want to build a small system

Hi, i'm from aruba its a small island in the caribbean close to south america(venezuela)
what i want to adchieve is a small system that can give me 12 hours of of airconditioning (800w 220volt 16amp)
In aruba its whole year long summer climate lots of son barely any rain so i quess its about 6-7 sunshine peak hours and about 11 hours sunlight
the problem i'm having is i dont know were to start
i tried calculating and came up with this
10.8KW daily so i need 5 1000 ah deepcycle batters with 1500 inverter but couldnt get to figure out what solar panels i would need i know im wrong in my calculation but suppose its right i'm still lost how to calculate solar panel's i need for this system
I would like a full explanation how what and howmuchs stuff i need for this system pls help me
Thanks in advance


  • BB.
    BB. Super Moderators, Administrators Posts: 33,476 admin
    Re: want to build a small system


    I am not an expert here, but I will take a stab at some of your questions (these are just suggestions, please ask more questions and research what is available in your area).

    The first thing to find is how much power you use per day--Remembering that your power usage may vary by month, winter, summer, etc... (except Aruba where it is probably beautiful the year round). These numbers need to be fairly accurate. Miss-understanding can dramatically affect the size of the system you will need. Also, conservation is the first place you should start before contracting to install a solar PV panel system. It will almost always be cheaper to save a kWhr than to try and generate a kWhr. Example below using the numbers you supplied there is a 4x difference in power consumed per day.

    New energy efficient refrigerator, airconditioner, fluorescence lights are probably the first place to look at conservation.

    800w * 24h/day = 19.2 kWhr/day
    220vac * 16amps * 24h/day= 84,480 Whr/day=84.5kWhr/day

    Next, are you grid tied or not (will you have AC power from a utility, or will you need batteries)...

    If you will be connecting to the utility grid (they act as a very big battery)--you will need to talk with your power supplier--in the US, it took major laws to force the utilities to allow "net metering" (where you generate somepower and consume some power and at the end of the month (or year) you either pay the utility for any net power you used, or if you are positive, you don't owe any money--except for a minimum connection fee). Grid Tied is usually the cheapest and best solution, assuming that you have reasonable utility power available.

    Batteries: You are your own utility system and proably have a lot of experience with a generator system. Generally, most people plan for the batteries to supply power for three days of no sun--you can do more or less, but there are some issues (fewer batteries will probably require you to run an generator more during poor weather and the batteries may not last as long because you are charging/discharging much more often and much deeper--More batteries are expensive and much more maintenance and the huge amount of current from a large rechargeable battery bank is huge--shorts are very dangerous). Also, plan on only discharging your batteries to 50% capacity--discharging more will damage the long term life of your batteries. Plan on replacing your batteries every 7-15 years (depending on battery quality, usage, and maintenance).

    19.2kWhr/day * 3 days * 1/50% = 115.2 kWhrs of storage (about 115 good sized car batteries)

    Generator: If you don't have utility power and need electricity 24 hours per day, 7 days a week; you will need a generator. Generally, a generator is most efficient near full load--so having it charge your battery bank on dark days is a good use for it. You should be able to run it much less and save fuel too (with or without solar panels).

    Next, you will have to decide how much you wish to spend for solar panels... They are not cheap right now (world demand is up)... Assuming $6 per watt (US price is probably a bit less), you are looking at $6,000 USD per peak kWatts of production... Now, you are in a very sunny area so you probably will get better power than I--plus you will have less deviation between winter/summer than I (and you may even be a bit cooler). For now, without any further information on Aruba, lets assume that you get, on average the same as a good Solar Power Day for me.

    My 3.5 kW peak panel system will generate roughly 19.2 kWhr per day (nice warm sunny month of May day, grid tied).
    3.5kW * $6,000/kW = $21,000 USD worth of panels. What would the price be in Aruba??? I don't know.

