Designing a system for a recreational building

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ferret
ferret Registered Users Posts: 2
Hi Everyone,

Reading through some of these threads, I realize I know even less than I thought I did. Ignorance was bliss while it lasted!

My husband and I plan to begin building what we are calling the Tiki Bar by our pond with sandy beach. It is a cement block square building we are working some energy efficient ideas into. Because of it's location on our property, running grid electric to it isn't the most feasible option. This building will function as a place for entertainment with the ability to be a place of refuge if the grid ever went down for an extended time. It will probably only be used a few days a week, primarily during the day for music and holding cold beer and possibly a few evenings for card games and a football game on TV here and there.

I'm so happy I found this site to ask all these questions BEFORE we were already locked into our design with construction, so here we go:

Our anticipated electric needs:
Ceiling fan (as needed)
smaller size fridge/freezer (beer, ice pops, other entertaining food needs)
LED ceiling lighting
TV/Dish - for husband's football watching with buddies
basic music speakers (daytime while enjoying beach)
...and while we are dreaming: nighttime landscape lighting.

My understanding is because our needs are not every day, that it will mean a larger battery bank for the nights when the usage is heavier, correct? The only thing which would have a regular pull would be the fridge and landscape lighting. From there, believe I just need to do the math to ensure the number of batteries don't drain below 50%?

I am looking at getting some 250w mono 24v panels. The current game plan is 3 or 4.
For the charge controller, I think I need a 30 amp?
Inverter: I want the ability to grow the system. What would the minimum be, and what would happen if I went with something in the 3000W range? Does having the larger inverter protect the system against surges?

Thank you so much in advance for your help - and your forgiveness in what I'm sure will be stupid.. er.. learning questions from myself.

Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,477 admin
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    Re: Designing a system for a recreational building

    First thing--Get a Kill-a-Watt type meter. And log your power usage (plug in for 1-7 days and get your peak watts and average kWH per day).

    Note that Watts is a "rate"... Like gallons per hour.

    Watt*Hours is an "amount"... Like gallons pumped.

    So when you say you need 3,000 Watts--That could be a 3,000 watt load (a lot of power) for X numbers of hours.

    Or that could be 3,000 Watt*Hours per day--Which would be enough to run a refrigerator, some lights, and a stereo.

    In any case, your "cost effective" solution would be to dig/trench a larger diameter conduit (or a couple) so you can pull AC wires and other communications cables (Internet, phone, cable tv, etc.). Even if it is a few hundred feet and under some walkways.

    Pure off grid solar power is expensive and difficult to justify for a limited amount of usage or as pure emergency backup power.

    And as soon as you add some larger items, such as a refrigerator, you are talking about a 3.3 kWH (3,000 Watt*Hour) off grid power system. This is a $5,000-$10,00 system (very roughly). And you will replace batteries every 3-7 years or so, and inverters/charge controllers every 10+ years.

    We are more than happy to help you design a system to meet your needs--But I want to make sure it is cost effective.

    You can make your beach area installation a real backup/emergency power shed--And again run some backup solar/generator AC power back to your home so you can keep it running during emergencies (some lighting, refrigerator, water pumping, etc.). You might find that a more practical solution (you can run some loads from your "free" solar power in your home).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • ferret
    ferret Registered Users Posts: 2
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    Re: Designing a system for a recreational building

    Thank you for your reply!
    The building and everything in it will be newly constructed/purchased, so there isn't a way to log power usage. I understand how to do the math on usage. I know the fridge will be the biggest draw. Minimum daily pull will be around 1000-1300 watts (mainly because of the fridge), with maximum pull being around 4000 on days we are using the building for a few hours of entertainment.
    I probably should have clarified more about the location. We have a 20+ acre property. It really isn't feasible to run wires from the house to the location on the pond where this building will be.
    We want the ability to grow the system if our needs increase, so we would rather go big first on items like the inverter than have to get a bigger one in another year.

    Right now, my energy use expectations are something like this on a typical week:
    Monday-Friday: Max 1500/day (fridge, landscape lighting)
    Saturday & Sunday - Max 4000/day (any combo of: fridge, landscape lighting, tv/dish, fan, speakers, interior lighting)

    I see right there that 4 250w 24v mono panels won't meet my daily needs... which are basically for the fridge. The good news is that this building will see more summer seasonal use, so I don't have to account as much for winter daylight loss. Are there any efficient refrigerator models out there that would help to drop my daily needs? Again, the fridge/freezer is really more for drinks and snacks. Technically we could turn it off during the week if needed...
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
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    Re: Designing a system for a recreational building

    Let's put the Citroen Mehari in reverse for a moment, listen to the ukeleles, and think again. :D

    If you have a target power need of 4kW hours (the hours is important) that is a lot of power. This isn't a small system anymore.

    Running lights and a fan is easy. Adding a TV it gets a bit bigger. Adding a refrigerator and you've just gone into household size.

    So the next question is: how much devastation will be incurred if the power goes off because there isn't enough? Tropical breeze, or typhoon?

