!2v - 240v 'inverters' How Much power do they use?

[Gareth111278]

Hi

I don't suppose anyone has a formula to work out how much 12V power an inverter pulls for a given 240V out put [watts].

I assume that with no 240V load they pull next to nothing, is this correct?

I'm thinking of getting one to run the TV whilst off hook up. (wish now i'd got one with an external 12 transformer!)

Cheers Gareth

 

[RogerL]

I think they're about 80% efficient so you'll use 1.25 watts input for each watt output - that means 1Ah at 12v will drive 9.6 watt hours at 240v.

 

[rayc]

Gareth, I have a 19" LCD TV/DVD combo which is rated at 38W.

If powered from the inverter it 'uses' 38W but this time the current is drawn from the battery not the mains.

At mains voltage it draws [38W divided by 240V] approx 0.2Amps.

The current being drawn from the battery if via an inverter is 38A divided by 12v which is approx 3 Amps. There is a small current draw from the inverter even if there is no load but it should not be significant.

I assume you mean that you wish you had bought a 12v /240 TV so that you would not need an inverter when not on EHU. I doubt there is significant difference in battery life by either direct or inverted method.

 

[rayc]

Gareth, I have a 19" LCD TV/DVD combo which is rated at 38W.

If powered from the inverter it 'uses' 38W but this time the current is drawn from the battery not the mains.

At mains voltage it draws [38W divided by 240V] approx 0.2Amps.

The current being drawn from the battery if via an inverter is 38A divided by 12v which is approx 3 Amps. There is a small current draw from the inverter even if there is no load but it should not be significant.

I assume you mean that you wish you had bought a 12v /240 TV so that you would not need an inverter when not on EHU. I doubt there is significant difference in battery life by either direct or inverted method.

As Roger has said, much clearer than me, check the power of the TV in watts on its rating plate and get an inverter that is rated at 25% above it. If it is not LCD, but an older tube type, then the power will be appreciably more and there will be a switch on load that the inverter may not be able to supply.

 

[Gareth111278]

Thanks guys , its a 19inch LCD so i expect about the same power use as yours Ray, I'm after a 300w in verter on ebay so it should have loads of power for the TV.

I'm going to silverstone to the GP for 3 days off EHU so i'm taking a tiny gernerator , as i'm worried that its 'rough' output will blow up the TV i'm just going to use it to top up the battery via a battery charger.

I didn't want to be in the possition of having to have the genni running just to watch TV, but it looks like i'll hardly need it.

I've got 2x7Kgs of propane so that should keep the fridge going, cold beer is a must!

 

[RogerL]

If it's a cheap generator with rough output, it could well blow the charger!

If you're stuck with rough output, use the 12v output to charge the battery - it'll take longer but should be safer.

 

[Gareth111278]

Hi roger,

it hasn't got 12v out [its THAT cheep] I'd rather risk

 

[JeremyClarksson]

Using a high power appliance like the space heater/water heater will help smooth a rough supply but watch out for the thermostat operating and connect the heater first/disconnect last.

The generator note will change as it goes on/comes off load.

Check the voltage tolerances on the power supply a lot will happily tolerate 110V - 250V nowadays and the heater should smooth any spikes.

You might also consider a plug in voltage smoother as used on computers etc.

We use a generator infrequently and I would not want it on while watching the TV myself, usually leave the water off overnight and then run the generator with the heater on to boost the battery in the morning whilst doing water, toilets etc.

I also plug in our little charger connected to our second battery at this time too.

This in addition to our 20W of solar panels means we only ever use the genny once every few days for appx an hour.

We only do this at CLs or long rallies.

 

[rob_jax]

Just a few observations from the electronics side:

"You might also consider a plug in voltage smoother as used on computers etc."

A lot of people think that, but those devices are meant to do these things:

1. They will protect against spikes - the cheap ones only trigger their protection above 500 volts, as you go more expensive then the protection voltage gets closer to mains voltage

2. The spikes only last a fraction of a second - maybe 1/1000 of a second - and that's very different to a generator producing an over voltage of 350 volts for ten or so seconds

3. Once they are triggered by over voltage, they are dead - kaput. They work because a voltage sensitive device blows like a fuse.

So no - those voltage smoothers won't help you with a generator.

Robert

 

[Gareth111278]

Thanks guys, I thought surge protectors didn't work but thanks for conferming this.

I don't think i'll connect the genni to the caravan at all , just use it with a battery charger , if it blows its not the end of the world!

I might not need it anyway, when i was a student i used a caravan with a big tractor type battery running a TV and all my lights for a week at a time between weekend charges, it never seemed to run down.

Just one last question , if i use 50M of cable to get the genni away from other people's units will that sap most of my 350W output --- , Will that smoooth the output?? [Might only have enough current to run a LED!!!]

 

[rayc]

Gareth, I have joined two caravan type 25m leads together and there has not been any appreciable volts drop.

The actual volts drop will depend on the size of the wires making up the cable and the current being drawn. The higher the current and the smaller the cable the more the volts drop across the it.

In any event 50m of 2.5mm cable drawing say 5A will be negligable volts dop. [I regularly check electrical cables for resistance of less the 0.1ohm over 30m or so].

 

[Gareth111278]

Thanks Ray,

So its good that i'll get no volts drop [how we laughed in college at the tutor who refered to it as VD!]

But i can't rely on the cable to smooth my output!

Gareth

 

[ProfJohnL]

Hello Gareth,

Whilst all cables will sap some power, it depends on what gauge of cable you chose. The bigger the cross sectional area of the conductors the less power loss will occur. The majority of EHU cables are designed to fit the 16A CEE form blue plugs and sockets, If this is the cable you are considering (Orange 2.5mm") then you shouldn't have any significant power loss.

If possible keep your cable length as short as possible, as the cable its self represents a trip hazard, and of course its quite heavy.

 

[Gareth111278]

Hi John, Yes i've got 2.5mm with the standard blue plugs, I've got 2 x25m but i only tend to carry one in the caravan, as you say its heavy!

Gareth