    Here are some US maps of solar power radiation... Puerto Rico is probably the closest to you so you can use its information to approximate how it would perform in your area, month by month with several collector options:

    http://rredc.nrel.gov/solar/pubs/redbook/ (home page)
    http://rredc.nrel.gov/solar/pubs/redbook/PDFs/PR.PDF (Puerto Rico)

    Remember there are losses too... For a battery / off-grid system, you will probably need to assume you will need, at least, a 25% larger solar array to make up for charging loses of 20%:

    19.2 kWhrs * 1/0.80 = 24 kWhrs per day (and therefore, 25% more panels)...

    Lastly, you will need electronics. There are many vendors around the world that can help supply you with almost any configuration you can need. If you are 240 VAC 50Hz, the US vendors have a bit less available, but you can probably find it somewhere. The types of systems include:

    Grid Tie: Solar panels connect to inverter then connect to your fuse/breaker panel. Simple and cheapest (a 3 kW inverter is probably around $1,500 to $2,000 USD). Many options and sizes available for your needs. Will NOT be able to supply AC power if you utility lines fail. You will still need a generator or some other system for lights when the power fails.

    Off Grid: This is where you generate all of your own electricity. You will need a charger (solar is one type, wind another). And you will need an inverter (battery to AC). You will probably want a "Sine wave" type invert. Modified Sine wave (really modified square wave) inverters work OK for some applications, but can cause motors (like refrigerators, AC, Pumps) to overheat. Many off-grid (if not all) have options to attach a generator to supply power when the batteries are low.

    Grid Tied with Solar/Battery/generator Backup: This tries to give you the best of both types of systems. The ability of Grid Tie to act like a battery and reduce your power bills, and the ability of an off-grid system to supply you with power while your utility lines are failed. Probably the most expensive to install and the fewest vendors/models to choose from... May be hard to find in 50 Hz.

    Once you have chosen the type of grid/off-grid system you will need... If off-grid you will need to choose the size of inverter to handle your peak loads (grid tie systems do not need this--your utility handles all peak loads). You will need to add up all of the loads that you will have on at one time--and you must account for starting current too:

    Refridge+Freezer+lights+TV+well pump=peak power
    400 watts + 300 watts + 200 watts + 160 watts + 1,000 watts * 3 = 4,060 watts peak (ave load is 2 kWatts)

    Anyway, I am sure I have left a lot out, and I am certainly not the expert here--but these were the issues that I looked at when I installed a solar PV panel system (grid tied) at my home. The few prices I gave you are just to help you understand approximate size of the costs for a solar PV panel system. I do not buy or sell the equipment so I have only an approximate idea of the costs involved. But, it did help me with planning/justifying my system to my wife.

    Many of the issues you will run into will be specific to your location... Costs of materials, building permits, permits from the utility company to connect grid tied systems, how hot do your panels get (hot panels can generate 20% less power than cold panels--pole mounted panels are cooler than roof mounted panels), damage from wind, mounting your panels with at least a 5 degree tilt to allow rain/dirt to run off, theft of panels/equipment, etc.

    Solar power can be a great help in controlling your power bills... But it is not cheap. In the US, utility power is still, generally, less expensive than utility power (unless there are no power lines nearby). For me, where I saved the most money was in conservation--not from installing solar. For my system, it costs me approximately $0.17-$0.20 per kWhr but I pay the power company $0.11 per kWhr. My bet is that power cost will continue to rise and my power costs are now fixed for the next 20-30 years (right now a flat $6.00 per month electrical connection charge/minimum rate).

    Also, look at your entire energy budget... For example, if you use fuel for hot water and/or other heating, you may actually save more money by installing a solar hot water system first... The panels are much more efficient and less expensive--although, I do worry about maintenance with all of the hot water pumps, storage tanks, etc.

    Good luck!
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2
    System2 Posts: 6,290 admin
    Re: want to build a small system

    Hi, thanks for your reply

    To give you a better view on my situation I will explain
    I'm curently conected to the grid, electricity in aruba is very expensive it's about $0.50 per KW thats 0.90 our curency which is florins (fls.)
    I get bills of $1000 avg per month thats more and a minumun salary wage here
    I have contacted my utilty company and they will take off my electricity if i put in solar cus they like to have monopoly cus there is only one of them here.
    Personally im willing to spend money to gives my whole house electricty off grid so I know I will need batteries I understand that part.
    but I dont want to rush into things and fail. Thats why I want to test out with solar first my choice of test is having an ac unit run for 12 hours daily and Here comes the problem.
    I don't know how to calculate how many panels and there wattage I need and batteries.
    One thing I know for sure is that my Ac unit is 220volt and 800w.

