The 90 ah merely refers to the amount of power available over time. So a 90 ah battery could provide 10 amps for 9 hours. As you rarely use car electrics without the engine running and providing the power, charging rather than taking from the battery the ah rating is largely irrelevant.
The main use of the battery is to start the car and for this you need a certain level of Cold Cranking Amps (CCA). That is the amount of power that can be delivered for a short length of time in order to turn the engine over
"
CCA is a rating used in the
battery industry to define a
battery's ability to start an engine in cold temperatures. The rating refers to the number of amps a 12-volt
battery can deliver at 0°F for 30 seconds while maintaining a voltage of at least 7.2 volts."
I am sure all car batteries sold today can deliver a high enough CCA to start the engine.
Car batteries should all be one size unless you are driving a huge 4x4 or similar.
I have used The Battery Guys in the past and had good service and a good battery. Next day free delivery.
HERE
www.thebatteryguys.co.uk