You make a good point. They really ought to be sold with Wh prominently displayed. I'm talking about 1200Ah@12V, although in reality there's a good chance that I'll do 600Ah@24V or 300Ah@48V. It's all the same 14.4kWh and will provide the exact same amount of usable power (ignoring some conversion efficiencies).

I'm using Ah because I'm used to talking to people about 12V battery systems, and Ah is the most prominent number when talking about battery capacity (when the voltage is already implied or agreed upon).

You're absolutely right though, and I should have been more clear about it.

Amp hours are what the batteries are specified in, right down to cells that are 1.5V, 2V, 3.9V etc.. Yes, once they're in system you want to know kWh capacity of the system as a whole, but it makes sense that people think of what's written on the batteries first.