For running computers, laptops and the 'better quality' LED lighting, you do not need a pure sine wave convertor, and a square wave is more than adequate. After all, these devices convert the AC into a DC signal and then using switch mode supplies, convert the DC to the voltage required, so effectively the cost of the sine wave is totally lost in these applications.

Pure sine wave convertors are suitable to run motors and microwaves, as they place less strain on this equipment, which requires a sine wave to be efficient.

As Andy says, first work out your total consumption, add 50% for inefficiencies and to allow the inverters to run at 50% of their rated value, as the full rating figures on most inverters is for short periods, next calculate the number of hours you wish to run on battery power, multiply by 2.5, as you want to not drain your battery's to less than 60% of capacity, to allow over 800 cycles or more out of them, going lower reduces the cycles in logarithmic fashion. Sit back calculate, and then recover from the shock , and work out what you can afford. Get a genny as a back up for the extra time you need, it will be cheaper.

If you are not sure of the final figures, place the numbers here and we will do the final calculations for you.