Basic Questions about solar system.

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System
System Posts: 2,511 admin
in General Solar Power Topics #1
I understand what I need for a solar system. Solar panels, controler, batteries, inverter. What I need is a good explaination of how things work. After the power comes from the solar sells. Does the power come from the batteries to the inverter to the appliance or from the solar cells to the inverter to the appliance or both ways. And I want to know about hook-up to the grid. Or what would help me more is for a 6000 watt, 12 volt system how many solar cell, how many batteries, etc.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,443 admin
    #2
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    Re: Basic Questions about solar system.

    For an off-grid system... Think of the battery as the "heart" of the system and all things relate, mostly, to the battery and not to other interconnected devices/loads/etc...

    The inverter takes power from the battery (battery needs to be rated to power the inverter and for the length of time it will run per day/whatever).

    The charge controller (solar, wind, generator, etc.) charges the battery--The size of the Charger and Panels has to be large enough to completely charge the battery (plus load) over a period of ~1-3 days.

    Certainly, if everything is sized correctly, the solar panels, through the charge controller, will power both the load/inverter and charge the battery at the same time (or else you would never charge the battery on a sunny day).

    However, there are secondary effects... For example; since a standard wet cell battery is about 80% efficient, it is better to run your heavy loads during the day with full sun then to charge the battery during the day and discharge it at night and loose another 20% power by "going through" the battery to time shift your loads.

    Also, a battery only have a certain number of discharge/charge cycles it can support before it "wears-out". So, again, using your heavy loads when you have solar/wind to run them directly (while charging the battery) will help the batteries to last a bit longer too.

    To a degree you can counter these secondary effects by adding 20% more panels to make up for battery losses, building a larger battery bank (less cycling, longer life), getting AGM batteries (90% efficient vs 80%), adding alternate charging devices (wind turbine to solar panels, etc.)... But, usually all of that does cost more money.

    If you have utility power available (and Grid Tie is allowed by your utility), Grid Tie is a great way to build a very cost efficient solar power system. With Grid Tie, your solar panels connect directly to a special type of inverter (no batteries). This "Grid Tied" inverter takes every bit of power available from your solar panels and "inverters" to your local utility power (typically 208-240 VAC 50/60Hz depending on your location). This power is simply driven backwards to your fuse/breaker box and will power your local usage--and if there is enough, drive your utility meter backwards on bright/sunny days (depending on the size of your panels, your power usage, and the type of utility meter / installation you have). A grid tied system's major drawback is "if there is a power failure, your system is dead also--to provide for safety of the utility workers"...

    If you want backup power with Grid Tied--you will need to build a hybrid system that is basically an "off grid" type system with inverters that support grid tie operation.

    And, in the end, for solar/RE power, it almost always it is better to spend your first money on conservation (more efficient lights, appliances, turning off unused TV's, Computers, with a power strip switch instead of letting them sit on "standby" 24x7, etc.) than to throw more money at a solar power system.

    Just to throw some very wild numbers about (assuming US)--Your utility power is typically around $0.10 to $0.30 per kWhr...

    Grid Tie Solar installed, maybe around $0.25 per kWhr (with CA rebates, you can get down to $0.15 per kWhr).

    For an Off Grid system, you are probably looking at an overall lifetime average cost of $1.00 per kWhr (could be more or a bit less--really depends on lots of different things)...

    To design your system, you need to know how much power you will be using. 6,000 watts doesn't really tell us much. If that is peak power (say A/C and/or Well Pump)--you would need a good sized inverter to run that--but is that 5 minutes per day or 24 hours per day.

    We would also need to know your W*Hours (Watts * Time, and a kWHr is simply 1,000 x W*Hr). A typical home in the US probably runs 300 to 600 kWHrs per month--and if you have A/C or other large uses, it can run 1,000-2,000+ KWhrs per month.

    A full off-grid system starts getting real expensive and complex to build when the monthly usage is much over 100 kWhr per month (can be done, but requires lots of panels, batteries, large inverters, backup generator, etc.).