    If you can help me with calculation I would appriciate it very much.
    Thank you

    PS there is nothing about solar electricity here in aruba so I will have to import everything.

  • crewzer
    crewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    Re: want to build a small system

    Here's a link to an previous discussion about solar energy and small A/C's. This might give you an idea of what's required to specify and design a system.

    Jim / crewzer

  • BB.
    BB. Super Moderators, Administrators Posts: 33,476 admin
    Re: want to build a small system


    I feel very sorry for you... at $0.50 a kWhr, I can just go by a Honda generator and $3.00 a gallon (yes, I know that it cheap for you) gas and generate my own... For less than 300 kWhr/month we pay about $0.114 per kWatt Hour

    Assuming that your power bill is $1,000 USD / Month and $0.50 per watt:

    $1,000 / $0.50pwKh = 2,000 Kwatts per month

    By the way, for more than ~1,000 kWhrs/month we would pay $0.35 per kWhr (basic residential rate schedule)

    2,000 kW/M divided by 30 days / month = 66.7 kWhr per day for your total load.

    800 watts * 12 hours/day = 9,600 Watt*hours per 12 hours user per day = 9.6 kWhrs (per day).

    Remember, the actual amount of power used by the AC unit may be less than 800 watts per hour--but you would need to purchase a meter like a kill-a-watt meter here (this is for 110 vac 60 Hz--you will need one for 220 vac 50 Hz--I would guess).


    Looking at your total power usage (assuming I correctly understand everything you have typed correctly), you are consuming a lot of power. Purchasing a meter like the Kill-A-Watt above (or, for years I even used an old Utility electric meter that wired a plug and outlet to it... then I just plugged my major appliances into it for 1 week each to get an idea of their average power usage).

    This is not a criticism, just an observation, your home is using quite a bit of power for the average US home in a temperate climate (assuming your temperature averages are around 32C high and 25C low). The one AC unit accounts for about 15% of your energy usage... You have other AC units or other large loads?

    To run this one AC system, using really rough estimates would probably indicate:

    (9.6 kWhr/day) / (4.25 kWhr/1 pk kw) * (1/0.80 eff) = 2.8 kW peak (solar panel size)...

    Please be aware that the above is just a starting SWAG (scientific wild a$$ guess) based on a good production day for my system and 5/6 derating factor for SF in May vs a rough average for Puerto Rico) (that is the 4.25 kWhr per 1 peak kW panel rating). A battery based system of that size will, roughly, be able to power your AC for 12 hours per day of good weather.

    Since you have AC power available (assuming that the Utility doesn't cut you off because you have solar panels in the back yard), it probably is not worth making the system any larger since you can just charge from the 220 vac mains in bad weather. And you can probably make the system 1/2 size (~1.5 kW peak) and still see how it performs for your area (you will still need 220 vac power to recharge your batteries some) and you can get actual power readings over several months or 1 year and then plan your full system with the exact numbers required.

    Battery wise, you can run with a minimum sized 1 day supply of ~10kWhr * (1/50% discharge) or 20 kWhr of batteries--again, don't spend to much money until you see it run for a while (another warning, generally, you will want to replace all batteries at the same time and not mix old and new batteries on the same string as the old batteries and new batteries will not charge/discharge the same--with the new batteries quickly degrading to the same performance/life as the existing batteries).

    For batteries, you probably can find them locally (electric forklift company or marine supply might be good local places to look for your "training batteries"--you could even try to find reconditioned ones too).

    The size of inverter you will need is not that large (perhaps 2-3 kWatt with a good surge rating). To run your Air Conditioner, you will need probably a "True" sine wave type inverter which are several times the expense of a modified square wave (modified sine wave) inverter.

