Can I get battery capacity by knowing my need for powering load?

Let's assume we have some load with current I to run for time t. Can we  get needed battery capacity C?  For this let's use Peukert's law t=H*(C/(I*H)^k  where H is battery rating of  discharge time and k=1.2 for most lead acids batteries. Normally H value is around 20 for an average sealed lead acid battery.  For those who doesn't know where to get it, please  put 1 in the last Hrate field. Very often manufacturer claim H=20. I disagree as I haven't seen true 20C rate battery. Here s the capacity C(Ah), solver for lead acid battery, where I-load current, t-time to run the load, H -is battery discharge rating.