    A Grid Tie system does not use batteries (lots of costs saved), does not have the losses of batteries and inverters)... So, for example my 3.5 kW watts of panels will generate an average of 400 kWhrs per month (near SF CA). Summer is >500 kWhrs per month; dead of winter 150-250 kWhrs per month depending on weather...

    I will stop here--because I am not sure where I have confused you yet, and don't really know what it is you need... ;)

    -Bill

    Lastly, I should add about 12 vdc battery...

    This is really based on how much power you need... If you need 6,000 Watt*Hours over 24 hours, that is 250 watts or about 25 amps at 12 VDC... If you need 6,000 watts, that is about 600 amps at 12 vdc--Wiring up batteries to support 600 amps is pretty difficult--so, normally, a large system would be wired for a 48 VDC battery bank--which would be around 150 amps or so. Much more reasonable current to manage. (P=V*I ; power=voltage*current... Double the voltage, 1/2 the current, etc.).
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • System2
    System2 Posts: 6,290 admin
    #3
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    Re: Basic Questions about solar system.

    Boy, that was a really nice description.

    :)

    thanks.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #4
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    Re: Basic Questions about solar system.

    a more simplified version.
    off grid-
    >the solar electric panels or pvs generate the electric power from the sun.
    >the charge controller regulates and conditions the power from the pvs.
    >the batteries store this power.
    >the inverter is an inverse converter that takes the smaller dc power and recreates it into the more usable household energy form of ac.

    on grid has similarities to the offgrid and depending upon whether battery backed or not it may skip a few of the steps as a straight grid tie system will go from pvs to the inverter. ultimately of course, grid tie adds one step of going out onto the grid. these inverters for straight grid tie (no battery) have a wide high voltage dc input range unlike the battery driven inverters with a smaller dc voltage.
    note-the inverse converter or inverter was a name given to a specific type of converter that instead of going from the normal high voltage ac down to a (usually lower) voltage dc it took the dc and made it into ac or the reverse of most other converters.
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  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    #5
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    Re: Basic Questions about solar system.

    As is so often the case, I second all of what Bill says. I would add however, that batteries are a very expensive option IF you have the grid available. Most people get caught up in the idea of having uninteruptable power (UPS) during a grid failure, and design the battery system in to make this work. In many cases it would be WAY cheaper to use a generator for the UPS, take the money saved and add in additional panel capacity.

    Batteries are high maintenence items, that have short (relative to the rest of the components) life span. The times that the grid goes down in most climates is rare. Of course if you are talking a doomsday scenario that is another issue. (Not something I worry about).

    Congrats on thinking forward, do your reading on this and other sites/pubications and you will soon figure out what you wish to accomplish. The only other admonition is, avoid the ready, fire, aim!

    Icarus
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  • System2
    System2 Posts: 6,290 admin
    #6
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    Re: Basic Questions about solar system.
    icarus wrote: »
    As is so often the case, I second all of what Bill says. I would add however, that batteries are a very expensive option IF you have the grid available. Most people get caught up in the idea of having uninteruptable power (UPS) during a grid failure, and design the battery system in to make this work. In many cases it would be WAY cheaper to use a generator for the UPS, take the money saved and add in additional panel capacity.

    Batteries are high maintenence items, that have short (relative to the rest of the components) life span. The times that the grid goes down in most climates is rare. Of course if you are talking a doomsday scenario that is another issue. (Not something I worry about).

    Congrats on thinking forward, do your reading on this and other sites/pubications and you will soon figure out what you wish to accomplish. The only other admonition is, avoid the ready, fire, aim!

    Icarus

    Personally I got solar when I did because the power company shut me off when I asked them for two days to get the money... that was in really cold weather and besides I don't have balance without light due to a brain injury.

    So, when they took away my heat and light, and I didn't even have a working phone without electric, I decided that I was not going to trust them in the future... nor pay them if they didn't want the money in two days as I had offered to pay.

    So, now when they call me, I laugh...

