How Many Kwh Can I Expect

Discussion in 'Solar Power' started by Robert Clark, May 9, 2015.

  1. Robert Clark

    Robert Clark Funster Life Member

    Joined:
    Jul 12, 2014
    Messages:
    5,608
    Likes Received:
    9,779
    Location:
    East Sussex
    Hi Guys

    Is it possible to calculate roughly how many Kilowatt Hours of energy you would get from two 115A leisure batteries when converted by an inverter to 240V?

    Once I know this, I can calculate how many days I can manage 'off grid' based on our anticipated power consumption;

    1 hour of 250w towel rail = 250w
    5 minutes of 800w toaster = 67w
    10 minutes of 1200w hair dryer = 200w
    10 minutes of 1200w Nespresso coffee maker = 200w
    Daily Total = 717w (assuming I am calculating this correctly?)

    Thanks
    Robert
     
  2. Peter & Elaine

    Peter & Elaine Funster

    Joined:
    Aug 30, 2012
    Messages:
    3,967
    Likes Received:
    6,462
    Location:
    Roobarb land
    image.jpg
     
    • Like Like x 1
  3. pappajohn

    pappajohn Funster Life Member

    Joined:
    Aug 26, 2007
    Messages:
    29,463
    Likes Received:
    16,905
    Location:
    YO11 2BD
    2 hours

    717w ÷ 12v = 59.75ah × 2hrs =119.5amps....or near enough 50% of your total capacity.

    50% discharge is the recommended limit without causing battery damage.
     
    • Like Like x 2
  4. JeanLuc

    JeanLuc Funster

    Joined:
    Nov 17, 2008
    Messages:
    2,782
    Likes Received:
    1,857
    Location:
    Warwickshire
    Firstly, I assume your calculation is your total daily usage.
    You have calculated watt-hours correctly at 717Wh. As this is to be supplied by an inverter running off 12V, you divide the Wh by 12 to get Ah.
    717 / 12 = 59.75 Ah.
    Then allow for inverter inefficiency - say 10% loss so divide the previous answer by 0.9
    59.75 / 0.9 = 66.4 Ah

    So allowing for no more than 50 % discharge (unless you have gels batteries in which case you can go to 80% discharge), your two batteries will support 115 Ah of use (2 x 115 x 50%) so you have some spare capacity - but not a lot if you want to run things like blown air heating, lighting, TV etc as well as your inverter. You would be able to last 1.7 days assuming no other consumption apart from the inverter (115/66.4).
     
    Last edited: May 9, 2015
    • Like Like x 2
  5. Bartyfixedit

    Bartyfixedit Funster

    Joined:
    Jun 11, 2014
    Messages:
    193
    Likes Received:
    290
    Location:
    Maidstone Kent
    The numbers you are quoting make this a nonstarter.

    To take the towel rail alone. 250 W divide by 240 V is roughly 1 amp. One amp out of the inverter will require 20 A in. 20 A on a 230 amp hour bank will pull the voltage down very quickly. I'm pretty sure you will find that load alone will shut the inverter down within an hour.
     
    • Like Like x 1
  6. Jimbohorlicks

    Jimbohorlicks Funster

    Joined:
    Sep 3, 2012
    Messages:
    640
    Likes Received:
    822
    Location:
    Cheshire
    Why would you want a 250w towel rail on for an hr per day-it wont dry the towels or put out any significant heat?

    Would you be putting anything back into your battery bank? by solar or aqternator for example?
     
    • Like Like x 1
  7. pappajohn

    pappajohn Funster Life Member

    Joined:
    Aug 26, 2007
    Messages:
    29,463
    Likes Received:
    16,905
    Location:
    YO11 2BD
    The towel rail will run for 5 hrs and consume approx 110amps inc loses.

    With a reserve of 115ah the voltage may still be 12.2/12.3v......far above the inverters shut down voltage which may be as low as 10.5v under load.
     
    • Like Like x 1
  8. eddievanbitz

    eddievanbitz Trader - Funster

    Joined:
    Oct 4, 2007
    Messages:
    3,548
    Likes Received:
    11,048
    Location:
    Taunton Somerset
    Easiest way to calculate AC Wattage into DC Amperage is divide by ten for a rough idea, without looking for the calculator.

    Don't forget to anticipate only half of your battery capacity is usable, and that you will also have your regular DC consumption, things like lights, pump, heating, TV etc
     
    • Like Like x 3
  9. Robert Clark

    Robert Clark Funster Life Member

    Joined:
    Jul 12, 2014
    Messages:
    5,608
    Likes Received:
    9,779
    Location:
    East Sussex
    Thanks Guys for your input- yes the daily total quoted was the maximum we would expect to use per daily session.

