Off-grid system questions

Hello all, first of all i appreciate any input you can give on this topic. I have been researching solar power/wind power systems. I am currently in the planning stage of building a house in san felipe mexico. I am looking at doing a completely self contained off-grid system for the house. I want the system to handle all the normal appliances i am accustomed to in my phx house plus a couple more. A/C, pool filter, water heater, 110v RO system, stove, refrig, freezer, lights etc. Like i said i am still in the planning stage of the house but i think i am looking at a 400amp panel. So with that what is the best approach to laying out and planning the system. I have lots of roof space availible as it is a flat roof. the area gets 360 days of sun a year with a desert climate. What watt panels would be needed, how many, what type of inverters, batteries etc. would it be better to go with more panels such as 130 watt or less of the 200 watt. thank you for the info.

Jake

Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,433 admin
    Re: Off-grid system questions

    First question for you Jake,

    How much electricity do you expect to use per month? And, how much are you willing to spend. I am going to make some basic assumptions just to get the discussion started--and please don't take anything I type as bad intentions on my part. Just trying to get the exchange of information primed.

    I am going to try and do this backwards, talk about really rough costs first, then give the talk about conservation...

    Basically, it sounds like you could be using on the order of 1,000 kWhrs per month (possibly even 2,000 kWhrs per month). So, lets price a 1,000 kW/month system with 3 day battery backup. The numbers below are SWAGs (guesses) but are close enough for a first pass estimate.

    Just as a rough approximation, assuming you will get around 5kW pk system / 1,000 kWhr/month. Since you are off grid, assume 80% efficiency for battery charging and another 1.17 times this for panel rating (hot panels in real world don't work as well as standard test conditions) . So, 6.25 kW peak power system with 7.3 kWatt worth of panels per 1,000 kWhrs per month.

    So, assuming $10 / watt installed (US pricing, don't know Mexico's tariffs), call it $62,500 per 1,000 kWatt hours per month. You can probably knock 1/3 off if you install the panels and inverters yourself.

    Batteries, usually, try for 3 days of bad weather loading, and never drain batteries more than 50%...

    1,000 kWhrs/Month * 3 days/30d/month * 2 for battery capacity = 200 kWhrs of batteries storage

    Let us just pick a nice large 2 cell, very good quality battery from here:

    http://store.solar-electric.com/su413am4vo.html Surrette 4-KS-25PS 1350 Amp-Hour, 4 Volt $1,036 list price

    200,000 / (1,350*4) = ~37 batteries * $1,036 = $38,332 list for storage batteries (assume 25% discount if you ask). Weight of batteries wet is 315 lbs each or approximately 5.8 tons of batteries. If you wish, you can use 1/3 the number of batteries for 1 day of storage--would need a generator more, and batteries probably won't last quite as long.

    Generator, you can shop around for a nice diesel or other fuel generator to make up for the bad days, peak loads, and the occasional solar panel electronics failure emergency (should not be often). Assume that the batteries will need to be replaced every 7-15 years (these batteries are probably closer to the 15 year mark if you take care of them).

    So, right now you are around $100,000 per 1,000 kWatt*hours of solar electricity per month with 3 day of battery backup (+/- 30%). I am not an installer, don't know Mexico's supply of solar panels, etc., but this is probably a number you can start with for planning purposes. If you plan on using 2,000 kWhrs per month, double the numbers.

    Cost per kWhr of electricity assuming 30 year system life:

    ($100,000 + $38,000 for 1 battery replacement set) / (1,000 kWhr/m * 12m/yr * 30 yr) = $0.38 per kWhr over 30 years

    The above numbers seem to be pretty reasonable as a first shot of an estimate. If you really shop around (buy used where you can), do as much work yourself as you can (or if labor is less expensive where your home will be), you can probably cut 1/3 to 1/2 the price. However, the costs of duty/taxes/shipping/etc. may just push you right back to the list price estimate above (my numbers are probably high if you purchase large quantities of panels/electronics/batteries)... Don't know.

    You asked about what size of panels--You should, mostly just look at what you can buy and what the price is (solar panels are hard to get right now because of world demand). Fewer large panels are probably easier to install and wire. Smaller panels will probably be cheaper to purchase. There will be lots of other decisions that you will have to make (12, 24, 48 volt system, where to install the panels, what size wire will you need, what kind of batteries, what kind of panels, what kinds of chargers/inverters, generator, etc...). The list will seem to be endless--but you do want to look at your options up front before you commit any large amount of funds.

