If you have an off-grid toilet with the 12 volt fan operating from a battery, then the best way to keep the battery topped-up is to use a solar panel.
For two toilets, or one toilet in a less than perfect position for sun, we offer a 40 watt mono-crystalline flexible panel. This gives a steady trickle charge suitable for replacing the power consumed by the fan. We would recommend the use of a solar controller with this panel to avoid overcharging on sunny days.
Please note that for best performance these panels should be mounted at approximately 30 degrees to the horizontal and should face as near as possible to a southerly direction.