I fail at batteries

So apparently my math was way off. Rather than multiplying the input voltage by current to get watts, it seems I need to multiply current by output voltage. In this case a difference of 668 volts. This means that each inverter is 3.4 watts, rather than 0.06 (which seemed suspiciously low to me anyway). But the real lesson I learned here is that rather than try to do all the calculations I should just buy a bunch of batteries, let it run, and see how long they last.