    Now, for the second part--going solar means conservation. Yes, you can throw lots of money at the problem of generating electricity and make it work--however, there are lots of secondary issues that make large solar systems difficult to maintain. Once a month or so, you will need to check the water in all of those batteries (above example is ~74 cells), clean with baking soda/water, and check cables. An array of ~40 175 watt solar panels need to be mounted at an angle in a strong frame (wind storms) and protected against vandals and theft. Typically, battery backed off-grid systems run at lower voltages and will have larger gauge copper wire (to reduce losses), again, an expense to install and protect against theft. Batteries will need replacing in 7-15 years. Backup generator is large, needs fuel and service.

    In general, you will be much better if you spend your money on conservation than on solar generation. Try to eliminate the heavy loads. The more electrical loads you reduce, the less you have to generate, and the less heat you bring into the home, the less AC you will need. Also, insulation, solar hot water (domestic hot water, space heating, pool heating) is usually cheaper and more efficient than just using Solar PV electricity for electric hot water, etc...

    I am grid tied here just south of SF Ca--don't need A/C, don't need that much heat (mild climate) and I am able to get my electricity down 200-300 kWhr/month (TV, fridge, freezer, computer, 4 people home schooling / home office--gas heat, cooking, hot water, drier). The off grid guys I read about tried for electricity usage below 100 kWhr/month. Now, instead of a $100,000 installation, you are talking about a $10,000 installation and $4,000 to replace the batteries down the road.

    That means when you design your home, that you need to follow all of the standard "green" home design and construction practices you can. Building into a hill, few west facing windows, south overhangs to prevent solar gain in summer, double pane windows, extra thick insulation, energy star appliances, either choosing not to install a pool--or at least looking at two speed motors and keeping backpressure low, looking at wind power if you are in a windy area, and on and on...

    You will find that many folks that are off-grid are even to the point of placing power strips on each appliance that uses standby power (and I do this too even though I am on-grid for some loads like the HDTV receiver, washer/drier, copier, laser printer, computer UPS, and a few others). This even can include TV's, cell phone chargers and other wall wort's. If you are aiming at 100kWhrs/month, that is only 139 watt load 24 hours per day, 30 days per month. And those little 7 watt nigh lights here, 5 watt wall wart there, 100 watt security light in the back yard, etc., all begin to add up real fast. If a home is running 1,000 kWh/month, that is a constant load of 1,390 watts--the little 7 watt loads here and there just don't matter as much then.

    You should probably look at Home Power Magazine (maybe get some back issues) for some ideas. You can download the entire previous month's issue for free to inspect and see if it is helpful for you.

    http://www.homepower.com/

    As you have further questions, please come back and ask them... There are others here that can give you much more detailed answers as they arise.

    Good Luck!
    -Bill

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2
    System2 Posts: 6,290 admin
    Re: Off-grid system questions

    Just a small tip.  You said you get 360 days of sun per year in a desert climate and have a lot of roof space.  My advice to you is to use thin-film modules.  They work better in hot climates than the "normal" panels do, but you will need a lot of roof space.  You might want to check out the Kaneka 60W modules at www.kaneka.com
  • System2
    System2 Posts: 6,290 admin
    Re: Off-grid system questions

    thank you that gives me an idea. to start with, i figured probably around 50-75k for the system plus or minus 20%, I figure i can save a fair amount on install since i am a General Contractor in AZ and i have electricians that work for me, and with some proper planning i can start to work the cost of some of the materials into some other jobs. Such as wiring, panels, etc. But obviously i would like to try and save cost where i can. I am planning on making the house as energy efficient as possible. block walls that are also framed out and insulated. Low-e windows etc. also planning on utilizing natural lighting where availible and maximizing usage of energy efficient lights energy star appliances etc. But as far as on a hill, and north south exposure, it wont happen as it is more designed for the ocean and beach exposure. There is some winds so i think also using some wind power is a viable source.

    i am not sure on usage right now i am not sure what i will need as i have never really paid attention to my current usage on my house in phx. The other thing is that the house will not be used year round. there may be several months a year that the house remains empty. I will have to take a look into what kind of battery back-up as far as days needed. I would think going with a 1-2 day battery back-up with a generator, Cause like i said the area gets a ton of sun. rainfall is 2-5" a year with 360 days of sun. The summers are in the 100s so a/c will be necessary but with the newer systems and higher seer ratings i am not sure their estimated usage.

    so i also wanted to know if the house is closed up and not used for months at a time, will that effect the batteries, cause i have been told that you should check them monthly and maintain them. what if any will be adverse effects of leaving them unattended with the power off?

    i am not sure on mexican availiblity of panels, my initial thought was just getting them in AZ or CA and driving them down.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,433 admin
    Re: Off-grid system questions

    It sounds like you are on the right track. With proper design, you may be able to get away with 500 or so kWatt*hours of electricity if you are 1/2 way careful with the design and appliance usage. A good place to start would be to look at your home electricity bill.

