brain ache electric problem

[deep denial]

Please help straighten me out on this presumably very simple little question. This concerns power invertors. If you run an appliance of, say, 240 watts off an invertor from a 12 v supply, that presumably represents 240/12 amps, ie 20 amps drain on the battery. But looked at from the point of view of the appliance running at 240volts, it's drawing 240/240 ie 1 amp. So Which is it - 20 amps or 1 amp???? where have the other 19 amps gone?????? /forums/images/graemlins/confused.gif

 

[stevebrassett]

The power remains constant (ignoring losses), rather than the current. Think of it in terms of water pipes. If the width of the pipe represents the voltage, then if the second pipe is a lot wider, then the flow needs to be a lot less to move the same amount of water.

In other words, the 240 watts remains constant, and the voltage and current vary.

 

[deep denial]

yes, but how many amps are drain ing from my battery?

 

[Conrad]

The formula for watts is voltage multiplied by amps whatever the voltage current or situation.

So if the watts (power) stays the same and the voltage goes down current will go up and vica verca.

This is why the pylons run at such large voltages because the large voltage keeps the current down and hence the size of the conductor.

 

[deep denial]

yes, that's my problem! how do I calculate how many amps are draining from my battery - using 240v , or 12v??? (and why)

 

[Conrad]

20 amps you was right in the first place

 

[stevebrassett]

Correct - 20 amps, I.e. 240 / 12.

 

[deep denial]

Ok, thanks, I think I've got it - there are 2 separate circuits. phew!

 

[Mal113]

In the first place lets consider that the appliance was running on 220 volts AC only, then the wattage of 240 watts would require a 1 amp current flow (approx) Now you wish to us the appliance via your inverter and therefore have to supply the same wattage to the appliance but this time your supply voltage is (approx) 20 times less at 12 volts, therefore to supply the same power 240 watts you will need 20 times the amperes as the voltage remains constant. So if you were to measure the current while the appliance was plugged into the inverter you would see 1 amp on the 240-volt side and 20 amps on the 12-volt side. This is approx as inverters themselves will consume power and cheaper models are less efficient. The system must remain in balance on both sides of the inverter; always remember there is no free lunch when it comes to power consumption.

 

[VicS]

Yes you are correct 20amps BUT inverters are only about 85% effeicient

So to deliver 240 watts at the output it will consume 240 /0.85watts from the 12 volt source ie a bit in excess of 280watts.

That means a current of almost 24 amps.

Your inverter my be less than 85% efficient, if so the current will be even higher.