    The 'frige will probably use a bit over 1kW hour all on its own. The TV & DVD player will pull close to 200 Watts (+/- you can make some adjustments) so if you watch a 2 hour movie you're near another half a kW hour there. Lights ... use LED's and they become nearly insignificant. A fan can pull 50 Watts so in 10 hours another half a kW hour.

    Looks like you could probably do it on 2kW hours per day. I run a whole household on a bit more than that, including Internet (via satellite) and water pumps.

    Once you have that power number you do not start with picking panels. Off-grid systems do not work that way. You start with picking a system Voltage and batteries. In this case I'd choose 24 Volts knowing that the loads will require about 100 Amp hours per day. That means a minimum 200 Amp hour bank, and it wouldn't hurt to try it with the cheapest batteries you can get; GC2's @ 220 Amp hours 6 Volts. If you have to up the capacity by doubling the bank you can do it sooner rather than later and all will be fine, at minimal expense.

    Now you have to charge them. Normally we'd pick on a 10% peak current rate, and that may still be the answer here although you could get away with a bit less as most of the power drain will be at night; no big loads during the day that need to be compensated for. On the other hand if you expand the battery bank you need to expand the panels as well.

    So 22 Amps * 24 Volts / 0.77 = 685 Watt array on an MPPT charge controller. Four 250 Watt panels would more than cover that, in fact. Should yield about 32 Amps.

    Aye, there's the rub. Three of those panels would cover the need and run on a 30 Amp MidNite Kid or Rogue 3048 just fine (750 Watts yielding 24 Amps). But if you need to expand ... you need batteries, panels, and a new controller. So you have to think about that question of "what happens if we run out of power?" very carefully.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,477 admin
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    Re: Designing a system for a recreational building
    ferret wrote: »
    Thank you for your reply!
    The building and everything in it will be newly constructed/purchased, so there isn't a way to log power usage. I understand how to do the math on usage. I know the fridge will be the biggest draw. Minimum daily pull will be around 1000-1300 watts (mainly because of the fridge), with maximum pull being around 4000 on days we are using the building for a few hours of entertainment.

    How much money do you want to spend--A crass question--But it does really help us to focus where to best put your limited resources. $5,000-$10,000 for such a system (as described) would not be out of the question. I can bury a lot of wire and or buy a $1,000 Honda eu2000i and run it on ~1-2 gallons of fuel per day when the power is needed.
    I probably should have clarified more about the location. We have a 20+ acre property. It really isn't feasible to run wires from the house to the location on the pond where this building will be.

    It is OK--We do listen, but most of us here are terribly "cheap" and don't like to waste other people's money either.
    We want the ability to grow the system if our needs increase, so we would rather go big first on items like the inverter than have to get a bigger one in another year.

    For many different reasons, it is actually quite difficult/expensive to "grow" an off grid solar power system. It is like buying a small car, adding a larger gas tank and engine, then bigger brakes, a trailer hitch, etc. and turning it eventually into a semi-tractor/trailer.

    You can "adjust" a system size (add to it) by about a factor of 2x -- But beyond that, many times you have to rip out much of the existing equipment/wiring and start over.
    Right now, my energy use expectations are something like this on a typical week:
    Monday-Friday: Max 1500/day (fridge, landscape lighting)
    Saturday & Sunday - Max 4000/day (any combo of: fridge, landscape lighting, tv/dish, fan, speakers, interior lighting)

    That is pretty much the power needed to run a full off grid (very energy efficient) home (using propane, etc. for heating/cooking/etc.).
    I see right there that 4 250w 24v mono panels won't meet my daily needs... which are basically for the fridge. The good news is that this building will see more summer seasonal use, so I don't have to account as much for winter daylight loss. Are there any efficient refrigerator models out there that would help to drop my daily needs? Again, the fridge/freezer is really more for drinks and snacks. Technically we could turn it off during the week if needed...

    Refrigerators are typically the largest single load most people will drop on a "smaller" off grid power system. And they are highly variable in energy usage. In summer, running in a 90F environment, they use much more power. Making ice, adding more food/drink, opening the door, etc. all adds to the loads. Letting stuff sit around "warm", then turn on for the weekend--Refrigerator uses ~2x its rated power for the next 24 hours to cool everything down.

    Engineering design is a "cruel" profession. All these "nice things" you want are scuttled by the engineer's derating everything for reliable/safe operation in good times and bad (sunny/dark/hot/cold/kids playing around system/etc.).

    Batteries are just about as cruel... The demand respect and proper charging/discharging. You under charge, they die. You over charge, they die. You forget to add water for a couple months, they die. You use well water to fill batteries, the die. You have lose/dirty connections, they die. A battery "dies" for randome reasons, and it kills nearby/connected batteries.

    Get the theme? :D

    And off grid system will require a certain amount of daily/weekly/monthly maintenance. The battery banks are pretty unforgiving. A genset--You change the oil every 50-100 hours (for small genset, no oil filter), and provide fresh fuel (and/or add fuel stabilizer) and away you go.