    For airconditioning, take a look at ground source heat pumps for the AC (and heat/domestic hot water). Being a contractor in the Southwest, you probably already know much more about them than I do.

    http://www.eere.energy.gov/femp/procurement/eep_groundsource_heatpumps.cfm

    If you are going to be gone for several months at a time, but leaving some lights and such running, or don't want to have as much maintenance, then look for AGM type batteries. They are maintenace free and will last a long time (and are more efficient to boot).

    This web site and their store (Wind-Sun) is located in Arizona (Flagstaff) so they may be a good place for you visit first. Solar panel supplies are tight right now, so to get the ones you want, you may have to order them a few months in advance of your need.

    Lastly, just as an FYI--you can always start with a few panels and build out from there. For the batteries, since you generally don't want to mix old an new (because they will charge/discharge at different rates as they age)... So, ideally, you will want to install all of the batteries at one time as a set.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2
    System2 Posts: 6,290 admin
    Re: Off-grid system questions

    well i have pulled out my latest elec bill and looks like i use a lot of energy a month here. i just bought the house and it is 20 years old so i am in the process of updating everything but right now it is looking at around 1500 kwH per month for usage. But i have 2 old a/cs for the house, a pool pump and filter that is also 20 years old that i am replacing. I am updating the lights and appliances. but i really had no idea. So i guess i will see how much i can reduce the energy here to what to expect in Mex. cause climate wise it is similar. minus the ocean part.

    as far as the maintainence free batteries is there a draw back to them or do they just cost more or do they have less of a total lifetime?

    So i guess another thing i am asking. Is to do a 1,000 kwH a month system what is needed. Like i am not aware of the math involved to figure out how many panels or what inverters plus how many batteries. etc are needed. i saw your math for the batteries of 1350 amps x 4 (for volts i am assuming) and the 200kwH for the 3 day storage for a 1,000kwH that equals 37 batteries. But how do i figure how many panels. i dont think i completely understood the figuring of the panels. do i need a 62.5KwH/h peak power systyem to generate a 1,000kwH a month system. and how do you figure what makes a 62.5kwH peak power system. I have only seen up to a 5500 watt inverter. So i will look into those back issues of homepower to try and figure some of this out. But if someone wants to give me the down and dirty of figuring systems needs i would appreciate it.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,433 admin
    Re: Off-grid system questions

    Good Afternoon Jake,

    A-Ha!, my cruel trick of talking about the extremely high costs of solar power first got you to look at conservation actually worked. :-D

    It is very easy to pay the $150-$200 per month summer electric bill and not look at what the underlaying numbers really mean. You will start looking at questions such as, if you save $10 in power (roughly 100 kWhrs at $0.10 per kWhr) per month, that is $120 per year. If you get 5% return on your money, you can spend 1/0.05 or 20x$120=$2,400 and more or less break even on the "investment". And, from the rough numbers above, a 100 kWhr/month system will cost roughly $10,000 to install (plus extra maintenance and such). So, saving energy vs generating using solar off-grid energy is roughly a $10k/$2.4k = 4x better investment (probably 2x-4x range).

    The drawback to the AGM batteries are that they cost more (2x?). But for the average user, they may last twice as long too, and they are roughly 15% more efficient in charging than flooded cell lead acid batteries. I am not the expert here--others can tell you the facts better than I. A few acid splashes and spills, replacing a corroded battery cable, and such--I probably would tilt towards AGM if I was off grid.

    The 1,000 kWhr per month system system was just a SWAG (scientific wild ass guess) on my part to use nice round numbers for calculation and comparisons to an electric bill. Will you need a 1,000 kWhr or even 1,500 kWhr system--it depends on you. As I have said before, conservation will be your friend in this whole project. I would suggest that you read around about folks that live off grid in your area (desert) and see what they have done and how much power they use. In the end, this really will be a life-style choice for you.

