It does have solar panels, batteries are for when it's got the Earth between it & the Sun & they wouldn't have lasted the 3 weeks or so since launch otherwise.
The momentum wheel is for attitude control. Can't use gas jets etc as there's no room for fuel & that could affect the sails as well.
Now it's deployed the next year is to attempt to change it's orbit using just photons from the sail. The momentum wheel will be used to tack, just like a traditional wind sail - so when it's orbit takes it towards the sun it's edge on so it has no effect & when going away from the sun it's full on to get the full benefit of the photons.