How do you work out

The load is 100W.

Amps = watts / volts so

Amps - 100/12.8 = 7.8Amps.

Plus add 15% for ineffeciency = 8.5A

So in one Hour you will have used 8.5 Amp Hours.


Im other words get LED or Flouros, you will have only used about 1 - 1.5 Amp hours for similar light.

For a fully charged battery, we'll assume 13v.
The capacity of the inverter (300w) doesn't really come into the calculation, only the load.
If the inverter was 100% efficient in conversion, then the input watts would be the same as the output watts.
Therefore, with Amps=Watts/Volts,
Amps = 100/13
= 7.79A
so if you're running it for an hour, you'll have used 7.79 Amp-Hours.

But the real inverter conversion efficiency is 80-90%, so let's assume 80%; the draw will be
7.79/0.8 = 9.6 amps.
Also, the inverter has a fixed power consumption regardless of the throughput (when you have no output load, there will be some current draw from the battery), so max efficiency is usually at max power; it will be a bit less efficient at small load current.

Not sure why you'd run a 100w bulb from an inverter tho :-)

Gerry

As above, but best way is to measure it with an ammeter.
Out of interest, my 300Watt inverter pulls 110 milliamps when idle (connected with no load).

Agree with Phil the only real check is with an ammeter and voltmeter.

One of these will answer all your automotive questions exactly!

No if's, no but's, no maybe's.

All you need is someone to give some hints on their use (provided you don't know yourself)

Otherwise on the calculation front others have supplied some great information.

G'day Jedo,
I am sure you are not planning on running a 100w light, it is the calculation of power consumption you are after.
I use a clamp dc amp tester, I think Dick Smith or Jcar had them for around $100, they used to be closer to $300.
It has a range of 400A dc. and you put the clamp around one wire.
This is useful for checking say a laptop computer power consumption, as it is variable depending how flat the batteries are. The dell laptop I use pulls 12A dc when the battery is low. This is handy to know when your inverter has a 10A fuse and you dont expect it to use that amount.
It has a lot more applications on your vehicle, you can check the charging rate of your alternator without disconnecting anything.


Cheers,
Bill

Thanks all
So if I'm running 3 x 100W light bulbs I just multiply by 3...
It was the calculation I was after.
Jedo