This is to some extent unknown territory for me and others might give you a better answer. I suspect that a panel that will fit on your mower will not produce anything like enough output to do what you want, but you need to check this with an actual panel or at least a specification for one. If you draw current the voltage will drop. If you can guarantee a minimum current draw then you can see what the maximum voltage would be at that current, under best case sunshine conditions. That's about as good an answer as I can give, you need to experiment.