Solar design help needed for a newbie

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System
System Posts: 2,511 admin
in Solar Beginners Corner #1
Greetings,

As I research solar, my head spins with the knowledge out there. I think I have a good concept of my power needs but realistically I do not have a clue where to start. I have looked at basic systems out there but I'm thinking that I should invest in high storage batteries based on my circumstance rather than fill the roof with panels. I just want to make a smart long term decision and I am hoping that this forum can help me work through the details of my design.

Problem set: I have an RV in an RV park that has 30 amp power to it. We have built an awning enclosure (with CA State permit) that is 720 sq ft and has a pitched roof with great sun exposure. The RV opens to this room, think of it combined as a small house. The State has dictated that we must use alternative power to have electricity wired in this awning enclosure and I need to develop a design that I can submit to the State for a permit. We use this RV most weekends of the summer. Typically arrive Friday night and leave Sunday night so 48 hours total of use each week thus leaving 5 days of sun capturing without usage.

Goal: I would like to develop a solar package that will power (4) 52inch ceiling fans for the entire time that we are there. In addition we wish to use a whole house fan when first arriving for apprx 15 minutes to equalize the inside and outside temperatures. We would probably do this each night as well when the temps (hopefully) are lower.

My watt usage estimate: 9479 watts for the weekend.
ceiling fans at medium=48watts 4fans x 48watts x 48hrs = 9216watts
WHF = 350watts = 350 x .75 = 263watts

Is my estimate valid? where do I start in developing a design that meets this need?

Thanks much,
Linda
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
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    Re: Solar design help needed for a newbie

    Welcome to the forum.

    First off, it's important to get the nomenclature right.
    Watts are what things use. Over time they become Watt hours. This is what you need to supply.

    So a 48 Watt fan used for 24 hours = 1152 Watt hours.
    Times four fans = 4608 Watt hours.
    The whole house fan @ 350 Watts for 15 minutes is 87.5 Watt hours (350 * 0.25 hours).
    All totaled: 4695.5 Watt hours per day. Call it 5 kW hours, because you never really know with motors. They have higher surge demands on start up, and will use +/- power depending on other conditions.

    You want to use a daily figure because you should aim for recharging daily.

    The good news is, if this is all you need to run the inverter doesn't have to be very big as the total is 550 Watts. A 1 kW inverter would be plenty. The bad news is that 5 kW hours is a lot of total power, so you should plan on a 24 Volt system to keep the battery bank "manageable". You are looking at at least 416 Amp hours (5000 Watt hours / 24 Volts = 208 Amp hours, times 2 for maximum 50% DOD allowance). The standard "golf cart" batteries @ 225 Amp hours could be used here: eight of them would give 450 Amp hours @ 24 Volts.

    To recharge such a bank you'd try for current of 10% of the Amp hour capacity: 45 Amps. To get that you'd need a fair amount of solar panels: 45 Amps @ 28.4 Volts charging = 1278 Watts less "typical" 77% derating = 1660 Watt array. That's no small system.

    You may think it strange the way I've upped some things. But trust me; loads are nearly always bigger than you calculate and they will grow no matter how conservative you expect to be.

    Is this any help?
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  • System2
    System2 Posts: 6,290 admin
    #3
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    Re: Solar design help needed for a newbie

    Thank you, this is a great start and I very much appreciaate the help with my nomenclature and my solar education. I will digest your words more fully this evening but one question before I need to leave for a meeting. Since I have 5 days of sunshine with no drainage, would it be more cost effective to have more batteries?
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #4
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    Re: Solar design help needed for a newbie

    No.
    Back when batteries were cheap and solar panels were expensive the plan was to use as little panel as possible and charge the batteries "slowly over time" (e.g. the days of the week for weekend-use cabin). This resulted in short battery life.

    Now that panel prices have come down and battery prices have gone up, it is much better to recharge those batteries as quickly as possible after they've been depleted. This results in longer over-all battery life.

    Likewise you may have heard of planning a battery bank for 'X' days without sun (more correctly; without fully recharging). This tends to result in an over-sized bank and a need for a lot of excess panel which is not fully made use of during normal operation.

    One plan which may work for your situation is a dual bank, where you can charge half at a time. This requires a bit of extra wiring and a somewhat expensive charge controller, but could save money on panels. Fully-automated function of this type tends to be very technical.

    Keep in mind I've only made some generalized suggestions. When you get down to specifics, you will have to compensate for your particular installation's shortcomings. Like panel temperatures, wire runs, and insolation.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,447 admin
    #5
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    Re: Solar design help needed for a newbie

    Before we go too far... What are your needs?

    You have utility power and want to:
    • save money?
    • go green?
    • emergency backup power?
    • go off-grid to be green?
    • go off-grid because grid is going away?
    • State of California has some sort of Renewable Energy requirements to allow you to stay?
    • Do you need a backup genset?
    • Will this always be a weekend home.
    What you are talking about (3+ kWH per day?) is not a small system... Before you spend any money--lets talk and make sure you are spending it wisely and it will meet your needs.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #6
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    Re: Solar design help needed for a newbie

    Bill;

    Read the first post. :p
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  • solar_dave
    solar_dave Solar Expert Posts: 2,397 ✭✭✭✭
    #7
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    Re: Solar design help needed for a newbie

    Most likely a grid tie setup will not work because of the service entrance sizing, I think that a battery solution is in the cards to power your adds, do you need to run it at night? You could be harsh on the batteries and run them a bit overboard on the discharge side with 5 days to recover them, Sacrilegious I know guys. That will shorten your battery life but lower you casts as well.

    Perhaps a smaller grid tie to run the meter backward for 5 days and you draw the grid excess would work.
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