    (by the way, you are correct, the 4 was 4 volts for the two cell battery I did the example with).

    Regarding the individual choices in the system (how big, how many, how expensive)... They all really, again, boil down to the life style choice. For example, you first mentioned a 400 amp service. Assuming 240 VAC, running 24 hours per day, 30 days per month--ready for this?

    kWhrs/month = (400a * 240 V * 24 h/d * 30 d/m) / (1,000 w/kw) = 69,120 kWHrs/month ($7,000 to $21,000 per month at my power rates).

    So, while you may have a huge circuit panel, in reality, you are not going to use nearly that much power continuously. And, in fact, you are going to make sure that you 1) use as little peak power as possible and 2) use as little continuous power as possible. Inverters (DC battery to AC for home use) are not going to be the most expensive part of the project. The Solar Panels and Batteries are. Using too much peak power just forces you to use heavier wiring--especially for the batteries as their 24 vdc (for example) voltage will be 1/10 of your 240 volt AC mains (and therefore your batteries will need 10x the size of wire).

    So, first thing is to understand how much your continuous and peak electric use will be. That is how you will size the inverter (or, many times inverters as you may need two 120 vac inverters connected together to get 240 vac for your heavier loads). And to do that, you will need to list every appliance, light, computer, well pump, pool pump, A/C etc. that you intend to use at the same time. You will look at name plates, and probably want to purchase a kill-a-watt meter ($34 at Amazon or other places) to measure each load you will want to use... Then include a "fudge factor" for motor starting current... It may look like this:

    300 watts = Fridge (peak staring est. 900 watts at 120vac)
    200 watts = Freezer (peak starting 600 watts at 120vac)
    1,000 watts = house lighting (peak = 1,000 watts at 120vac)
    1,000 watts = pool motor (peak = 3 kWatts at 120vac)
    2,000 watts = well pump (peak = 6 kWatts at 2400vac)
    ============
    4,500 watts max continuous power (possibly two 120 VAC 3kW inverters if the 110 vac loads are distributed across the two legs)

    300 watts = Fridge (peak staring est. 900 watts)
    200 watts = Freezer (peak starting 600 watts)
    1,000 watts = house lighting (peak = 1,000 watts)
    1,000 watts = pool motor (peak = 3 kWatts)
    6,000 watts = well pump (peak = 6 kWatts) (assuming that only one high power device cycles on at one time)
    ===========================
    8,500 watts peak power (each inverter will need to be able to provide approximately 5 kWatts of peak load)

    So, now you have the inverter rating (may be 120 vac, 240 vac or a mix). If you can run the well on a timer when the pool and house lighting is off/low, you can use a smaller inverter(s), etc...

    Next, you will need to calculate/measure your average load per day or per month, and it may vary by season. For example, your loads may be higher in summer (AC, pool and the home is occupied) vs winter (no AC or pool, home not occupied). Fridge, freezer, lights may be more constant across the year. So, at this point you will need to size the batteries. As per my previous example, you will take your daily or monthly kWHr loads, the number of days you wish to run without charging, and times 2 for not discharging your batteries below 50%. Also, you don't have to assume worst case--for example, you may not need your AC and pool during poor weather. However, I would not recommend going under 1 day of storage of heavy load (with 2x discharge safely factor)--hard on batteries.

    Battery kWhr storage = Work (kWatt*hours per day) * Days w/o charge * 2

    Now you will need to choose your batteries and do a couple sample calculations to see how much you will want to spend. Typically large batteries are going to have fewer cells to refill, fewer batteries to wash, and fewer cables to buy/maintain. However, they are heavy. You can probably stack 50-100 lb batteries by yourself or with a friend. 300 lb batteries, you are will need some serious help and/or a small crane to toss those guys around.

    Also, you will need to pick a voltage to run your battery stack at... 12vdc is common for small systems (lots of accessories and choices). But, for heavy loads, 8.5kWatt peak load would need around 850 amps to supply the energy. Not going to happen. 24 vdc is popular, and you can get 48vdc. But even at 48vdc, you are still talking about 200 amps of current. So, you are going to need to pick even numbers of batteries to make the bank. (2 x 12 vdc = 24 vdc, 12 x 4 volt = 24 vdc, etc.)