    Just to give you a "nominal" system design... You could possible get away with a 1/2 size array and battery bank (a bit more maintenance/monitoring, a bit less capable and less money up front--Your choice). A quick design based on "nominal" design (at least give you the information to price out and size installation).
    • 4,000 WH per day * 1/0.85 inverter eff * 1/24 volt battery bank * 2 days storage * 1/0.50 max battery discharge = 784 AH @ 24 volt battery bank
    Just to give you an idea, that is ~24 x 6 volt @ 220 AH "golf cart" batteries (not that I would tell you to use GC batteries--This is a pretty good size battery bank). You are right on the cusp of needing a 48 volt battery bank. I.e., if you want a larger system in the future, you should pick 48 volts right now so you can keep the AC inverter, DC backup charger, get the correct solar charge controllers, etc.

    Then sizing the array based on the capacity of the battery bank 5% to 13% rate of charge--10% rate of charge makes for a pretty reliable/easier to manage system (big battery banks need big solar arrays even if the loads are light):
    • 784 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 2,952 Watt array nominal
    And we need to double check against the amount of sun you get for your area... Using PV Watts for Athens Georgia, fixed array, tilted to 34 degrees from horizontal:
    Month    Solar Radiation (kWh/m 2/day)
    1      3.69     
    2      4.77     
    3      5.40     
    4      5.82     
    5      5.70     
    6      5.69     
    7      5.69     
    8      5.61     
    9      5.31     
    10      5.51     
    11      4.34     
    12      3.80     
    Year      5.11
    

    Looks like 4 hours of sun will cover you for most of the year (looks like summers are hazy around Athens?).
    • 4,000 WH per day * 1/0.52 overall system eff * 1/4 hours of sun per day = 1,923 Watt array
    So a 1,923 to 2,952 Watt array would be "justified" for this system.

    For the charge controller--If you choose a MPPT type controller (more expensive, but recommended for larger systems):
    • 2,952 Watts array * 0.77 panel+controller derating * 1/29 volts charging = 78 Amps
    That would "fit" one ~80 amp MPPT charge controller (~$600).

    Need an AC inverter--Probably a minimum of 2,000 Watt. Too large of AC inverter can waste power. Besides the refrigerator (which needs around a 1,200-1,500 watt minimum AC inverter for a modern Energy Star full size refrigerator), you will need to add up the power for the lights/stereo/etc. A 4,000 Watt inverter would be on the large size for this system (and battery bank).

    An off grid system should have a means of backup power for charging during bad weather--A genset would be in the range of:
    • 784 AH * 0.13 rate of charge * 29 volts charging * 1/0.80 charger eff * 1/0.67 charger Power factor * 1/0.80 genset derating = 6,893 VA rated genset
    That is not a small generator and will need significant amounts of fuel to run it (call it ~1-2 gallons of gasoline per hour).

    All the above is not trying to scare you off--But showing what a true "nominal design" off grid power system would look like. I could see cutting the battery bank/generator by 1/2 -- And you could go with the smaller array. Batteries will last ~3-7 years typically (if used or not).

    We try to make the system support your needs. If a system does not (misses by "this much")--Then it fails. Of course, your system is (in general) not a critical use system--But if it dies at 9pm on a beautiful summer evening--What good was it?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • bill von novak
    bill von novak Solar Expert Posts: 891 ✭✭✭✭
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    Re: Designing a system for a recreational building
    ferret wrote: »
    We have a 20+ acre property. It really isn't feasible to run wires from the house to the location on the pond where this building will be.

    I am wondering why this is. Your property, if square, is about 1000 feet on a side. A 1400 foot trench (i.e. if your house is on one corner and your tiki hut is in the other opposite corner) with 2" conduit and 4 runs of 10ga wire is going to run you around $5000, and you will have far less headache than with an RE system. And I suspect the run would be shorter than that since it won't be worst case.

    Then once you have that in you have some much easier options. Your four wires in the conduit are going to be grid AC, backup AC, neutral and ground. Put an RE system with battery in the tiki hut and set AC in limit to say 10 amps. That will limit the power the system draws to 10 amps and make up the difference with battery power. When load drops batteries will be recharged. Since the batteries won't be cycled much they will last far longer.

    Then when power does go out you have a bit of power available from the final conductor for basic backup in your house (lights, fans, radio, sump pump etc)

    Better yet put a larger system on your house and just run the power out to the tiki hut. Much simpler overall, and the tiki hut can still use an inverter/battery to reduce voltage sag and help with starting refrig compressors etc if you want to.

    Not trying to talk you out of the RE system, just trying to understand your situation.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
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    Re: Designing a system for a recreational building

    A couple of other ideas:

    Get a cheap generator and a Kill-A-Watt. Set up your Tiki bar and run it from the gen through the meter for a while. That way you'd have a precise power usage number and be better able to determine what is the most economical route to go.

    Which would be either run the line for grid power, or install a full solar system, or install a small solar system and use the gen if/when needed.

    I have to say this project has caught my interest. I'd like to convert my end room into a similar 'vacation spot' and right now it is -30C with 45cm of hard packed snow on the ground outside so anything of a tropical nature seems appealing. :D