboptimal panel angle.
    Seems like this would be the case, otherwise "12 volt panels" would provide very little useful charge in the summer.


    So, "12 volt panels" in parallel get no benefit from MPPT controller.

    What if they are tied in series? 85W+100W=185W, 17.6v+18.9v=36.5v. Isn't that similar to one 250W, 30.1v panel?

    And how the heck do you do the math to estimate 12v amps output?
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: How much solar does one need?

    It looks like this:

    A "12 Volt" panel that has a Imp of 7.9 and a Vmp of 17.7 is rated as 140 Watts.
    On a PWM controller that Vmp 17.7 is high enough to deliver the necessary Voltage to charge the battery, and the current is limited to 7.9 Amps. It will be 7.9 Amps if the battery is down to 12 Volts (effectively 95 Watts) or if the loads and battery demand that current at Absorb Voltage of 14.8 (effectively 117 Watts).

    On an MPPT controller the formula is different because any Voltage 'overhead' not lost in wiring can be converted to current. Thus at 12 Volts you get potentially 8.9 Amps or 107 Watts of power. At higher charging Voltage there is less Voltage 'overhead' (differential) between the panel output and the battery Voltage so the current there changes little over PWM.

    Adding panels in series increases the Voltage and to some extent then the Voltage 'overhead'. In essence if you are going to lose 2 Volts in the wire that's all you'll lose, whether it's from 17.7 or 36 Volts at the array. But by the same token too high a Voltage makes the controller work more to adjust it resulting in power loss in the conversion.

    From the example an 85 Watt 17.6 Vmp 4.8 Imp panel in series with a 100 Watt 18.9 Vmp 5.3 Imp panel would actually be 36.5 Vmp 4.8 Imp because the lower current panel will limit the current: 175 Watts, or 10 Watts less than predicted. The MPPT controller will not make up for that.

    As a current source PV likes to meet its Imp point and allows its Voltage to do whatever. Connected directly to a battery (or even a resistive load) the Voltage will be at battery level and yes the current flow will try to bring that up to panel Vmp, which is why you need the charge controller.

    The main advantage to an MPPT controller is not the slight current advantage to be had under otherwise equal conditions but rather the flexibility it allows in array design such as the use of 'wrong' Voltage GT style panels on an off-grid system which are usually much cheaper per Watt. When you get near 400 Watts array size it becomes much more economical to use those panels + MPPT controller.
  • MegaBiteMegaBite Registered Users Posts: 8
    Re: How much solar does one need?
    If you can't up the system Voltage due to needing 12 VDC then consider higher capacity 6 Volt cells. For example L16 size with a 390 Amp hour rating. This would reduce your DOD to around 25% daily. But you would need a significant increase in PV to charge those: at least 600 Watts on an MPPT charge controller.

    As it is your 85 Watt panel puts out about 4.5 Amps peak, which is only a 'maintenance' charge for 200 Amp hours. If you want to reduce generator run time you'll need a lot more solar panels.
    Could you provide example of panels and controller?

    For example, 2 X 250 watt, Vmp 30.1 volts Imp 8.32 amps would need a 45 amp MPPT controller? and provide ?? amps?
    Or, 2 X 315 watt, Vmp 36.5 Imp 8.71 would need a 45 amp MPPT controller? and provide ?? amps?

    Maybe a couple of other configurations?

    TIA for all your help!
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: How much solar does one need?

    For charging 390 Amp hours of 12 Volt battery you want 39 Amps peak current @ 12 Volts as a minimum. That works out to (39 Amps * 12 Volts / 0.77 typical efficiency) 607 Watts.

    Possible configurations:

    Two 315 Watt panels (Vmp 36, Imp 8.7) in parallel. 630 Watts total on an MPPT controller would yield (630 * 0.77 / 12) 40 Amps peak current. (Note these panels only come in full pallets so not a good choice).

    Three 220 Watt panels (Vmp 30, Imp 7.3) in parallel. 660 Watts total on an MPPT controller would yield (660 * 0.77 / 12) 42 Amps peak current.

    Four 190 Watt panels (Vmp 36, Imp 5.2) in parallel. 760 Watts total on an MPPT controller would yield (760 * 0.77 / 12) 48 Amps peak current.

    Five 130 Watt panels (Vmp 18, Imp 7.4) in parallel. 650 Watts total on an MPPT controller would yield (650 * 0.77 /12) 41 Amps peak current. On a PWM controller would yield (7.4 * 5) 37 Amps peak current.

    Note that last one I compared MPPT output with PWM output because it is the only configuration of the examples that could use either. The MPPT gives about a 10% gain in charge current in theoretical numbers. Actual performance will vary according to the situation of the installation.
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