jet can you explain the " Read the data sheet thing " & saying No to SWOMBO .
On nearly all elec appliances they will have a little silver coloured sticker with some info my fan here has
220-240VAC 50Hz 50W
So the item draws 50watts at 220 - 240VAC
What we need to know is how that relates to 12v power so how many amps will that cause the inverter to take from my battery bank.
So step 1 - is that watts figure (50) below my inverter capacity in this case yes

Step 2 - take the 50W and divide it by 12 (this is the basic voltage number of our batt bank )I know that's low ect but using 12 is the easiest) 50 / 12 = 4.16A DC.
So roughly it will take 4.16A out of my battery bank to power the inverter to run that fan.
we estimated 4.16A at 12v but because my battery bank had 13.6v in it that figure was actually 3.24A real time tested which is better again than my basic divide by 12 rule.
So as you can see if you factor off the worst case voltage of 12v then anything above that is cream. and you simply read the silver sticker on the appliance and you divide by 12 presto you have a rough idea.

Also to note was because i had solar panels conected to the batts the start voltage was 13.6 and with inverter running fan it only dropped to 13.45v as the solar picked up the output and supplied to needed load for the inverter.As before that I was in float mode and the solar reg was wasting away power as the batts were full.
Which brings me to the next point ..where possible use your inverters when there is a charge being applied to your batts weather that be alternator or solar panels , the higher nominal voltage of the batts will help look after you batt bank.
DISCLAIMER- I am not suggesting/advocating that you power your larger than 100w inverter while driving along , that is a personal choice and i won't go into that, what I meant was like at the cape when lara wanted to use the infamous hair straightener i would start the car to warm the engine and plug inverter into the car then and let here use hair straightener, this way it had a higher voltage instead of the overnight settled voltage of the aux battery after it had run fridge all night. So in essence the alternator was power the inverter not the aux battery.
here is the pic of the watt meter running the fan ,
Jet