    At the same time, you can probably size your generator for what you will want to run. Generators are, relatively cheap to purchase and it will be the fuel costs that will hurt you in the long term. The less solar/wind source you have, the more generator time (and fuel) you will need. Worst case, the generator would need to supply your worst case load (say 4.5 kW + starting load rating) and, for example, charge your battery pack in 10 hours (for three day storage, following the 1,000 kWhr/month example) approximately {[(1,000 kWh/m) / (30 days)] * [(3 store + 1 charge day)] / 10 hours to charge)} + 4.5kW = 17.8 kW min generator capacity. As always, there are losses in all of these conversions (AC to DC to battery charge back to AC) and so the numbers above are +/- 20%--you can play with them as you want (smaller battery bank, more time to charge batteries, etc.).

    Lastly, I would calculate the amount of Solar/Wind power that you will need. And here, the variables are, zero solar, you will just be running the generator. Enough solar for winter, and plan on running generator more during summer AC season, etc... Your choice (based on the numbers that you have calculated/measured for your needs). Once you have picked the amount of power you want, you will need to look at solar radiation tables for your area (or someplace nearby like southern Arizona)... The calculations I did earlier were based on my numbers fudge factored with some solar radiation numbers for southern Arizona/New Mexico/Texas. I am sure that somebody can do better for you--however, I would leave that until you better know your actual energy consumption for this new home.

    But, just using my numbers--during summer my 3kW system/inverter peak (3.5kW worth of BP 4175 solar panels, 20 of them, -500 watts due to real life performance and inverter efficiencies) will generate very close to 20 kWhrs per day right now (late of May) on clear 70 degree days. That is 600 kWhrs per month. Yours may be better (more sun, better orientation towards south, may be worst, my area is much cooler than yours). During the winter, my best days were around 12 kWhrs per day (360 kWhrs per month)... However, that would be assuming that I had clear weather. In reality, May 1 through May 30 this year my monthly was 504 kWhrs/May 2006 (or 16.8 kWhrs per day average), and for December 2005 148 kWhr/December (4.9 kWhr/day). Both December and May have been cool and very wet/overcast months for us. And with solar, you are going to get variability that you just don't have with utility power. According the the Solar Radiation numbers for San Francisco, there should be, roughly a 1.9:1 ratio between May:December electricity production numbers--From above, mine are 3.4:1... So, take any numbers with a grain of salt. Was it because of the bad December weather, my array points SSE, or some tree shading in winter--yea, probably all of that. You can make estimates--but they will not always agree with real life.

    You can take the the solar radiation tables (kWhrs/m^2 per day) * area of the collector * efficiency of collector * temperature derating * charging efficiency = kWhrs per day per collector. And divide that back into the number of kWhrs per day that you will need for the number of panels. It does probably get close in theory for the number of panels you will need for that time of the year.

    Or you can just use my numbers for 3.5 kW of panels (20 kWhr per day, or 16.8 kWhr per day, or 12 kWhr per day, or 4.9 kWhr per day) as your starting point. For solar panels, you can start with a 3kW array and see how it performs. And add more panels/chargers as needed down the road as needed for your area. And you can try different panels/chargers as price/availability changes over time. And since you are charging batteries, you can multiply my numbers by 0.8 for battery charging losses (assuming flooded lead acid batteries).

    You also will need to put the arrays into strings (series and parallel) arrays of panels to connect into the chargers. There are maximum voltages and current to worry about too. And all of this depends on the tilt of panels, high and low temperatures, and such... I will point you to an example that Xantrex uses for their Grid Tied Inverter--does not apply for your off-grid application, but it shows you some of the the different things you will need two look out for.

    http://www.xantrex.com/support/gtsizing/index.asp?lang=eng

    Using my system, I have BP Solar BP 4175 panels, GT 3.0 inverter, minimum temperature of 32F, max of 95F, and with two strings of 10 (20 panels), the PTC rating is 3,500 watts of panels, but the more conservative CEC rating is only 2,933 watts--which is much more realistic. I typically see 2,500 to 2,800 watts on my system, and I have seen 2999 watts once or twice on cool days just after the sun came out of some clouds.

