Cabin solar upgrade

MinnowMinnow Registered Users Posts: 15
Hi folks. I'm new here. I've started reading the Solar Beginner section, but there's a lot to learn and I'm looking for some advice.

My cabin has been inching toward solar. Phase 1 was a simple cabin entirely run on propane.

Phase 2 occurred when we had a trip where the temperature never dropped below 85 degrees, even at night. I wasn't willing to further heat my cabin with propane lights. I was in a hurry and didn't spend a lot of time on research but went out and bought two 75w solar panels, two 6 volt deep-cycle batteries, a Morningstar ProStar PS15 charge controller, a bunch of 12v CFLs, and a roll of 10 gauge wire. I lucked out and the results worked fine for years.

Phase 3 was when my wife wanted to be able to write on her computer. That resulted in a Samlex 1000w Pure Sine Wave inverter, which has also worked fine for years.

Now, however, I think it's time to plan a bit and learn some more. Our jobs now allow us to go up for longer periods, but we need extended access to computers plus access to the Internet. We put in a satellite modem but the solar couldn't keep up and we had to use the generator at times. The weather was bad but, quite frankly, I'm pretty sure it wouldn't have been adequate anyway to run lights, two laptops, the modem and a network.

How do I go about sizing this properly? Am I on the right track below?

On the AC side, the modem draws 2 amps, the Airport 0.3 amps, my computer's power pack maxes at 2.34 amps but generally is drawing about .8 amps (per a Kill-A-Watt), my wife's computer maxes at 1.5 amps but generally is drawing .3 amps. 2 + 0.3 + 2.34 + 1.5 = 6.14 amps worst case, or about 3.6 amps with a more typical usage on the computer...let's say 5 amps average. 5 amps of 120v AC is 50 amps of 12v DC. I haven't found a specification for the inverter, yet, but if I guess it's 80% efficient, then I need more like 62.5 amps of DC for about 8 hours of the day.

In the evening, if every light in the cabin is on, we draw 9 amps of 12 volt current for maybe 4 hours.

So I need 536 amp-hours of capacity per day. That's where I get a little stuck. *smile* What's a reasonable amount of battery capacity to provide this without destroying the batteries?

On the panel side, we're only there in the summer. Sun charts show a high of 7.2 hours of sun, a low of 2.7 hours of sun, an average of 5 hours of sun. Would that mean I'd need something like 1000w of panel?

Another thing to add into the mix is that we use a generator to fill up our water pressure tank. While it's running, the generator puts out 25 amps of AC. The pump uses a small fraction of that. I thought about buying something like an IOTA DLS-45 charger and using the "extra" power to add charge to my batteries.


  • icarusicarus Solar Expert Posts: 5,436 ✭✭✭✭
    Re: Cabin solar upgrade

    Welcome to forum,

    There re some very smart, very experienced folks here who are more than willing to give good advice,, and. Few of us with perhaps not so good advice.

    You have learned one of the basic rules of off grid PV, (and in that world, so the lesson is likely to stick!) that rule is,, all loads grow with time. Ours started with a single reading light, moved on to a couple of CFLs, and now is a fully wired small house with ISP, radio, lap tops, fridge lights even!

    The next basic rule of off grid is,, all hardware choices stem from a proper load calc. Add up all your total daily/weekly loads, and thier durations. Find the total AH/WH. Loads, as well as the peak current draw of both the average, as well as the spike loads,, like pumps etc.

    The load calcs will determine the (roughly) battery size, which in turn will determine the cooler size, and thrive of the PV. The loads will also determine the size and design of the inverter, and a genny based charger. Trying to fit the wrong components into a system often times leads to square pegging. For example, because one might have an 1000 watt inverter onemay choose to use it, even the it may not be the best choice for but one simple example.

    My final simple rule (which is probably not final!) for off grid is this. To determine the net, out the inverter amount of power that a well balanced PV system can produce is this: take the name plate rating of the PV, divide this gross number by 2 to account for all cumulative system loses. Then multiply that number by 4 to account for the averge hours of good sun one might expect, per day, on average over the course of the year. So, for example, a 1000 watt system might look like this. 1000/2=500*4=2000 WH/day.

    Just as an aside. We live in a small off grid house. We have 400 watts of PV. On a routine basis we use between 500-800 WH per day. Most days we regenerate that power with out problem,, and with 450 ah of battery, we can go three days with no sun, and on the the third day, in full sun we can recapture nearly all three days worth in the real world it is actually better, since most days you seldom get zero out put. That said, after a number of partial grey days, the battery might go down 45 ah, then back up 30, then down 45, the down 30 for a few days. Then in that case,, it is time to ruthe genny for a few hours.

    Bottom line, is we rarely run the genny,, perhaps once a month or so for a few hours.

    Good luck, keep in touch,


    PS. I would steer you to read the following links. The bottom line with the batteries is to decide how m any days of autonomy you wish, then design th battery bank around this load. I personally like to draw my batteries down no more than 15-20% per day. Others permit bigger draws,, but as the following FAQs show, battery longevity (ccycles) is related directly to DoD average. One can make a calc between DoD and longevity and price. For example a smaller battery bank is cheaper up front. Drawing it down more, but shortening it's life may be good economy, or false economy. I downsized my current battery bank, even though I increased the loads. My calc was if the smaller battery lasted 5 years, and the bigger one 8, the smaller one would be cheaper per kwh. It has turned out to be true,, in that the smaller one is five years old,, and is in good shape still.
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Cabin solar upgrade

    if your intention is to not use the generator at all and go all solar then this is large and expensive. the >500ah per day draw would dictate at least double that for a battery bank and this would up the minimum charge to the batteries from pv as well. think about the >500ah this way. if you are getting full insolation for 5hrs (maybe you do get 7hrs max, but i wouldn't count on it) this would require >100a over 5hrs. now pv isn't rated in real world so dividing that by 77% reveals at least about 130a in 12v pvs. bein the battery bank shouldn't be drawn below half these figures are now doubled and you'd need at least 260a from 12v pvs. i hope i thought this out correctly, but even if i didn't you are getting the idea that this will be huge.

    many items do not need to be on all of the time and that is especially true of the satellite stuff. conservation and some compromise will be needed unless you intend to put in a full blown solar system no costs barred to cover the loads. in the off season the solar would be well oversized to maintain the batteries as long as you don't have a situation where the pvs will be buried in snow the whole season.

    you will want to be able to charge your batteries with the generator regardless if it is needed to be run daily due to limited solar charging or not so you should get a charger.

    ultimately you will need to make some decisions that we can't make for you. you must decide what will be drawn and for how long as well as what will be available from solar to put charge back with. the generator will be needed to possibly run some of those chores as well as charge the batteries when you go overboard with their use.
  • MinnowMinnow Registered Users Posts: 15
    Re: Cabin solar upgrade

    Thanks. There's clearly quite a lot to think about here.
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