calculation for imverter

Discussion in 'Motorhome Chat' started by Daz n Tina, Jul 28, 2015.

  1. Daz n Tina

    Daz n Tina Funster

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    Can anyone tell me how to calculate the power used from 240v to 12v.

    I have just purchased a small fan 16 w max 240 volt.

    I have 2 # 90 watt batteries and a 1800 w inverter.

    Is it safe to leave the inverter on say overnight?

    Any help will be cool.

    Thank you
     
  2. JeanLuc

    JeanLuc Funster

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    The basic rule is amps = watts/volts (and you can rearrange this formula to calculate any one variable if you know the other two. When using an inverter which is supplied by 12V DC you have to use that lower voltage rather than the 230V rating of a mains device, since it is 12V that is supplying the inverter.
    So in the case of your fan:
    16W / 12V = 1.33 amps
    that is the current that the fan will draw from the batteries via the inverter. In fact it will be a bit more than this since no inverter is 100% efficient. Normally they are 90% if you have a decent one. That means the 1.33 amps should be divided by 0.9 to give a figure of 1.5 amps rounded.

    It is not a good idea to leave an inverter switched on when not in use as they all draw a small current when idle; that will drain your battery. Larger inverters and lower quality ones will have a higher drain when at rest. An 1800W inverter is huge overkill for a small fan as the combination will be rather inefficient. You would be better off with a small inverter such as a 150 watt or perhaps a 300 watt at most. Of course, if you want to run large mains items you will need the big inverter but to go back to original formula, if you were to use the inverter to its maximum, the calculation would be:
    1800W / 12V = 150 amps (ignoring the inefficiency for the moment).

    You state you have 2 x 90 watt batteries but I suspect you mean 2 x 90 AmpHour (Ah) batteries. That means they can each deliver 90 amps for one hour or 45 amps for two hours for example. It is not a good idea to discharge your batteries below 50% of their capacity in order to prevent unrecoverable damage so your 2 x 90Ah batteries can safely deliver 90Ah without risking damage.
    Your 1800W inverter at full capacity would draw 150 amps as we calculated above so you could run it for a maximum of 30 minutes. In practice, most people use large inverters for items like hair dryers and microwave ovens and these tend to be used for short periods of time such as 10 minutes.

    The other issue to consider is that you then have to put back the charge you have removed from the battery and that means mains recharging, a very long drive or plenty of solar power and bright sun (or a combination of all).

    For a full understanding of inverters, have a look in the resources section (tab at the top) where there is documentation explaining it all.
     
    Last edited: Jul 28, 2015
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  3. SomeoneElse

    SomeoneElse Funster

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    As a simple rule of thumb, I allow one amp at 12V for every 10Watts at 230V.
    This works well for reasonable loads on my 1500W pure sine wave inverter.
    However for such a small load 16W on a large 1800W inverter, a different estimate is probably needed.
    My inverter uses around 1,5A when switched on with no load attached.
    Your 16W load would need a similar current, so say 2 times 1.5A ie 3Amps.
    That 3AH or say 30AH over a 10 hour night.
    You need to ensure you can replenish that amount during the day ready for the next night.
    Then allow for other loads.
     
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  4. gloworm

    gloworm Funster

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    Daz, think it would be less trouble to get yourself a Punka waller.

    Eric
     
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  5. TheBig1

    TheBig1 Funster Life Member

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    Buy a low wattage 12v fan and get more from your batteries
     
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  6. Daz n Tina

    Daz n Tina Funster

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    I heard there were a few I could pick up in Calais.
     
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  7. Daz n Tina

    Daz n Tina Funster

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    I know, I have been waiting for a call back from company's selling the easy breeze but they don't seem to respond.

    As were off to Blane's next week I thought a low wattage fan will suffice if it is to hot.
     
  8. Daz n Tina

    Daz n Tina Funster

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    Problem solved today. Went down the motorhome switched on the inverter, the sound of a fuse popping. Checked inverter and 5 of the 6 , 32 amp fuses blown. Changed them still don't work.

    Tina of to the barbers for a grade 3 skinhead tomorrow, 1 x 1000watt hair dryer for sale.

    Will I replace the inverter, no.

    Happy days. :):clap2:
     
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  9. aldhp21

    aldhp21 Funster

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    I ordered one off Amazon.com and has it shipped from the states. Cheaper than buying one in the UK and it only tood about a week to arrive.

    http://www.amazon.com/Fan-Tastic-01...TF8&qid=1438628575&sr=8-2&keywords=12volt+fan

    Cheers
    Alan
     
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