# Watts Volts Amps equation HELP!



## bigcats30 (Aug 13, 2012)

Right guys I'm happy that -

Watts divided by volts = amps (and all the other ways)

But when working out what comes out of an inverter do I use 12v or do I use 230v?? (IE Inverter connected to the battery)

Because obviously the amperage is vastly different for the two (12v/230v)

I get the feeling I stick to 12v equation as the inverter is still drawing the watts thats required to run the item. But I'm unsure.

(I haven't bought an inverter yet to test it)

Cheers

Cat


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## airstream (Jan 7, 2009)

*rough guide*

Hi,
Rough idea is 12 volt amps are 18 to 20 times the the 230 volt amps

Ray


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## safariboy (May 1, 2005)

To take an example if you are using a 250V inverter to run a 500W appliance the current through the appliance is 500/250 = 2A.
The current on the 12V side will be 500/12 = 41.6A. Actually the inverter also uses some power and so the 12V current will be a bit larger.

If the appliance is turned off the 12V current falls to just power the inverter.

This is why you must wire the inverter with short thick wire straight on to the inverter.


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## listerdiesel (Aug 3, 2012)

It gets a bit involved as the inverter will introduce losses, and the output will be affected by the power factor of the load.

12V side of things is DC so simple Volts X Amps = Watts is fine.

240V side of things is AC, so Volts X Amps = VA, multiplied by the load power factor will give Watts.

Resistive loads such as filament bulbs and kettles are Unity Power Factor = 1, so you can directly convert VA to Watts, but if the power factor is less, then you start to see power losses , being bad with inductive loads such as motors etc.

Peter


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## bigcats30 (Aug 13, 2012)

Danke all


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## listerdiesel (Aug 3, 2012)

Just a little illustration of how Power Factor affects things:

When I worked in the Film & TV industry, we built four new soundproofed generators, we assembled the engines and alternator/control panels, and we then took them out to Sweden on new Bedford chassis-cabs for the soundproofing cabinets to be built at a company in Stockholm.

The units were rated at 50kVA each, so we could put 50kW of filament lighting on them which was a full load.

When we tried Metal Halide lighting, which has a large electronic ballast, we expected to see a reduced output as the Power Factor of the ballasts was quite low, and in fact we saw just over 30kW of lighting at full load.

This was the same generator that an hour before had been happily running at 50kW of filament lighting.

I built the control panels and had included a power factor meter in anticipation of the problem, and with the Halide lighting it showed just over 0.6, which multiplied by the available 50kVA gives 30kW.

The lighting guys didn't believe their eyes, and it took a while for them to accept the difference.

A mixture of both types of lighting came to be the best, as the power factor of 1 or Unity for the filament lights helped raise the low PF of the halide lighting to about 0.75.

At the consumer end of the market, it can have significant effects on small inverters running something like a computer or similar, which I pointed out in another thread.

Peter


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## premiermotorhomes (Feb 13, 2008)

Good morning,

Don't forget when calculating draw on the batteries that inverters are not an efficient form of energy conversion, you could be looking at 50% to 90% efficiency level depending on the load placed on the inverter, the lower the load the lower the efficiency.

Regards,
Chris


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## robinpompey (May 7, 2011)

Not being technically minded if you need very think wires connected to the inverter what about the cigarette socket that you plug into. I have a Bessecar E769 running on a Fiat Ducato base. Will the installed 12 v circuits take an inverter?


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## listerdiesel (Aug 3, 2012)

Cigar lighter sockets are fine for small inverters that take under 10A, but for anything bigger, say 150W and above, a dedicated power point is better.

Peter


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## robinpompey (May 7, 2011)

Listerdiesel Thanks


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## fionn (Nov 29, 2011)

Hi all

I need to install an inverter but have not got a clue what to do.

I want to run a tv/DVD player of it so kids can watch while im on the road and also be able to charge iPad and iPhone whle driving.

What size inverter do I need and any other advice would be much appreciated.

Kevin


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## listerdiesel (Aug 3, 2012)

For small appliances, a 150W should be fine, but try and get a pure sine wave inverter if you can, they are more expensive but will perform better on most devices.


Check also that the TV/DVD player doesn't already run off 12V, Avtex and many others come with 12V input.

Peter


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## fionn (Nov 29, 2011)

Thanks listerdiesel.

No the tv doesn't run of 12v. Are the inverters easy to set up or would I need an electrician?

Another question what do most motor homers do for drying hair? My lovely wife has my head wrecked that she needs a hair dryer ! 

Am I right saying that we can use a hair dryer once we are on site and hooked up?


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## listerdiesel (Aug 3, 2012)

Most small inverters are plug and play, they have an integral 13A socket.

A small hair dryer is going to be more than a 150W inverter would run, but once you're on hook-up your MH sockets will be available.

Peter


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