# calculation for imverter

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

1. ### Daz n TinaFunster

Joined:
Jan 8, 2014
Messages:
819
1,968
Location:
Emsworth
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. ### JeanLucFunster

Joined:
Nov 17, 2008
Messages:
2,991
1,994
Location:
Warwickshire
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
• Like it x 5
3. ### SomeoneElseFunster

Joined:
Apr 22, 2013
Messages:
592
391
Location:
Cotswolds
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.

• Like it x 1
4. ### glowormFunster

Joined:
Dec 30, 2008
Messages:
946
1,773
Location:
Cheshire
Daz, think it would be less trouble to get yourself a Punka waller.

Eric

• Like it x 6
5. ### TheBig1FunsterLife Member

Joined:
Nov 27, 2011
Messages:
8,058
10,419
Location:
Dorset
Buy a low wattage 12v fan and get more from your batteries

• Like it x 2
6. ### Daz n TinaFunster

Joined:
Jan 8, 2014
Messages:
819
1,968
Location:
Emsworth
I heard there were a few I could pick up in Calais.

• Like it x 2
7. ### Daz n TinaFunster

Joined:
Jan 8, 2014
Messages:
819
1,968
Location:
Emsworth
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 TinaFunster

Joined:
Jan 8, 2014
Messages:
819
1,968
Location:
Emsworth
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.

• Like it x 1
9. ### aldhp21Funster

Joined:
Feb 17, 2009
Messages:
289
1,157
Location:
Cheltenham
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

• Like it x 1