Let's have a look at Sam's numbers again:
They'll probably draw about 60 watts or so, so about 1/4A at 240V, that converts to roughly 5A at 12V so a 20 AH sealed lead acid battery, run down to 50% (the inverter will shut down when the voltage drops to 10V which represents, very roughly, 50% charge being used) would give you about 2 hours. But it's impossible to calculate with any accuracy and you may get three hours, or maybe only one....
He's estimated that the two items you've specified will require about 60 watts. (He's guessing because you can't find any information about such things for the Ibiza anywhere.)
Watts = Amps x Voltage. So 60 watts at 240 volts is 0.25 amps
but you'll have a 12-volt battery supply so this time the equation is 60 = ? x 12. Thus the ? is 5 amps.
A typical Leisure battery will be 100ah - that's 100 amp-hours. So
in theory you can draw 5 amps for 20 hours.
But as Sam has pointed out you can't drain a battery flat. He's suggested 50% which gives you 10 hours.
BUT these are simplified theoretical figures. We don't know the actual current requirements in watts that the kit you're thinking of buying will have. Also the current-draw will be variable depending on the style, volume etc of the music. Then there;s the power requirements of the inverter. That itself will use some power in its operation/conversion processes. Most inverters have a cut-off to stop you draining the battery. The voltage at which they'll shut-down also has an impact on how long you can power the kit.
So it's not a precise science unless we have a lot more information. But the figures above give you a very approximate way of calculating some numbers.
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