Charging process of a battery
No, you can't assume that a given solar panel delivers constant power. The power that it delivers depends on both the amount of light it is receiving and the load that it is connected to.

Under full illumination, a given solar panel will produce a certain maximum open-circuit voltage. It can also deliver a certain maximum current into a short circuit. Since power is voltage times current, the maximum power will fall somewhere in between.

Connecting batteries of different voltages directly to the panel will put it at different points on its "operating curve". The one with the lower voltage will generally get more charge (amp-hours) &mdash; but not necessarily more energy (watt-hours). But your oversimplified calculation is not the right way to estimate the difference. You'll need to have specific data for the panel and the batteries in question.

A Maximum Power Point Tracker (MPPT) circuit is specifically designed to adapt a solar panel to a battery, varying the battery voltage (and current) so that all of the power available from the panel from moment to moment can be used.