It is, but that doesn't necessarily make it easier to use. If you have a factory producing batteries totaling x Wh/yr in capacity and every car needs a battery

It is, but that doesn't necessarily make it easier to use. If you have a factory producing batteries totaling x Wh/yr in capacity and every car needs a battery with capacity y Wh, then you can build x/y cars per year. The calculation is mathematically the same if the production is specified as z W (where z = x / 8766), but not as immediately obvious to understand.