I want to power a room with a car battery (or more), is it possible?

[LG]

I think you're sometimes getting volts confused with watts. The 20W light bulb sounds right. But laptops don't use 240W. More like 20-30W. But the power supplies are usually switchers and are rated for 100-240V so they can be used in Europe or US.
But if I were to take a stab, all that stuff in your room probably takes something like 200W. A typical car battery nowadays is like 60 amp-hours, meaning the battery can supply 60amps for one hour, or any combination of amps and hours where amps times hours equals 60. At 12V, this works out to 60x12 or 720 Watt-hours. So at 200W one car battery would last for 3.6 hours. And then just multiply that by the number of batteries and that's how long it would last, assuming continuous use. You would also need an inverter to convert the 12VDC from the battery to 120VAC.
Car batteries wouldn't be the best choice for this as they don't do well in deep discharge situations. Just a few complete discharges can kill them. Marine batteries or gell-cells would be a better choice.