best to use fridge through 240v via inverter or 12v direct ?

[Ian Paul]

If I connect my fridge which draws 80w direct to the battery it draws about 6.5 amps but connect via inverter by 240v it draws about 0.8 amps, it would appear the best option ?

Connecting my 500w oil radiator is watt divided by volts = about 2 amps, compared to expensive 12v heaters. So its a no brainer to use ...or am I missing something ??

 

[Wh05apk]

Watts are volts X amps, so running on 240v will be 240x0,8=19.2amps about 3x the power on 12v befirb you consider inefficiencies of the inverter.

Why are you running on 12v, can you not use gas or do you mean when driving?

 

[scotjimland]

Power , measured in Watts = Volts x Amps

Assuming you have a 3 way fridge.. example ..
When driving your fridge runs on 12v .. it has a 120 watt element. using 10 Amps from the vehicle alternator
When on EHU it has a 240v element rated a 125 watt using 0.5 amp
When not driving and not on EHU it uses Gas

You cannot heat a van using 12 v heaters. or 240v heaters via an inverter.

a 500w 240v heater fed from the battery using an inverter would use

500/12 = 42Amps.. .. 85ah battery effectively dead in less than one hour

 

[Richard n Ann]

Put simply, you're looking at the high side amps (240v amps) not what's actually being taken from the battery on the 12v side

 

[Ian Paul]

scotjimland thanks, so surely if I connect via inverter a 500w heater/240 = 2amps ? I have 120amph battery. I would only use the oil heater for about 15-20 minutes in the morning on a time before I get up. I have yet the a heater so any advice would be helpful, otherwise I will just keep to an old tular 140 heater and wear vest when it gets cold.

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[tonka]

YES... You are missing that your drawing from 12v supply and not 240v so your maths are all to cock.

500w divide x 12 is almost 42 amps and at 15 mins uses about 10 amps Plus loss for the efficiency of the inverter which could add 5-20% depending on quality.
Like what @scotjimland has said..

If you were connected to main EHU then your maths are right...

 

[138go]

What you could do of course is power the MH from an Inverter that way you will be charging the batteries as well as running the heater on 240v. In effect perpetual energy ..

Think I might patent that ...

 

[Lenny HB]

Easy way to work out what you are taking from your battery is the current at mains voltage times 20 = current at 12v.
500w heater is 2.08 a at 240v, 2.08 x 20 = 41.7 amps at 12v.

 

[pappajohn]

I connect via inverter a 500w heater/240 = 2amps ?

Correct... The heater is taking 2 amps from the inverter...... BUT...... The inverter is taking over 42 amps at 12v from the battery to produce the 2 amps at 230v.

Watts are constant while amps and volts are variables.... The higher the volts, the lower the amps (and visa-versa) for a given wattage.

 

[pappajohn]

I would only use the oil heater for about 15-20 minutes in the morning on a time before I get up

It will take that long for the heater to warm up and a hell of a lot longer to heat the van to any noticeable degree. I think you should be looking at an hour or more to make it worthwhile.

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[138go]

Why don’t you just fit a Diesel Heater and be done with it. Using electricity to generate heat uses a lot of power.

 

[Ian Paul]

diesel heaters cost £300 + fitting ? Unless you know better where to get them ? Last nite when it was cold 5 degrees, I put the tubular heater 150w on overnite for 7 hours It almost drained the battery, but the van was warm when I woke up. the solar will charge it up later.

 

[autorouter]

Current (amps) is the flow of electrical charge. Voltage is how much energy the charge can carry.

A 240 volt mains circuit can carry 240 watts per amp.

A 12 volt battery circuit can carry 12 watts per amp.

Heating effect depends on the watts. A 120 watt mains heater and a 120 watt battery-powered heater will have exactly the same heating effect.

The mains heater will take 120/240 = 0.5 amps. The battery-powered heater will take 120/12 = 10 amps.

An inverter is a device that changes the voltage from 12 volts DC to 240 volts AC.

You can run a 120 watt mains (240 volt) heater from a 12 volt battery by using an inverter. The current (amps) from the inverter to the heater will be 0.5 amps. But the current from the battery to the inverter will be 10 amps. In fact it's more like 11 amps because the inverter wastes about 10% of its energy in the voltage conversion process.

I put the tubular heater 150w on overnite for 7 hours It almost drained the battery,

150 watts for 7 hours is 150 x 7 = 1050 watt-hours. From a 12volt battery that's 1050/12 = 87.5 amp-hours. As you say, that's almost draining a 100 amp-hour battery. You'll need a very good solar setup to fill up the battery in one day.

 

[tonyidle]

You'll need a very good solar setup to fill up the battery in one day.

Indeed. Or even a week.

 

[pappajohn]

the solar will charge it up later.

Unless of course, the solar develops a fault or it rains all day... Then you still have a flat battery the following evening.

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[138go]

You will be very lucky if you get 7 hours out of the battery tonight. 100w panel on a bright sunny day will give about 5 amps tops in Uk..The battery will just flatten sooner. Trying to heat a MH using battery power with an Inverter is not really the answer.