working out 12 volt draw through inverter

What I do is measure the DC current coming from the inverter as the lappie is the only thing I run from it. But the lappie is 230 volts 1.5 amps, so should pull 1.5 through the inverter plus some inverter current, but it doesn't, it pulls about 6 So it 's hard to tell when thinking of a new computer what will fit and what won't. I tried an imac recently and that pulled 10 amps from the inverter.

Surely you mean INTO the inverter? Its output is 230VAC, however if you are measuring that current (which can't be more than ~1.5A AC), then the meter may be reading wrong because it is not on a DC range or not true-RMS (I assume the inverter is a stepped wave, ie "pseudo" sinewave). IMO all you need do is design your new PC to match (or not exceed) the laptop's power consumption - ie, if the laptop is 75W, then the PC should be 75W or less - I don't see why you need to know the 12V current. The PC consumption can be increased a bit if you use a DC-ATX supply or a dc-dc converter rather than an inverter (depending on why you are specifying that power limit).

The inverter is a 1kw pure sine wave. Far bigger than I need I know but all I've got now. And I'm sorry, measuring the 6 amps is the flow form the batteries to the inverter, not coming out of the inverter. That's what i always measure to work out what effect a device has on the batteries. I see your point about sticking to a wattage that I know is safe. The problem is this one is 230 volts at 1.5 amps which is 345 watts. An iMac is rated at about 171 watts, yet when switched on the drain from the batteries to the inverter is 10 amps. Luckily I had one to try before buying. I have considered using a dc to dc converter, but I recall when I had my old Dell. It drew 3.5 amps using it's mains charger through the inverter, and almost 5 amps using the 12v Air charger!

So you have measured 1.5A on the inverter output to the laptop's plugpack?

Hmmm. No I haven't. I only ever measure what goes in. I don't think the inverter is bust though as I used to have a 600w pseudo sine and the current draw was almost the same on that one too. Is it worth measuring what's coming off the inverter? Will it help in any way?

So where is that 1.5A from? As per before, plugpack ratings are not the same as their load ratings... Why worry about the AC current? All you need to know is the laptop's DC input, else the inverter's DC input. But surely the laptop has a power consumption figure whether on some label, or manual, or spec sheet?

This is getting confusing so perhaps best to go back. The laptop charger unit has stamped on it input is AC 100-240v, 1.5 a. So that's 240 x 1.5 = 360 watts. The output is DC 19v 4.7a. 19 x 4.7 = 89.3 watts. Measuring the load to the inverter from the batteries when the laptop is in operation, I measure a load of 6 amps coming from the DC 12v batteries. 6 x 12 = 72 watts. So, I've got 3 completely different figures, but I thought watts would always be equal? Now, lets say I want a new computer and I know that it runs at AC 240v 2 amps. 240 x 2 = 480 watts. Because all of the other figures are inconsistent, how can I tell how many amps will be drawn when using the new computer?

The 240V x 1.5V is totally irrelevant so forget that figure. (Forget entirely what the plugpack says. That is only useful as an upper limit to what a load could draw.) Since KP has a point about energy not being created out of nothing, I suspect your output 19V 4.7A is also from the plugpack data and not from actual measurement. That leaves you with 6A probably at ~12.5V, hence 75W. I would therefore assume the laptop itself takes ~60W (assuming 80% inverter efficiency). And again, what is the PC's 240V @ 2A based on - it's 480W PSU rating? Irrelevant! What is the PC actually taking? But a PC with a 480W ATX supply (or any PC for that matter) is unlikely to draw as little as a 60W laptop. You need to design your PC with equivalent low-power laptop chips & components. And if the PC is drawing 480W, then you have your answer - the PC is drawing about 8x what the laptop draws. PS - get a dc-dc converter for the laptop and bring the inverter & plugpack as a backup. DC laptop converters are available for ~$30 upwards. (I bought a 120W 12V DC converter WITH AC input (that combination is rare else more expensive) for $29. Granted, it runs hot, but it works well.) PPS - KP - have you considered writing a paper on that energy theory?

I think some old guy beat me to it! If you KNOW the current on the DC side and you KNOW the current on the AC side you can calculate the actual efficiency of the inverter. You can then determine roughly what an actual AC load will do to the DC side. As oldspark said the numbers listed on the power supply for current is absolute MAX current under worst operating condition. I would expect the current to drop in half when you use 230VAC as opposed to 100VAC.Kevin Pierson

KPierson wrote: I think some old guy beat me to it! If you KNOW the current on the DC side and you KNOW the current on the AC side you can calculate the actual efficiency of the inverter. You can then determine roughly what an actual AC load will do to the DC side. As oldspark said the numbers listed on the power supply for current is absolute MAX current under worst operating condition. I would expect the current to drop in half when you use 230VAC as opposed to 100VAC.

Is there a formula for calculating the efficiency then?