    Others here can give you a much better idea of the exact equipment you will need to purchase... I quick look on the web says that you have 60Hz -- so purchasing from a US manufacturer / supplier may save some money for you (depending on your import taxes/fees/punishment).

    Because I purchased Xantrex Grid Tie, I know a little about its off-grid products. Not a recommendation--just a place to start with a, relatively simple yet complete Off-Grid system that also allows you to tie in to the Grid for charging or servicing (has an internal transfer switch between the inverter and the 220 vac mains). Also has a generator control option.


    Just a quick search on the web shows it to be around $2,500 USD.

    The only problem with this unit is that it is a 120VAC output... You would either need to purchase a 110-220 transformer (should not be too expensive) or a 120 vac air conditioner.

    Other vendors (and other models) should offer you the 220 VAC output if that is what you need.

    Again though, looking at your total electricity usage, going on a conservation kick would be my first suggestion. If air conditioning is a major source of your electricity bill, then insulation, central air conditioning, and/or looking a "ground source heat pump":


    would be where I would start. If you have water (pool, stream) or land where can drill/bury pipe, the ground source heat pumps can use something like 1/2 of the energy of a typical air conditioning system...

    Arookto, I will stop here for a moment. Please ask questions as I am not sure exactly what your knowledge is and where you will want to go from here.

    As a suggestion, if you have an engineering school nearby (or government sponsored environmental department), you may be able to contact them and work out a mutually beneficial agreement (engineering student gets degree, school gets an article in the paper, you write check and get free labor and maybe even some donations).

    Take care,

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: want to build a small system

    is aruba 50hz or 60hz at 220vac? this would one determining factor in choosing your inverter. with bills as high as yours you would definitely benefit by going solar. how many kilowatthours do you use in a month is another factor needed to be known. your payback period will definitely be shorter comparitively to ours.
  • System2
    System2 Posts: 6,290 admin
    Re: want to build a small system

    Thanks to all for the sugestions it's helping me get better knowledge of solar which i lack
    Now to the question before my last month kw usage was 2761 Kwh thats what the bill say
    To give u better view at my usage i will tell what kind of stuff i have hooked up
    here we have 50-60 hz 120 and 220vac both 220vac only uses for ac units
    i have 4 ac units 2 is working everyday about 12 hours each
    pool with electrick pump
    boiler for water like to shower with warm water hehe
    dryer and wash machine both electric
    a 60" tv 3 more 20" tv's
    2 direct tv units
    those are the heavy loads i think others are lights and no watage apliances

    hope this info might help you
    thanks for all the help
  • BB.
    BB. Super Moderators, Administrators Posts: 33,476 admin
    Re: want to build a small system

    In terms of conservation:

    Look at heavy loads that run multiple hours per day... Measuring their load (as opposed to just looking at the name plate rating) and think about what you would want to do to save energy... These are suggestions--you may have already done these or, at least have a reason for not doing them):

    Don't run the pump 24 hours per day (look up pool care sites and see what they say is appropriate).
    Replace pool motor with two speed pump and run mostly on low speed.
    AC: Look at your current AC units (grills clean, no leaves/dust). Filters clean.
    AC: Look at new units (don't know what is in Aruba--but in US units older than 10 years should probably be replaced)
    Fridge/Freezer: Units older than 10 years, may want to replace.
    Are you using electricity for hot water Boiler and (heaven forbit) heating the pool? If so, solar water heaters would be a big savings.

    Lights and other low wattage appliances are important--but probably only will affect your bill once you are below 1,000 kWhr / month (and really, probably when you are in the 300 kWhr per month area). Big yard lights (Halogen, spot, area lights, security, fancy home lighting) can eat up a lot of power too (especially if they are on all night). Timers and replacement with energy efficient lights will help.

    If you have bright/hot lights in air conditioned spaces (again 300 watt, 500 watt floods/spots, or lots of 75 watt bulbs, etc.), changing those out to to fluorescence lights (and other types) will save twice, once the energy to run them, second, the energy to cool the room where they are at... Same thing with large refridgerat or in air conditioned space... New ones use less energy and will save AC too.

    Anyway, I have to go right now. Type at you later!

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset