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Formula for PV Panels and Inverter

Discussion in 'Photovoltaics' started by sunnybouy60, Jan 12, 2006.

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  1. sunnybouy60

    sunnybouy60 Guest

    Hi guys, I was wondering if someone could explain to me (in simple terms)
    the forumla to determine how many PV panels are needed in a PV system?

    I understand that Total Watts x Hours Usage Per Day = Total Watt Hours

    and that

    Total Watt Hours / Voltage System ie 12v = Total Amp Hours

    and that

    A Deep Cycle battery should only be discharged 50% so double the battery
    capacity to cover the Amp Hours

    now.... how do I determine how many PV panels I would need???

    I know it is dependant on the amount of sun per day and my geographical
    location. Also what is the forumla for determining the size of the Inventer
    that would be needed? Can someone help me out here?

  2. S.C.F.

    S.C.F. Guest

    want the anecdote?
  3. SJC

    SJC Guest

    That is a bit like asking "how big is a fish". You make the system the
    size you want to make it. I have seen grid tie inverters as small as 1KW
    and they may make some smaller. If you buy a 1kw sine wave grid tie
    inverter and use less panels, it will only supply to the grid what the
    panels can provide. If you have more panels, then the inverter will
    supply to the grid only what it can invert from DC to AC.
    If you are not going on the grid you can just charge batteries and
    use a square wave inverter to power small tools and those inverters
    start at 100w or less.
  4. S.C.F.

    S.C.F. Guest

    want the anecdote?
  5. Chuck Olson

    Chuck Olson Guest

    There's a website that you can use to calculate what will approximately
    cancel your electric bill, averaged over a year's time. It's and it is useful to find a solar installer.

    Generally, there are two different types of PV you can plan on - - grid tied
    or stand-alone. Sounds to me like you want a grid-tie system, so you can
    forget about batteries. The grid is your energy storage facility with your
    electric meter keeping track of what you put into storage versus what you
    take out.

    Now that we have that out of the way, you might want to think about the
    future and your growing needs, and couple that with the fact that all that
    energy derived from sunshine is free (the cost of the equipment adds to the
    value of the home, so arguably that cancels out). Once you reach that
    mindset, all you need to figure is how much roof area you have to play with,
    and if it's facing a good direction. I have a 560 sq ft garage roof facing
    south so I'm going to load that roof up with 36 panels, 170 watt each for a
    little over 6 KW peak output, and roughly 25 KW-h per day storage. (With
    that amount of energy, I can probably try to get my Prius converted to
    plug-in and charge up an extra large battery in it overnight to go 30 miles
    the next day on electric power only.) The state will contribute a little
    over $14,000 to help me pay for the PV system, and federal adds another

    As the utility keeps track of things, you only have to "pay the piper" once
    a year if your usage exceeds your generation. Unfortunately, if your
    generation exceeds your usage, the piper says "forget about it" and you get
    to think of new ways to use your excess energy, like an electrically-heated
    hot-tub, or whatever improves your outlook on life.

    Have fun,


  6. I don't think that's really accurate. If you put $30K of panels on your
    roof, that only adds $30K to the value of your home for someone who wants
    to have that system. I could put a $30K gold plated custom bathtub in my
    home, but it won't have a whole lot of value to someone who only takes
    showers or doesn't need any "bling" with their bath, so I shouldn't expect
    to realize anything like $30K additional resale value on my home because
    of it unless I sold my home to a pop star.

    Realistically, all you can expect to get back in additional value of your
    home is the production capacity of the PV array * utility rate * years of
    expected lifetime - maintenance/repair costs. If you spent $30K on a PV
    system that generates 5000 khw per year and your utility rate is 10c/kwh,
    that's $500 a year. Using those numbers you could expect your home's
    value to increase by at most $5-10K (and many buyers might even consider
    it a negative just because they won't want to hassle with something they
    don't understand) Now if you find someone who is interested in PV they
    might buy your home just because of that system, but even then I doubt
    you could get back what you put into it. After all, for someone who is
    into PV they'd probably rather buy a home without it and add it themselves
    after a little research to find out what they wanted from it versus buying
    it "used" and built to someone else's specifications.

    I'm not saying it is dumb to do PV, you just shouldn't fool yourself that
    it is a good investment today, unless you live in a place like CA with
    very large rebates and high utility rates, or you believe utility rates
    are going to go up way way up in the next few years. You definitely
    shouldn't fool yourself into thinking it is going pay for itself through
    increase in the resale value of your home!

    I am interested in doing PV on my new home, but since it is at least 4-5
    years from being break even (or close enough that I'm willing to do it
    anyway) I'm going to build it "PV ready". (Which to me basically consists
    of making sure the roof could take the weight of being fully covered by
    panels on the south side, and running some metal conduit up to the roof
    from the basement to allow me to run some extra thick copper wire up
    there later)
  7. sunnybouy60

    sunnybouy60 Guest

    Too funny SolarFlaire...

    Ok I must not have made my question clear here, I appreciate all of your
    input. Could someone just give me a breakdown for the following?

