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  • Help me build an off-grid solar system

    I have spent hours reading and learning about solar systems, but I can't seem to find a scenario quite like mine. I'm an IT consultant and I work remotely for various customers. I rarely visit an actual office (yeah I love my job) and therefore it doesn't really matter where I'm working from. I'm single and have no kids so I bought an Excursion and 30' travel trailer and I plan to explore the country for the next couple of years. My RV has a front bunkhouse, and I tore out half of it and built myself a working area with a desk. I have four devices, all of which use DC power.

    Two laptops at 19.5v each (~30w each on AC power)
    An LCD monitor at 12v (30w AC)
    A desk lamp at 12v (7w AC)

    I can get car laptop chargers from amazon and I can easily wire a 12v plug onto the end of the lamp and monitor power cords. I would like to keep this system separate from the rest of the RV for now. Basically, it will be solar panels, a charge controller, batteries, and a 12v DC "power strip" to run all my devices. I don't need an inverter. There's also the possibility of adding a third laptop (security requirement for big government contractor known for building airplanes whose model numbers start with a 7) so I'd like to oversize the system to start with.

    My usage in a 24-hour period is roughly 70 DC amp-hours right now (that's with 2 laptops). Another laptop will use about 20 DC amp-hours per day.

    The equipment I've picked out so far:
    2 x 300W solar panels
    Morningstar TS-MPPT-30 charge controller
    2 x Lifeline 12v 105 AH Deep Cycle Sealed AGM Battery (GPL-31T)

    According to various solar calculators, I should be able to make almost 100 Kwh per month in the winter (in southern CA, AZ, NM or FL). I also plan on having a small generator to charge all the batteries and run the RV (except the AC and hot water heater) when I'm off ground power.

    Is this going to be sufficient for my needs? Any glaring holes or obvious omissions?

  • #2
    We design these all the time for RV and tiny homes.

    Assuming you've done your loads list right (I didn't check your math), that's 70Ah x 12V = 840Wh

    If you are mounting the panels flat on the roof, you have to use sun hours based on flat angle. In LA, that's as good as 7.89 hours in June, and as bad as 2.73 hours in Dec.

    840wh / 2.73 hours / .67 inefficiencies = 459W of solar needed for Dec. 840wh / 7.89 hours / .67 inefficiencies = 158W of solar needed for June. March - Sept won't be worse than 5.12, so we can use that as Spring - Fall. 840Wh / 5.12 / .67 = 244W.

    70ah x 1 day x 1.04 temp compensation x 2 (50% depth of discharge) = 145ah 12V per day, but that's beating the heck out of the battery, I recommend doubling or tripling it to make your batteries last more than a year or 2.

    Both your battery and solar needs to be bigger than your plan.
    Solar Queen
    altE Store

    Comment


    • #3
      Originally posted by Amy@altE View Post
      70ah x 1 day x 1.04 temp compensation x 2 (50% depth of discharge) = 145ah 12V per day, but that's beating the heck out of the battery, I recommend doubling or tripling it to make your batteries last more than a year or 2.

      Both your battery and solar needs to be bigger than your plan.
      He already plan to have 2x105 AH 12 volts batteries, so he have enough batteries. and he is going to have a small generator to charge the batteries when necessary.

      Comment


      • #4
        Originally posted by Amy@altE View Post
        We design these all the time for RV and tiny homes.

        Assuming you've done your loads list right (I didn't check your math), that's 70Ah x 12V = 840Wh
        Don't assume I did anything right.

        If you are mounting the panels flat on the roof, you have to use sun hours based on flat angle. In LA, that's as good as 7.89 hours in June, and as bad as 2.73 hours in Dec.
        What if I incline the panels to 10 or 15 degrees and always park so the panels are facing south? Or even better, I'm thinking of mounting them such that I can go up on the roof and adjust them. I don't plan on relocating the RV more than once or twice a month at the most.

        840wh / 2.73 hours / .67 inefficiencies = 459W of solar needed for Dec. 840wh / 7.89 hours / .67 inefficiencies = 158W of solar needed for June. March - Sept won't be worse than 5.12, so we can use that as Spring - Fall. 840Wh / 5.12 / .67 = 244W.

        70ah x 1 day x 1.04 temp compensation x 2 (50% depth of discharge) = 145ah 12V per day, but that's beating the heck out of the battery, I recommend doubling or tripling it to make your batteries last more than a year or 2.

