# Confused about DC to AC & amps

Discussion in 'General Electronics Chat' started by hargsnz, Oct 31, 2013.

1. ### hargsnz Thread Starter New Member

May 29, 2011
3
0
I need some help to figure out the amps used in the following situation.
I have a hot water heater element on my boat that is 240v 1500 watts. I want to use an inverter on my battery bank ( 360 amps total ) to convert 12V DC to 240v AC. If I do the maths on the amps drawn using the 240v it says 6.8 amps.

So it's all good.

Then I was looking at heater elements that are native 12v 600watts. That maths says 50amps. So that's not good.

So is it correct to do the amp calculation only after the conversion to 240v?

I'm confused.

2. ### ronv AAC Fanatic!

Nov 12, 2008
3,291
1,255
You need to make them both watts. Then solve for amps at 12 volts. A lot.

3. ### panic mode Senior Member

Oct 10, 2011
1,320
304
you always deal with power (and energy).

if there was two heaters rated for 1500W and one is for 240V and other is for 12V, both would produce same heating effect.

however, one operating at 240V would need only 6.25A (P=V*I, so 1500W=240V*6.25A)

one operating on 12V would need to draw 125A (12V*125A=1500W). this means you need thicker cable/wires.

now suppose you get an inverter and suppose it's efficiency is 100%. then you would get 6.25A on 240V circuit (output of inverter) but 125A at input of inverter (12V circuit).

now the problem is that only 100% efficient energy conversion is mater/antimatter reaction. since you don't have that
you have to live like rest of us....and inverter efficiency is not 100%, maybe some 60%.

1500W/60% = 1500W/0.60 = 2500W

so to get 1500W heater operating at 240V, you would need to draw total of 2500W from 12V source. the difference of 900W is wasted (converted to heat actually, but not where you want it). so if you plan on running 240V devices from 12V, you have to pay penalty in energy conversion so your 12V battery will have to supply 208.33A. That is quite a bit more than 125A we discussed earlier.

conclusion: if you are powering something from different voltage (higher or lower), you are going to spend more energy than if you are powering it directly. if the 12V is what you have, choose the 12V heater!

hargsnz, anhnha and strantor like this.