    5000 Watt Hours per day total
    416.6 amp hours
    4 - 210 Amp Hour Deep Cycle Batteries

    How many PV panels and what Wattage do I need to support the above
    configuration and how do you calculate the number/wattage of the panels

    What size Inverter would be best suited?

  8. Guest

    *Appraisal Journal, October 1999.
    "All solar systems are exempt from property taxes. The National Appraisal
    Institute states that the value added to your home is 20 times the value of
    electricity saved in the first year. A solar installation is one of the few
    ways that you can increase the value of your home without increasing your
    property taxes."

  9. I'm aware that many states (including mine) exempt solar from property
    taxes. I was talking about actual resale value of the home, which in
    many places has only a very loose relationship to the taxable value of
    the home.

    Given that I'm paying about 9c/kwh and there are no rebates in my state,
    if I installed a system capable of generating 5000 kwh per year in my
    locale (i.e. a bit over 3000 "peak" watts) that's worth about $450, so
    multiplying it by 20 that's $9000. Unfortunately we're about a factor
    of 3 from a fully installed grid-tie system for $3 per peak watt. So
    even at that (IMHO somewhat optimistic but I guess NAI knows better
    than me) 20x generation valuation its only a 33% payback. Stuff like
    remodeling of older kitchens and bathrooms is supposedly a 90%+ payback,
    and even stuff that's not a very good investment like adding a pool in
    more northerly climates is still a lot better than 33%. So again my
    point is that you have to do solar because YOU want to do solar, not
    because you figure even if you are only in the house for a few years
    you'll get most of it back when you sell your house, because you won't.
  10. Guest

    My $500 12'x32'x16' tall plastic film sunspace was exempted as "HVAC space."
    Let's not tar and feather sunspaces with PVs :) The economics are different,
    adding valuable floorspace to a house while collecting heat energy at 5 cents
    vs $5 per peak watt.

  11. Steve Spence

    Steve Spence Guest will help you.

    Your inverter needs to be sized to the loads you are trying to power.
  12. George Ghio

    George Ghio Guest

    Who says. 70% would be safe, 80% pushing it, Depends on haw soon you
    cane replace the charge.
    ROT - 60% of your load i.e. 1000Wh = 600 W inverter. Unlikely that you
    will run more than 60% of your full load at once. Decide with care as
    inverters cost money
  13. Steve Spence

    Steve Spence Guest

    70% discharge really shortens battery life. Shame on you george for
    misleading him.

    so does everything but not a whole lot. 2500 watt MSW's are only $269
  14. wmbjk

    wmbjk Guest

    Don't forget that a deep discharge combined with a relatively large
    load may cause the inverter to trip off due to temporary low voltage.
    On paper that next 20% of battery capacity below 50% discharge might
    be plenty to vacuum up some cat litter, but in reality one may have to
    reset the inverter before breaking out a broom and dustpan.
    No no no. Inverters should be sized for power. Imagine a system
    supplying say, a standard submersible well pump for 10 minutes each
    day. Call it 150Whrs per day. Your RoT 90W inverter owner will have to
    use the bathroom at the filling station. And in the example you used,
    that 600W inverter wouldn't be able to support some common household
    appliances, even though 1kWhr per day might otherwise be sufficient.
    In the OP's example of 5kWhrs per day, your RoT would recommend a 3kW
    inverter. Which would crash if say, a water pressure booster pump
    started while he was nuking a sandwich. Or would be far too large if
    the system was say, running a couple hundred Watts of lighting 24-7.
    In fact, I can't see any reason for this RoT to exist. If one knows
    the daily consumption, then he already knows which loads and
    combinations will be running when, and can choose the inverter

  15. George Ghio

    George Ghio Guest

    Read the post. Clearly says (with typos) Depends on how soon you can
    replace the charge. 70% discharge is common for max DOD for X number of
    days autonomy.

    It does not say "Run your batteries down to 30% every day".

    70% does not "Really shorten battery life" unless you do it every day.

    My daily DOD is currently 12%. Still it would not worry me to pull the
    batteries down to 70% after ~6 days of no sun.

    Shame on you steve for misleading him.
    We know you buy second best to cut costs but even at that price why buy
    an inverter just to buy another one due to poor choice in the first place.
  16. George Ghio

    George Ghio Guest

    This from Wayne who has admitted that he can chew through 10kWh in four

    If you want to be taken seriously you can start by documenting your system.

    You can't though can you?
  17. George Ghio

    George Ghio Guest

    The point is that 50% is bunk.

  18. George Ghio

    George Ghio Guest

    Oh dear. Truth, Wayne has not got a clue.
  19. George Ghio

    George Ghio Guest

    Let's see:

    George - posted total energy use for all fuels + solar -

    Wayne - posted weight of batteries and inverters -

    Conclusion Wayne knows next to nothing.

    Looks good to me
  20. George Ghio

    George Ghio Guest

    Ah yes, Windsun is a battery retailer. The reason for 50% max DOD is
    that if you swallow that line you end up buying 30 to 50 percent more
    batteries than you need and the battery retailer makes 30 to 50 percent
    more money.
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