        Both your battery and solar needs to be bigger than your plan.
        I'm not following your math. I understand 840Wh / 2.73 hours = 351 watts. That's the energy I need to get out of the panels during the day in. Why are you dividing by .67? That would increase the output of the array, not decrease it due to efficiency loss. Multiplying it would seem to make more sense. If something can theoretically put out 100 watts, but suffers from a 20% efficiency loss, we multiply 100 by .2 to figure out the loss.

        I don't understand what you are doing with the battery calculations either. Can you explain that more?

        Comment


        • #5
          FWIW your needs are so small, you have no need for any solar or the expense that comes with it. Even if you use solar smart money is to use an Electronic Battery Isolator (a whooping $50 worth) to enable your vehicle alternator to charge the House Batteries when the engine is running. Your alternator can generate more energy in 30 minutes than the panels can do in a couple of days. You can spend the extra $500 to $1000 on solar if you want, but does not gain you much if anything at all if you drive once every two days.
          MSEE, PE

          Comment


          • #6
            Originally posted by andym View Post
            Don't assume I did anything right.


            What if I incline the panels to 10 or 15 degrees and always park so the panels are facing south? Or even better, I'm thinking of mounting them such that I can go up on the roof and adjust them. I don't plan on relocating the RV more than once or twice a month at the most.


            I'm not following your math. I understand 840Wh / 2.73 hours = 351 watts. That's the energy I need to get out of the panels during the day in. Why are you dividing by .67? That would increase the output of the array, not decrease it due to efficiency loss. Multiplying it would seem to make more sense. If something can theoretically put out 100 watts, but suffers from a 20% efficiency loss, we multiply 100 by .2 to figure out the loss.

            I don't understand what you are doing with the battery calculations either. Can you explain that more?
            You can play with different angles changing the sun hours here, http://solarelectricityhandbook.com/...rradiance.html Note that they use angle off vertical, so 90 degrees - angle they use to = angle off horizontal, which most people reference.

            You are correct dividing by .67 increases the size of the array to compensate for 67% system inefficiencies. By dividing, I make the array bigger. So I can now go the other way, 459W of solar panels x 2.73 sun hours x .67 = 834Wh generated. Yes, it is not perfect, but close enough for rough numbers.

            You never want to use more than 50% of your batteries capacity, so you double the amp hours needed. 70ah x 2 = 140ah, using 50% depth of discharge. I couldn't find a life cycle curve of your battery, but usually you will see you can usually get 2x as many cycles, thus twice as long life, by using only 25% of the battery per cycle instead of 50%, which is why I recommended increasing the size of your battery bank even though you have 210ah planned. You also don't have any room for days without sun, I usually plan for at least 3 days, and then after 3 days, 50% DoD is ok, as long as it doesn't happen all the time. The 1.04 temperature compensation was because a battery's capacity decreases as it gets colder, they are usually rated at 80F. Since you are mobile, I used 1.04 for 70F as a worst case, as I figure if it gets colder than that, you'd head south. If I were designing the system, I'd do 70ah x 3 days x 1.04 temp x 2 (50% Dod) = 561ah. I'd also recommend using true solar batteries rather than a hybrid that won't handle deep discharges as well.

            I'll look at your loads list later to confirm that.
            Solar Queen
            altE Store

            Comment


            • #7
              I just reread your original post, I thought you had said two 100W panels, I see now it is 2 300W panels. OK, that's good for 600W.

              I can't do your loads list as I don't know how many hours you will have things on. Please tell me I am missing something where you said you will cut off the AC plug on the monitor to plug into DC. I'm hoping there is a power block that outputs 12V DC to the monitor, and you are just planning on taking that out of the equation.

              Since you list all of the watts in AC, do the loads list that way.

              laptops qty 2 x 30W x 8 hours? = 480 watt hours / 12V = 40 amp hours. etc

              What are the specs of the panels you want, I'm not sure that charge controller is big enough. They are rated at the output, so if you are putting in 16A at 24V in, you will be putting out 32A at 12V. Please list the Voc and Isc of the panels.
              Solar Queen
              altE Store

              Comment


              • #8
                Originally posted by andym View Post

                Is this going to be sufficient for my needs? Any glaring holes or obvious omissions?
                Cell phone charger - fairly small wattage I would guess. But if you spend an hour talking on the phone to your client, you'll probably need to re-charge the phone.

