It's also worth bearing in mind that the usable capacity of a lead acid battery is only about 50% of its nominal capacity and that to fully recharge the last 20% requires a float charge that takes a very long time. In my installs I tend to use LiFePO4 battery's, very expensive but you can use about 80% of the nominal capacity and you can bang loads of amps into it up to 100%. They're much better suited to leisure systems but maybe prohibitively expensive for some projects.
Whereas I agree that it can be much much lower than the stated battery capacity in Ah for lead-acid batteries, it all depends on the discharge rate. For slow discharge rates, typical of LED lighting and small device charging, the actual figures can be nearer 80 or even 85%. You need to apply Peukert’s Law to your discharge calculations.