I am installing a Lectra San and would like to know if there is a "rule of thumb" for the battery size I should have on the boat (similar to Peggy's holding size tank rule of thumb). The boat is an H 27 and it has a Guest shore powered battery charger. It does not have a holding tank (trade off for space for the Lectra San) and I fear running out of power and not being able to flush. Thus, I want to get this power thing right.... Also, any other advice on the electrical side of the install is greatly appreciated (I am electrically challenged, despite reading Nigel Calder's book several times). Thanks!