                Network access (cell-to-Wifi/ethernet bridge? Or a wifi-to-ethernet bridge that will allow you to have a higher gain wifi antenna up on the roof to point at the local starbucks?)

                Monitors (which Amy already touched on)

                Comment


                • #9
                  My comment would be to use 2, 6V 200ah batteries in SERIES to avoid the pitfalls of parallel batteries. Same total watt hours, just a better wiring arrangement.
                  Powerfab top of pole PV mount (2) | 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

                  Comment


                  • #10
                    Originally posted by Sunking View Post
                    but does not gain you much if anything at all if you drive once every two days.
                    I hadn't really thought of that. However, I don't plan on moving the camper every two days. Maybe once or twice a month. When I'm plugged into ground power at an RV park I have plenty of power. My concern is if I wanted to dry camp somewhere for a few days, but I can either run the motor (it's a diesel with two batteries and one alternator) or run a small generator.

                    Originally posted by Amy@altE View Post
                    You never want to use more than 50% of your batteries capacity, so you double the amp hours needed. 70ah x 2 = 140ah, using 50% depth of discharge. I couldn't find a life cycle curve of your battery, but usually you will see you can usually get 2x as many cycles, thus twice as long life, by using only 25% of the battery per cycle instead of 50%, which is why I recommended increasing the size of your battery bank even though you have 210ah planned.
                    You make it sound like I can either spend $600 every 2 years or $1200 every 4 years AND have the added weight and cost of dragging around twice as many batteries. That doesn't sound like a very good idea to me. Am I missing something?

                    Originally posted by Amy@altE View Post
                    You also don't have any room for days without sun, I usually plan for at least 3 days, and then after 3 days, 50% DoD is ok, as long as it doesn't happen all the time.
                    I can also run a generator for a couple hours to recharge on cloudy days.

                    [QUOTE=Amy@altE;138219]I just reread your original post, I thought you had said two 100W panels, I see now it is 2 300W panels. OK, that's good for 600W.[quote]

                    I was hoping nobody caught that.

                    I can't do your loads list as I don't know how many hours you will have things on. Please tell me I am missing something where you said you will cut off the AC plug on the monitor to plug into DC. I'm hoping there is a power block that outputs 12V DC to the monitor, and you are just planning on taking that out of the equation.
                    You don't know me, so it's forgivable, but no, I am not an idiot.

                    Since you list all of the watts in AC, do the loads list that way.

                    laptops qty 2 x 30W x 8 hours? = 480 watt hours / 12V = 40 amp hours. etc
                    Here's my question with that calculation. The monitor takes 30W of AC power. If I run the monitor directly off a battery, I start with 12v DC, the inverter converts that to 120v AC, then the power supply converts it back to 12v DC. That's horribly inefficient. I know inverters are usually about 95% efficient, but consumer grade power supplies can vary anywhere from 20% to 90% from what I have read. So while my monitor uses 30W AC, I doubt it's going to use 30W directly from a DC source.

                    What are the specs of the panels you want, I'm not sure that charge controller is big enough. They are rated at the output, so if you are putting in 16A at 24V in, you will be putting out 32A at 12V. Please list the Voc and Isc of the panels.
                    I haven't picked out a panel or even settled on a brand yet, but Astroenergy seems decent. They are supposed to be a top-tier Chinese manufacturer so the quality is good but an American made panel is significantly more.

                    This panel looks good: http://www.wholesalesolar.com/produc...6612P-305.html

                    Originally posted by foo1bar View Post
                    Cell phone charger - fairly small wattage I would guess. But if you spend an hour talking on the phone to your client, you'll probably need to re-charge the phone.
                    I charge my phones off the laptop so the usage is included there. I also have a wifi hotspot through my employer.

                    Originally posted by Mike90250 View Post
                    My comment would be to use 2, 6V 200ah batteries in SERIES to avoid the pitfalls of parallel batteries. Same total watt hours, just a better wiring arrangement.
                    What are the pitfalls of batteries in parallel?

                    Thanks for everyon'e help. I've learned a ton about this already (including whether I actually need a system like this) but obviously I have a lot more. In the future, I'd like to build an off-grid tiny house so anything I learn won't be wasted.

                    Comment


                    • #11
                      [QUOTE=andym;138299]I hadn't really thought of that. However, I don't plan on moving the camper every two days. Maybe once or twice a month. When I'm plugged into ground power at an RV park I have plenty of power. My concern is if I wanted to dry camp somewhere for a few days, but I can either run the motor (it's a diesel with two batteries and one alternator) or run a small generator.