    I purposely went through the calculations really fast just to get an idea of the system costs and to show you the price for a 1,000 kWhr per month power bill. The math is really not that difficult. So, the basics are (remembering your algebra):

    V=voltage in Volts
    I=current in Amperes (or amps)
    R=resistance in Ohms

    P=Power in Watts
    P=Power in kWatts = Watts / 1,000 (kilo=1,000)

    V=IR (voltage equals current * resistance); I=V/R; R=V/I
    P=I^2*R (I*I*R) in watts
    P=V^2 / R (V*V/R) in watts

    Work = P*Hours = Watt*Hours or kWHr = P*H / 1,000 (add missing "=" sign)
    Amp*Hrs = sort of like Work in kWHrs but without multiplying times Voltage (example 100 amp*hr 12 volt battery = 1,200 WHrs = 1.2 kWHrs). Many solar folks talk about amp*hr system usage--works fine if you assume the voltage is the same through the system (e.g., everything is at 12 vdc). If you are converting between 12v, 24v, and 120v, I prefer Watts*Hour or kWHr).

    I hope the above helps you more than it hurts. Once you have your power usage numbers (peak, continuous, summer, winter, etc.), then probably any solar equipment supplier would be more than happy to do the actual calculations and system design for you to make the sale (or for a nominal design fee if you wish to shop around). For the first time, please get the exact equipment needs from your supplier. You are setting up a power plant for your home, and while your local electrical contractor can probalby do most of the work--experience helps to pick the correct equipment for your exact needs. There is still much to be decided that I have not even typed about--battery temp sensors, generator controller, sycronized inverters, lightening protection, and the list goes on and on...

    Perhaps, one interesting way for you to start would be to install a smaller 1-2kW solar array on your current home (especially if you are going to keep it--although you can move the parts down to Mexico if that is where you will be moving/living--the batteries are the only part that may not be worth keeping from the small system--therefore, you may wish to use small and cheap storage batteries for the experiment). You can even take part of your home off-grid with a small backup generator too (or even use the utility as your backup "generator") and see how it all plays together (and gather some performance numbers).

    Take care,
    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2
    System2 Posts: 6,290 admin
    Re: Off-grid system questions

    that is some great stuff i am going to have to reread that to completly digest it all. But that is a good idea about adding a small system to my current home. I think it would help me figure a lot of things out just by doing. i am sure i will have other questions tomorrow.

    jake
  • BB.
    BB. Super Moderators, Administrators Posts: 33,433 admin
    Re: Off-grid system questions
    jakestorm wrote:
    But how do i figure how many panels. i don't think i completely understood the figuring of the panels. do i need a 62.5KwH/h peak power system to generate a 1,000kwH a month system. and how do you figure what makes a 62.5kwH peak power system. I have only seen up to a 5500 watt inverter. So i will look into those back issues of homepower to try and figure some of this out. But if someone wants to give me the down and dirty of figuring systems needs i would appreciate it.

    I forgot to address your questions exactly... So,

    The 62.5 kW peaks worth of panels, is a typo... That is 6.25 kW peak. To calculate the amount of panels needed, it is a several step process. First step is to understand how much power you will need per day, per three days (i.e., per battery storage ability), etc. and when that is (what month/season) and compare the to how much sun you get during the year. Depending on the season, you will designing the panel to supply your worst case load during the summer (longer hours, higher sun elevation--good things; but high temperature, bad thing, lowers solar panel output). The closer you get to the equator, the less the summer/winter sun angle affects your production (however, weather/clouds are seasonal, so need to account for that). Next, there is usually the efficiency of the inverter, charging system, and batteries to account for... For a battery backed system, 80% efficiency is a good number to assume when starting. So, your load * 1/0.8 = solar panel load.

    Next, you need to use a table/map to find out how much sun hours per days or kWatt hours / meter or some number that allows you to map the sun energy per year/season/month to your location. Then using panel efficiency and average temperatures, use that to understand how much power a panel can put out per day (i.e., a 175 panel may put out 150 watts for 5 hours per day in summer for you, or 150w * 5h = 850WHr or 0.85 kWHr per panel. Divide that back into your daily load and you know the number of panels you will need.

