Is my math right? Or is my load greater than my input?

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Steven Lake
Steven Lake Solar Expert Posts: 402 ✭✭
in Solar Beginners Corner #1
Hi all! I'm back! (*everyone runs for cover* T_T lol)

Ok, I'm not sure how many of you remember my last question about a PV system for camping, but I've now finally had a chance to start working on this (was waiting for the camping trailer to come out of winter storage) and it seems that I've overestimated the room available for me to work with. Actually, I'm about 2x's over the available space. So I've downsized a bit and need to know if the math will work on this now. Here's what I'm looking at for parts.

135w Kyocera solar panel (x1)
Misc parts (controller, wiring, etc)
Sundanzer 50liter fridge and freezer (1 each)
Trojan T105-RE 6 Volt 225 AH Deep Cycle Battery (x2 in order to get 12v)

Now, assuming 50% sun (in Michigan you're more likely to get 50% days than 100%), is what I've cooked up going to work, and will the solar cell and batteries provide enough power for the fridge and freezer? Ambient temperature where we'll be at is about 70f average across the day. (high ~80, low ~60) And if so, how much "breathing room" do I have in my amp hours? IE, if we were to run a couple laptops, and maybe a small TV, or recharge some power tools off an inverter for, oh say about 3 hours a day (I'm thinking that an 800watt inverter should be plenty for this application, maybe even as small as 500w) would I have enough capacity to do that and still run the fridge/freezer, or am I pretty much tapped out with just the two appliances? And if I have spare capacity, how much?

My math says that I'd have enough for up to 5 days with no sun, break even at 50% sun, and a surplus of 75 amp hours a day at 100% sun (x8hrs/day). I don't claim to be very good at math, so I just want to double check my figures against more experienced users and their results. Any input you can give is welcome as always, as you guys have been a huge help to me at ironing out the last details of this system before we go ahead with building and installing it. (or at the very least ordering the required parts)
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
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    Re: Is my math right? Or is my load greater than my input?

    Just my opinion but one 135 Watt panel isn't going to recharge a 225 Amp hour battery bank. The charge rate might peak around 3%. In terms of harvest it might gather up to 500 Watt hours per day. The 'frige says 114 Watt hours per day, the freezer 280. That's 394 total, so it might just do it. If you could shrink the battery bank down it might be okay: you only need about 65 Amp hours. Perhaps something around 100 Amp hours. Might be worthwhile getting an AGM since this will be portable/transportable.
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  • Steven Lake
    Steven Lake Solar Expert Posts: 402 ✭✭
    #3
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    Re: Is my math right? Or is my load greater than my input?

    So one panel wouldn't really provide enough capacity for this design? As for the batteries, as I mentioned before, the design is *supposed* to give me a comfortable margin for the two appliances *plus* some breathing room for adding an inverter that would get used a couple hours a day. That's kinda what I'm shooting for. It's good to know that I can at least maintain the basics on one panel. But if I'm hearing you right, I'd need a second to have the "breathing room" I want for the inverter.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #4
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    Re: Is my math right? Or is my load greater than my input?

    The way I see it the one panel looks like this:

    135 Watts @ 77% (typical efficiency) = 103.95 Watts.
    103.95 / 14.2 Volts charging = 7.3 Amps, 5% (recommended minimum) of 146 Amp hours.
    103.95 Watts * 5 hours "equivalent good sun" (max) = 519.75 Watt hours per day.

    I don't see it working with 200+ Amp hours or leaving any margin for even small inverter use.
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  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    #5
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    Re: Is my math right? Or is my load greater than my input?

    My quick napkin rule of thumb

    Take the name plate rating of the PV, divide that by two to account for all cumulative system loses, and then multiply that by four to represent the average number of hours one might reasonably expect to get per day, on average over the course of the year. Season to taste, remember your mileage will vary!

    In an RV application the number actually gets a bit worse since the RV is seldom if ever perfectly aligned to take best advantage of the sun. Additionally, most people prefer to keep the RV cool by parking in the shade, defeating any PV system that is fixed mounted.

    Done anywhere near correctly, all calculations originate from the load calcs. If you don't properly ID the loads, and their duration, the rest is only guess work.

    So, a 100 watt PV might produce this: 100/2=50*4=200 watt/hours /dayof power out the inverter. Enough to run a 100 watt TV for 2 hours. Or 2 ten watt CFL bulbs for 10 hours,, and so on.

    As an FYI, we live, off grid with 400 watts of PV, 450 ah of batteries. We routinely draw 5-800 wh/day. On a perfect winter day, we can get upwards of ~1/5 kwh, and indeed I have actually gotten ~2.2kwh, but on average,, we put the same 5-800 wh back in that we take out, plus a bit. Our batteries are big enough for ~ 3 days of no sun if we are willing to draw them down to ~ 60% SoC.

    Tony
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  • Steven Lake
    Steven Lake Solar Expert Posts: 402 ✭✭
    #6
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    Re: Is my math right? Or is my load greater than my input?

    Hmm, ok. So in other words, to have any breathing room at all in this design, I'm gonna need to add at least one other panel, possibly two. Well, that gives me something to work with. Now I just need to see if I've got the room to add the extra panel(s). ^_^;;
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