Ampere hour or Ah is the amount of energy a battery can supply.
A typical leisure battery for the caravan market in the UK would be 110 ah at the 20 hour rate This means that the battery will supply 110 amps at a usable voltage over 20 hours (In this case we take a usable voltage to mean at load above 10.5 volts). To understand this further take 110 and divide by 20 the battery should be able to provide 5.5 amps continuously for 20 hours.
If you take a battery and the Ah value is measured over a different time period then the value will change because a 110 ah battery over 10 hours should be able to provide 11 amps continuously over 10 hours. It is very important that the hours rate is used when sizing any battery for this type of application.
The performance of a lead acid battery will decrease the faster the power is drawn from it.In a business like County Battery Services one of the most common mistakes people believe is that the 110 ah battery will deliver 110 amps for 1 hour this is NOT the case unless the battery is specified 100 ah at 1 hour rate
Beware some manufactures misquote their Ah capacity or do not give an hourly rating with it.A good indication of the battery quality is the weight. The main component part and the most expensive is the lead if you can remove some of the lead you save money.