    Lastly, some brands/models of inverters can be tied to gether in synchronized strings to provide higher current and, also 240 VAC from two 120 VAC inverters. Or, using separate circuits, you could also use one inverter for the kitchen, another for the pool and well pumps, another for the lights, etc...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • rplarry
    rplarry Solar Expert Posts: 203 ✭✭
    Re: Off-grid system questions

    Jake
    I live about 150 miles south of where your proposed off grid home will be. I'm in Bahia de los Angeles, and have been off grid down here for 8 years. My system is nothing compared to what you are proposing, but I have been doing this long enough to know what works and what doesn't. BB has been giving you some great advice, and all that I would add is that you should strongly consider either AGM batteries or the larger GEL batteries. I use the GNB Absolyte llp batteries mod. 90a-13. They are very expensive at $2150 each (I have 3 of them) but batteries are the heart of your system and being maintenance free is a very big advantage. Also get the highest capacity panels you can get (175-200 watts) what ever you can get so that keeps wiring and mounting hardware to a minimum. Lastly use a really good controller such as the Outback MX60 so that you can wire as many panels in series and keep the panel voltage high and reduce the wire size down to something manageable.
    Good luck,
    Larry
    PS Also you will probably find that a 48v system will make a lot of sense for you as it will help to minimize wire size and of coarse you will want pure sine wave inverters.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,433 admin
    Re: Off-grid system questions

    RpLarry,


    Thanks for the compliments. :wink:

    Perhaps you can give some numbers that Jake can use to calculate how much a 1 kW worth of panels will generate per day, per month a couple different times per year--and are there days when clouds cut down production...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • crewzer
    crewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    Re: Off-grid system questions

    This map suggests average annual insolation for San Felipe (near the NW corner of the Gulfo de California) is ~5.0 hours/day (assumes a south facing array tilted up at angle = latitude, or ~31 degrees). Targeting net production of 1,000 kWh/month and assuming ~67% overall system efficiency (modules, wiring, charge controller, batteries, inverter, etc.), you’ll need an array rated at  (((1,000 kWh / month) / (30.5 days/month)) / (5 hours / day)) / 67% = ~10,000 W STC.

    HTH,
    Jim / crewzer
  • System2
    System2 Posts: 6,290 admin
    Re: Off-grid system questions

    so larry did you buy your solar system down in mex or did you buy it in the states and bring it down?

    so what does that annual insolation of 5 hours a day 67% etc etc mean? is that good or bad

    so what is the difference in using the different volt systems 12, 24, 48? what does each allow for you to do? benefits draw backs. you mention 48v allows for a more manage wire size? so what are we talking? Assume i use the garage that is on the south side of the house mount the panel on there i have 600 sq ft of roof space on the flat roof to mount the panels putting them at 31 degrees etc. and i then run all the wiring to the converter and batteries into that same garage. so from the panels to the batteries i would say a max of 30-40 feet. maybe half that once you figure inverters, controllers, batteries etc.
  • crewzer
    crewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    Re: Off-grid system questions
    so what does that annual insolation of 5 hours a day 67% etc etc mean? is that good or bad

    “5 hours/day” of insolation means that solar radiation equivalent to 5 kWh/m2/day is harvested. Since a PV module’s power spec is derived from 1 kW/m2, it’s mathematically convenient to state that a 100 W PV module exposed to daily insolation of 5 hours will generate energy equivalent to 100 W x 5 hours = 500 Wh (0.5 kWh)/day under STC conditions.

    An annual daily average of 5 hours is not the best number possible, but it’s pretty good.

    67% net efficiency means that net useable energy roughly equivalent to 67% of the PV array’s STC-rated energy generation will be available. It assumes a PV array operational efficiency of 85%, overall interconnect wiring efficiency of 97%, charge controller efficiency of 95%, effective battery efficiency of 94% and inverter efficiency of 91%.

    85% x 97% x 95% x 94% x 91% = 67%. YMMV :wink:

    HTH,
    Jim / crewzer
  • rplarry
    rplarry Solar Expert Posts: 203 ✭✭
    Re: Off-grid system questions

    Jake
    I bought most of my solar stuff in San Diego and brought it across the border a little at a time. I think NAWS has done some large installs in Mexico and could probably give you some good advice on how to get all the equip you need to San Felipe.
    As far as the different voltages, the higher voltages allow you to use smaller wire sizes, for instance if you did a 12v configuration and it required lets say 2/0 battery cable to run the length you needed, you could reduce the wire size in half simply by going to a 24v configuration, and in half again by using a 48v configuration. Another, even more significant advantage to going with a higher voltage system would be using less charge controllers. Lets say you were to put up a 3200 watt pv array. If you had a 12v system, it would take 4 MX60's to handle all that power, but if you went with a 48v system then just one MX60 could do the job, a savings of about $1500.
    As far as the solar insolation, I don't know how they come up with those numbers, but here it is not uncommon to have weeks if not months when there are no clouds especially in the spring and summer. Winter is another matter and just last February we had 11 days in a row where there was little or no sun, during that time I put 23 hours on my generator. Anyway BB's suggestion about giving you some numbers to work with got me off my butt and I went down to check my MX60 and I found that with 816 watts (9 US 64S and 3 Photowatt 80S) of PV I can produce 452 amp hours or 6.0 KWhrs on my highest days according the the logging screen. I also have 200 watts of mixed panels that run through a Solar Boost 50, but it has no logging capability so I can't tell you what they produce historically. My 816 watts STC input on the MX60 shows a net output of around 600 watts, so around a 75% conversion. The high temps down here really reduce the output of the array.
    Hope this help a little,
    Larry
  • BB.
    BB. Super Moderators, Administrators Posts: 33,433 admin
    Re: Off-grid system questions