    But... it's a bit cold. Though it was fairly cold ever since my furnace broke. Having had tetanus I'm not so able to do things as previously.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #7
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    Re: Basic Questions about solar system.

    everybody's purposes and needs in solar do vary. not everybody is keen on the idea of a generator either as availabilty and pricing of the fuel during a prolonged crisis makes that useless and it did happen to me too. it is true that that type of crisis doesn't happen often, but it does happen and is a big reason why many go with solar power. not everybody is doing this just to try to 'sell' electricity making the ups scenario a major reason for solar. the added abilty of the ups to reduce ones electric bill when batteries are fully charged made the system even more appealing. again this is very dependant on one's initial wants of a system as many utility systems are more reliable than others making a grid tie batteryless system appealing with its higher efficiency. that higher efficiency will be useless in many areas with frequent power outages.
    contempt of a utility is also a reason for whatever failures that utility may have in serving their customer's needs.
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  • System2
    System2 Posts: 6,290 admin
    #8
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    Re: Basic Questions about solar system.
    niel wrote: »
    everybody's purposes and needs in solar do vary. not everybody is keen on the idea of a generator either as availabilty and pricing of the fuel during a prolonged crisis makes that useless and it did happen to me too. it is true that that type of crisis doesn't happen often, but it does happen and is a big reason why many go with solar power. not everybody is doing this just to try to 'sell' electricity making the ups scenario a major reason for solar. the added abilty of the ups to reduce ones electric bill when batteries are fully charged made the system even more appealing. again this is very dependant on one's initial wants of a system as many utility systems are more reliable than others making a grid tie batteryless system appealing with its higher efficiency. that higher efficiency will be useless in many areas with frequent power outages.
    contempt of a utility is also a reason for whatever failures that utility may have in serving their customer's needs.

    I'm unclear... are you saying I caused my utility company to fail by my contempt for it...?
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  • Ralph Day
    Ralph Day Solar Expert Posts: 1,019 ✭✭✭✭
    #9
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    Re: Basic Questions about solar system.

    Neil,

    Did your last sentence mean that it is a self fulfilling prophecy, if you have contempt for something...a utility for instance...you will only recognize their failures and contempt for you? Or your contempt for them breeds their contempt for you? JUst puzzling, maybe profound (if i understood it). :blush:

    ralph
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  • System2
    System2 Posts: 6,290 admin
    #10
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    Re: Basic Questions about solar system.
    Ralph Day wrote: »
    Neil,

    Did your last sentence mean that it is a self fulfilling prophecy, if you have contempt for something...a utility for instance...you will only recognize their failures and contempt for you? Or your contempt for them breeds their contempt for you? JUst puzzling, maybe profound (if i understood it). :blush:

    ralph


    Yes, philosophically I think that's a good question. :)

    Meanwhile... I've built a bit of a tent, a la youthful ideas decades ago, and it's really toasty in here. This appears to be about the amount of space my laptop can heat pretty well.

    I'll watch a half an hour of news in a minute, then I'll watch Ugly Betty and Grey's Anatomy on my computer some time.

    So I'm all snug and happy.

    It was a great Thanksgiving.

    :)

    Hope yours was lovely, too.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #11
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    Re: Basic Questions about solar system.

    what i said was that your present contempt of the utility may very well be a good and justified reason due to the utility not fulfilling their obligations to the customers they are supposed to serve. i would classify one's life being put into danger as irresponcible of them seeing as how you had only told them you intend to pay, but 2 days later than they demanded. i deem your request to them as reasonable in light of your circumstances and you calling them to indicate your willingness to pay. they strongarm people this way and get to charge more charges in order to turn the electric back on. imho they are lucky you didn't sue them.
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  • System2
    System2 Posts: 6,290 admin
    #12
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    Re: Basic Questions about solar system.

    Oh. Thank you. I think a lot of people in positions of power like to exert the power.

    And yes, you're right. They're mean, then they charge extra.

    The best thing about getting the solar, was that it was exciting to plan it. And overall it works really well. I mean, it works great.

    It was fun to do that, and actually it's fun to figure out how to handle the cold. I think the tent thing is going to work rather well. :)

    Thank you, again.
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