We have a 80 Watt Solar panel fitted on our Coachman, last winter it kept the battery charged at over 13 volts, this was checked by the onboard metre and my multimetre, both readings were similar, they are very useful if you keep your van in storage as we do, on our previous van we had a Phantom Tracker that drained the battery in 10 to 14 days so we had to bring the battery home for charging for 24 hours and then take it back to refit it, a real pain, as we had to go to the storage to check more often than we wanted, no battery, no Phantom, no insurance