I presume you mean battery capacity. Batteries don't have a power rating as such, they have an output voltage, which if my memory of Year 8 electrical science, which was longer ago than I care to think about, is correct is basically the pressure of the electric charge. Different e-bike systems may be designed to run on different voltages, but in the grand scheme of things ois neither here nor there. The drive unit is legally capped at 250 watts maximum power output, which means the only remaining variable is how much charge the battery can hold. A 400 watt-hour battery has enough charge to sustain a 400W power draw form one hour, or a 1W power draw for 400 hours, or anything in between (or beyond) on a directly relative scale. A higher capacity battery doesn't alter the power output, that's governed by the power unit itself, it just allows you to run further on a charge, or be a bit less frugal on the boost. Basically, it's a bigger fuel tank.