So I thought I'd come here to explain some physics.
There are two important equations:
- p=iv - power (watts) = amps * volts
- v=ir - volts = amps * resistance
Note - an amp is a measure of how many electrons are flowing through the wire per second. 1 amp is A LOT of them - I forget the exact number (I'm a mathematician, not a physicist)
A volt is how much energy each electron has. A generator is only able to provide so much energy to each electron (12 volts = 12 joules per coloumb (a coloumb is a large quantity of electrons)) but the amount of them it moves along can vary, that's your amps.
watts is a rate of energy, it's in joules per second. So 5 watts for 10 seconds is 50 joules of energy. Your fuel tank could be measured in joules of energy it holds and your engine is the magical device that turns fuel into mechanical energy.
As you know cars run pretty much exclusively on 12 volts - or some other fixed number. This means if you want, say 600 watts, your alternator needs to be able to provide "i" in 600=12i, which is 300=6i, 100=3i, i=33.3 amps, which is A LOT of current. This is why wires melt more often in cars then they do at home. Where to provide 600 watts at 220 volts would be about 2.73 amps. Over 10 times less electrons carrying charge per second through devices!
So the more power you draw the more electrons the alternator has to drag through your wiring, the more energy it takes from the engine. A good "back of the envelope" value for how much energy an engine produces (mechanical of course, not heat) is 2,400 watts. If you draw 600 you've only got 1800 watts left, so you put your foot down harder (thus using more fuel) to get back up to 2,400 watts for kinetic energy.
Okay so that's the first half.
Car batteries are NOT BUILT FOR CONSTANT DRAINS they are good at one thing, and one thing only: short bursts of high drain (like a starter motor)
- Also they can retain charge for a long time. But I wont contrast with Lithium based batteries here
If I recall correctly, car batteries are good for 70amp hours (they can provide 1 amp for 70 hours, or 70 amps for one hour) if you draw about 3 amps (which would last about 23 hours)
remember p=iv, so p=3*12 = 36 watts for for 23 hours. That's not much is it!
Or we can do p=iv=70*12 = 840 watts, but only for an hour. (INACCURATE as car batteries don't behave this way, expect nearer 840 watts for 40 minutes)
- This implies a linear relationship. As I said car batteries are not good at small loads for long periods of time, they are good for short bursts of very high current. You get the idea though.
You want a laptop ~ 40 watt (depending on what it's doing, if it's charging its battery it'll draw more power, up to the charger's max rating).
Fridge ~ 90 watts
Mobile phone ~ 3 watts - lets ignore this.
You're looking at drawing about 130 watts. This'll require about 11 amps.
Your battery can do 70 amps, for an hour. Using this information that'd give 70/11 = 6.36 hours. Keep in mind it's not good at doing this for a long time
So I'd bank on it being approximately 5 hours until a full battery completely ran out.
IF however you get 3 batteries. You're nearer their comfort zone (3.67 amps per battery) which should give you about 19 hours. This is a more accurate number - You'd have to wire these up in parallel.