working out 12 volt draw through inverter

I use a laptop through an inverter, and the inverter draws 1 amp when working. The charger for the laptop says 100-230 volts 1.5 amps. However when using the laptop, it draws not 1.5 amps, but 6.5. What's going on? Is there a way of finding out how much current an item is going to draw when used through an inverter?

You've left out the voltage side of the equation. I would guess that your 1A output is at 120VAC and your 1A computer draw is also at 120VAC while your 6.5A draw is at 14.4vdc. Ultimately, you need to convert everything to power (watts) to have an idea of what the 12vdc side of the inverter will draw. 1A at 120VAC is 120 watts (Current x voltage = watts). Your wattage on the input side should be higher then your actual output wattage due to efficiency drops in the inversion process. However, at 6.5A at 14.4vdc you are only using 93.6 watts on the input. This makes me think that you are actually drawing less then 1A at 120VAC, otherwise you are actually creating energy inside your inverter and I believe there is a physics law somewhere that states that that is not possible!Kevin Pierson

Adding to KP's reply, a general rule of thumb (ROT) for 12V loads is divide the output power by 10 to get the approximate input current. FYI... That /10 instead of using the "correct" 12V or 14.4V etc is an easy approximation that factors in typical efficiencies of ~80% - eg, amplifier or inverter output versus their input power. FYI... Your laptop uses a max of 150W (100VAC x 1A), hence 150/10 = 15A input. You are under half that because the 1A plugpack is a standard size and over-rated for your laptop. The 1A "idling" current means your inverter consumes ~12W without anything connected, hence why people don't leave inverters on (for long) unless the battery is charging.   

The 1 amp output is at 230 volts, so that's 230 watts? But yes, 6.5 amps at 14.4 = 93.6 watts. Is that nearer to what you'd expect?

This can't be right. 1.5 amps at 230 volts, 1.5 x 230 = 345 watts. Then 345 / 12 volts = 28.75 amps.... Am I missing something?

No, that's right. Since 240:12V = 20, you'd expect 20x the 12V current compared to its 240V current (close enough to 230V). It's 10x for 120V systems. But remember, just because a supply can supply certain power does NOT mean it supplies that power. (IE - it only supplies what the load consumes.)

So really, there isn't an accurate method of knowing what anything will draw at 12 volts without measuring it inline?

Well, that's relative. Knowing the 230V power you can guesstimate the 12V current - ie, assuming 80% efficiency and deciding if it's at 14.4V or 12.6V etc. But you don't seem to know the laptop's load. It's probably around 90W but consumption varies markedly with laptop specs etc. It depends on why you want the info. If it's to design the distribution, near enough should be good enough. You could base that on the inverter's max output - eg, if 300W max out => ~400W in => ~30A hence a 40A circuit. But you could base it on the expected consumption, but you need to find out what the laptop consumes. EG, if 90W => ~120W input => 10A hence a 15A (or 20A) circuit. (Ensure the fuse is rated for or less then the cable in case anyone adds a load to the inverter. You want the fuse to blow, not the wire to melt.) If power (efficiency) is a concern, the a dc-dc converter is the way to go. And dc-dc converters designed for automotive use should handle cranking voltage dips etc - most inverters cut out around 10.5V or higher where as dc-dc convs can tolerate down to 8V and sometimes 6V. A dc-dc solution should mean ~20% less power consumed.

I need to know simply so I can design a PC that runs from 12 volts, but one that won't draw more than the one I have currently, which uses approx 6 amps.

6A at 12V? (72W) So what can you measure - DC Amps? Measure the inverter input current with the laptop running. Or find what your laptop consumes - maybe by measuring its input voltage and current (on the DC side). Dare I ask why it can't draw more than the present device?