    And every time you 1/2 the current, you can step down 3 wire gauges (half the copper area of the wire). A couple of examples. NEC allows 20 amps on 14 awg wire.

    14 awg 20 amps @ 12 vdc = 240 watts
    14 awg 20 amps @ 24 vdc = 480 watts
    14 awg 20 amps @ 48 vdc = 960 watts

    or

    14 awg 20 amps @ 12 vdc (480 watts)
    17 awg 10 amps @ 24 vdc (480 watts)
    20 awg 5 amps @ 48 vdc (480 watts)

    So, a 48 vdc system can use ~6 awg smaller wire diameter to transmit the same power...

    There are other issues too (such as length of wire, temperature, fill-factor in conduit, etc. that must be accounted for), but with 48 VDC you use 1/4 the copper (and, roughly 1/4 the price) or you can send the power 4x farther on the same awg as a 12 vdc system with the same losses. Or, just changing an existing system design from 12 vdc to 48 vdc, you will have 1/4 the current and 1/16 the wire heat loss due to "I squared" heating and wire resistance. (P=I^2*R)

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Off-grid system questions

    sorry bb, but i believe #14 is 15amps and #12 is 20amps. you could also use the voltage drop calculator to see what size wires work.
    http://www.wind-sun.com/smf/index.php?topic=1477.0
  • BB.
    BB. Super Moderators, Administrators Posts: 33,433 admin
    Re: Off-grid system questions

    I don't have the information in front of me from when I was working on this a few years ago... But, I thought I remembered, 14 awg had been up'ed to 20 amps (20 amp breakers) in many applications. 15 amp breakers for branch wiring has been standard for many decades (and in commercial I saw 20 amps branch breakers too--although it is possible that they were 12 awg runs)... The only thing I could find off hand:

    http://www.interfacebus.com/Copper_Wire_AWG_SIze.html
    The National Electrical Code [NEC] requires their own cable sizing for premises wiring. Refer to the NEC rules to determine building wiring, as this page relates to electronic equipment wiring. For reference, the ampacity of copper wire at 300C for common wire sizes:

    14 AWG may carry a maximum of 20 Amps in free air, or 15 Amps as part of a 3 conductor cable.
    12 AWG may carry a maximum of 25 Amps in free air, or 20 Amps as part of a 3 conductor cable.
    10 AWG may carry a maximum of 40 Amps in free air, or 30 Amps as part of a 3 conductor cable.
    8 AWG may carry a maximum of 70 Amps in free air, or 50 Amps as part of a 3 conductor cable.

    Somebody with a current NEC book can give the correct numbers--I was just trying to give quick sizing examples. There are more than just wire size that affect gauge--hence my disclaimer of "There are other issues too (such as length of wire, temperature, fill-factor in conduit, [add insulation type,] etc. that must be accounted for)..."

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • halfcrazy
    halfcrazy Solar Expert Posts: 720 ✭✭✭
    Re: Off-grid system questions

    14 gauge is rated to 20 amps but nec requires a 15 amp breaker for residential branch circuits. in commercial i am not shure but probably allowed to go to the 20 amp mark
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Off-grid system questions

    that's a contradiction for if they rate it at 20amps then put a 15amp breaker then they don't trust it at 20amps and are limiting it to 15amps through that 15amp breaker, therefore it is 15amps.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,433 admin
    Re: Off-grid system questions

    Welcome to what was part of my world for 20 years--regulatory approvals. Conflicting requirements that, at times, did not make a lot of sense. And add the fact that the regulations were frequently copyrighted, very expensive, sometimes had many multiple volumes, you had to meet to sell the product, and a boss that was too cheap to allow the department to purchase them... :|

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