Battery life

This depends very much on what type of machine it is.  Some machines have an extremely long battery life that can run a whole day realistically without needing recharging, others can be so short that their battery is little more than a way of keeping the machine running if it’s moving or someone trips over the chord.

• Some ultrabook type machines can have extraordinary battery life;  they are designed for mobility and use away from power points and often have lower powered processors, smaller screens and special “solid state” disks (SSDs) which don’t have a motor and use much less power.  Some can have more than 12 hours of battery life but this also depends on how the machine is used;  some can run for 8 hours on light use but 2 hours with heavy use so it’s hard to get a reliable rating on this
• Many laptops have removable batteries – typically larger machines have this feature.  This lets you carry more than one battery with you or replace a battery as it gets older and holds less charge;  however others have a fixed or internal battery which needs a technician to replace (e.g. Apple laptops are designed this way, many ultrabook machines are as well)
• Really large laptops tend to have moderately sized batteries – huge batteries are very heavy to carry and ultimately won’t give a large machine a particularly long battery life anyway

Generally I advise the following:

• When looking at a machine try to see its power chord and “power brick” – some are very small and easy to carry, others are disproportionately large and difficult to carry. 
• Work out how much you’re going to really need to use it away from mains power.  This may be less than you think – for example many modern aircraft and trains have power points for travellers to use, it’s still rare in some countries but is becoming more common.   Conversely however if you have poor battery life you may not find yourself using it away as much – this means you’re getting much less value for money from the machine you buy if you are too afraid to use it out and about.  If you’re writing an assignment or an article for a site it can be much nicer to do this outside or at the park;  here good battery life is its own reward.
• If you’re going to need long stretches away from a power point then a very long battery life or at least a user-changeable battery is essential.  Some laptops also let you clip in a larger or extended battery though these can add significantly to weight and bulk – look into this first, only a fairly small number of brands still offer this option