The pressure or electrical force between two points. The longer the distance between the two points (i.e. the cable/wire length) the greater potential for voltage drop.

The unit used to measure electrical current as it flows past a specified point.

The rate at which electrical energy is used. Watts are a combination of pressure and flow i.e. volts and amps.

To connect 2 batteries in series a link lead of a suitable grade must run from the positive (+) terminal of one battery to the negative (-) of the other battery, this will make a 24 volt battery system.

To connect 2 batteries in parallel a link lead of a suitable grade must run between each battery's positive (+) terminal. The battery negative terminals can either be linked in the same way or both batteries earthed to a suitable spot. Linking 2 batteries in parallel will keep the battery system/bank at 12 volts and double the Ah output, i.e 2 x 110Ah batteries linked in parallel = 220Ah

The power the battery can deliver for a given time under certain conditions is usually referred to as its CCA (cold cranking amps). There are various different methods of measuring the CCA, but the most popular standard in the UK is the SAE (Society of Auto-mobile Engineers) method. Basically the higher the CCA the better. 

It is perfectly OK to upgrade your existing battery to one with a higher CCA as long as the battery dimensions will fit your vehicle's battery tray and the batteries are the same voltage.

The Amp hour rating denotes the amount of energy that can be taken from a battery before the terminal voltage falls below 10.8 volts. This test is usually carried out over a 20 hour period (20 hour rate). Thus a 50 Amp hour battery can be discharged at 2.5 amps for 20 hours before the voltage drops below 10.8 volts (i.e. 20 x 2.5 = 50). With leisure batteries Amp hours is often abbreviated to amps (e.g. 85 amp leisure battery).

How much 12 volt power (Amp hours) do I need?

The simplest method of working your power comsumption is as follows:

Watts divided by Volts = current in Amps. Thus on a 12 volt electrical system a 120w pump will take 10amps (i.e. 120 watts/12 volts = 10 amps). In theory, using the above equation, you might think that an 80 amp leisure battery would be adequate to run the pump for 8 hours, but in practise by the pump will fail to run properly as the battery becomes completely discharged so a safety margin of around 25% should be applied with regards to the battery Amp hours required. See the equation below:

i.e. 10 amp pump to run for 8 hours = 80 amps + 25% safety margin (20Ah) = leisure battery of at least 100 Amp hours is required to run the pump efficiently.

It is also possible to do the equation the other way round by multiplying the volts by the amount of amps, i.e 12 volts x 10 amps = 120 watts.

Ok, let's show off. Here's another way 120 watts divided by 10 amps = 12 volts.

For a 24 volt battery system replace 12 with 24 in all equations and check all your low voltage appliances are rated at 24 volts.

Get it now?

The easiest way to explain this is to imagine your cables like a water pipes. If you try to force water under pressure through a pipe that wasn't designed to cope with that much pressure the pipe will almost certainly burst. The ultimate solution is to fit a bigger bore pipe.
When dealing with long cable runs towards the end of the run the voltage will drop, but the amps will increase. Using a cable to small to cope with the voltage drop could and increase in amps, in a worst case scenario result in fire. Thus, always upgrade the cable size to reflect the length of the cable run and fit the appropriate rated fuses. If in doubt consult a DC electrics qualified electrician.