We have a 2014 Jeanneau 44 DS, and I am amazed how the batteries deplete on a long sail, with just the electronics/radio/GPS/bilge running. I did a 10 hr trip last season and although the batteries weren't depleted I was happy to start the motor and charge a bit as we moved to a mooring. How do people do it leaving a boat on a mooring for extended periods? I notice not all boats on mooring have solar or wind generators set up. Do their owners come out once a week and check on them? What is a reasonable expectation of a battery maintaining the bilge pump alone? Weeks? A month?
We leave our boat on a mooring all season with no solar charger. With our coastal sailing habits and our boats simple electrical needs, we've reached a workable system of sailing with minimal charging via the alternator.
First, you may want to look into the bilge pump. Ours is raised about 10 gallons above the deep sump. In that position, the pump never comes on(I test it regularly).
Our manual bilge pump is what we use to clear the bilge. In a week, we might clear a quart of bilge water(stuffing box, anchor chain locker, deck leak?), or the pump won't prime. But that little amount of water can wreak havoc with an automatic bilge pump as it sloshes around. Mostly, the manual pump gives me an indicator that something is leaking if it clears more than normal.
Next, our power needs are pretty small in comparison to most. We have no refrigeration (there's plentiful ice where we sail). New devices (ipads, phones) with a small shipboard GPS don't require much power. Our biggest draw is an wheel pilot, but that's minimal compared to larger systems(our boat has an easy helm).
We like to sail to where we're going, as much as possible. Our house bank (2 Grp 27's) allow us to sail for a few days without worrying about charging, at all.
On the mooring in season - when I make a weekly, or so voltage check - the batteries read about an average of 12.5-6 V. Not fully charged(rare) but not undercharged, seriously.
The starting battery is always fully charged(we have a high-ish output alt).
I keep thinking I'll add a small solar charger but then we find a way to cut power usage and things are working well for us. And I'd worry about overcharging(not a problem now).
We get an average 5 years out of our house batteries which are, last I knew, $100 or less. It's a simple system that works for us.