    We already have the Inverter fitted (2kw) and tested everything on the list (not at the same time).

    Started off by turning on the towel rail (which does seem to work on 1 hours elec as its quite small). In reality the towel rail will be probably be running when we're driving in the mornings, so this won't affect the leisure batteries as we have a battery to battery charger.

    Ran all the other appliances listed for roughly the time quoted and the voltage showing on the inverter was still over 12V (was showing 12.1v) after the test.

    If we can get 2 days from the batteries then this would be great, as we tend to tour rather than stay in the same place for more than a couple of days.

    Furthermore we are conscious of our power consumption and manage our usage accordingly.

    Thanks again for the answers, can always reply on MH Fun for solving life's little mysteries

    Regards
    Robert
     
    • Like Like x 1
  10. Bartyfixedit

    Bartyfixedit Funster

    Joined:
    Jun 11, 2014
    Messages:
    193
    Likes Received:
    290
    Location:
    Maidstone Kent
    20A on a 230 amp hour bank is a C/11.5 load. The capacity at that load is going to be much reduced hence me saying less than an hour. It may be more but not very much. As to 10 minutes at 1200 W which is a C/2.3 load then in my opinion it will wreck the batteries before long.

    All of my van electrics run off an inverter and I have a battery monitor so I am speaking from experience.
     
    • Like Like x 1
  11. hilldweller

    hilldweller Funster Life Member

    Joined:
    Dec 5, 2008
    Messages:
    26,431
    Likes Received:
    25,098
    Location:
    Macclesfield
    12V is generally considered flat.
     
    • Like Like x 1
  12. Robert Clark

    Robert Clark Funster Life Member

    Joined:
    Jul 12, 2014
    Messages:
    5,608
    Likes Received:
    9,779
    Location:
    East Sussex
    When I turned the appliances off the voltage reading on the inverter/charger increased to over 12.5
     
  13. hilldweller

    hilldweller Funster Life Member

    Joined:
    Dec 5, 2008
    Messages:
    26,431
    Likes Received:
    25,098
    Location:
    Macclesfield
    Which is fully charged. Good batteries !
     
    • Like Like x 2
  14. pappajohn

    pappajohn Funster Life Member

    Joined:
    Aug 26, 2007
    Messages:
    29,463
    Likes Received:
    16,905
    Location:
    YO11 2BD
    True Brian....but that indicated 12.1v was under load not at rest.
     
    • Like Like x 1
  15. hilldweller

    hilldweller Funster Life Member

    Joined:
    Dec 5, 2008
    Messages:
    26,431
    Likes Received:
    25,098
    Location:
    Macclesfield
    >> the voltage showing on the inverter was still over 12V (was showing 12.1v) after the test.
     
  16. Robert Clark

    Robert Clark Funster Life Member

    Joined:
    Jul 12, 2014
    Messages:
    5,608
    Likes Received:
    9,779
    Location:
    East Sussex
    Sorry for not being clear, it was 12.1 at the end of the test when the load was still connected
     
  17. Bartyfixedit

    Bartyfixedit Funster

    Joined:
    Jun 11, 2014
    Messages:
    193
    Likes Received:
    290
    Location:
    Maidstone Kent
    If you want to work it out, here is a handy calculator.
    http://www.csgnetwork.com/batterylifecalc.html
    You need the peukert No for your battery for really accurate readings, which you probably won't have. Probably about 1.2 if you read the instructions to the calculator.
    Have a play with different values, it's quite an eye opener.
    I actually got from 1.5 to 3.8 hours to flat(50%) for the towel rail depending on the Peukert No.
     
    • Like Like x 2
  18. eddievanbitz

    eddievanbitz Trader - Funster

    Joined:
    Oct 4, 2007
    Messages:
    3,548
    Likes Received:
    11,048
    Location:
    Taunton Somerset
    Good link. Peukerts Law explains to us that as the battery voltage diminishes, the amperage will need to increase. Most good battery computers will automatically work this out for you, so the displayed results are acurate
     
    • Like Like x 1
  19. hilldweller

    hilldweller Funster Life Member

    Joined:
    Dec 5, 2008
    Messages:
    26,431
    Likes Received:
    25,098
    Location:
    Macclesfield
    No problem, this is MotorhomeFun not a technical forum.
     
    • Like Like x 1
  20. funflair

    funflair Funster Life Member

    Joined:
    Dec 11, 2013
    Messages:
    4,476
    Likes Received:
    4,722
    Location:
    Guisborough
    Watts=volts x amps, you can only use half your battery and you loose a bit through the inverter so let's call 2 115 ah batteries 1380 wh and you will get less if it's 240 volts through the inverter.

    Keep it simple for us thicko's.

    Martin
     
    • Like Like x 2
Loading...

Share This Page