                      You make it sound like I can either spend $600 every 2 years or $1200 every 4 years AND have the added weight and cost of dragging around twice as many batteries. That doesn't sound like a very good idea to me. Am I missing something?



                      I can also run a generator for a couple hours to recharge on cloudy days.

                      [QUOTE=Amy@altE;138219]I just reread your original post, I thought you had said two 100W panels, I see now it is 2 300W panels. OK, that's good for 600W.

                      I was hoping nobody caught that.



                      You don't know me, so it's forgivable, but no, I am not an idiot.



                      Here's my question with that calculation. The monitor takes 30W of AC power. If I run the monitor directly off a battery, I start with 12v DC, the inverter converts that to 120v AC, then the power supply converts it back to 12v DC. That's horribly inefficient. I know inverters are usually about 95% efficient, but consumer grade power supplies can vary anywhere from 20% to 90% from what I have read. So while my monitor uses 30W AC, I doubt it's going to use 30W directly from a DC source.



                      I haven't picked out a panel or even settled on a brand yet, but Astroenergy seems decent. They are supposed to be a top-tier Chinese manufacturer so the quality is good but an American made panel is significantly more.

                      This panel looks good: http://www.wholesalesolar.com/produc...6612P-305.html



                      I charge my phones off the laptop so the usage is included there. I also have a wifi hotspot through my employer.



                      What are the pitfalls of batteries in parallel?

                      Thanks for everyon'e help. I've learned a ton about this already (including whether I actually need a system like this) but obviously I have a lot more. In the future, I'd like to build an off-grid tiny house so anything I learn won't be wasted.
                      Oh good, I wasn't imagining it, you did change from 200W to 600W. You'd be amazed at some of the "interesting" schemes we've seen, I always have to ask what I think is the obvious questions, at the risk of asking the dumb questions. "Are your solar panels outside in the sun?" (I get the answer "no" to that one more times than I care to think about)

                      If you currently are using all of the equipment you plan on using, but with AC, you could also plug it all into a power strip and use a Kill-a-watt meter to measure how much power you are using in a day. http://www.p3international.com/products/p4400.html That would get you an accurate reading, and you could either subtract about 10% for the inefficiencies of the DC/AC conversion, or figure you can use the extra power somehow.

                      Wiring batteries in parallel can result in uneven charging/discharging, shortening the life of the battery bank. In a perfect world, one string is best, but there are plenty of people who will accept 2 or reluctantly even 3 strings. But one is definitely best.

                      It's not a 1 to 1 comparison, but yes, it's kind of a" pay me now or pay me later" with batteries. Buy a bigger bank in the beginning that will last longer, or a smaller one now that will need to be replaced sooner. Keep in mind most off-grid systems aren't mobile, so the added weight isn't always a big deal. Space is also a consideration. It's really just about setting accurate expectations.
                      Solar Queen
                      altE Store

                      Comment


                      • #12
                        If you are still planning on using AGM batteries (among other good things, their internal resistance is very low) parallel is to be avoided, because miniscule differences in wire resistance will cause them to be loaded and recharged un-evenly.
                        The math is here: http://www.smartgauge.co.uk/batt_con.html

                        Flooded batteries have higher internal resistance which helps swamp out the wire differences, but are still problematic.

                        Also, AGM have half the expected lifetime of flooded batteries, so the expensive batteries still die sooner, but what a ride !!
                        Powerfab top of pole PV mount (2) | 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

                        Comment


                        • #13
                          Originally posted by Mike90250 View Post
                          If you are still planning on using AGM batteries (among other good things, their internal resistance is very low) parallel is to be avoided, because miniscule differences in wire resistance will cause them to be loaded and recharged un-evenly.
                          The math is here: http://www.smartgauge.co.uk/batt_con.html

                          Flooded batteries have higher internal resistance which helps swamp out the wire differences, but are still problematic.

                          Also, AGM have half the expected lifetime of flooded batteries, so the expensive batteries still die sooner, but what a ride !!
                          True, but flooded batteries need maintenance, where sealed don't. And he's putting these in an RV, so he'd have to deal with venting. It's all about if he's willing to pay more for convenience. Most of my clients choose sealed for the convenience, knowing full well it is a more expensive option. A well maintained flooded battery will cost less and last longer than a sealed. A neglected flooded battery becomes an expensive boat anchor.
                          Solar Queen
                          altE Store

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