How Many Batteries do I Need?

It depends on how you use your boat.  And it's usually more than you can fit in the available space.  Ah, boating.  Compromises, compromises, compromises.

The ABYC electrical standards have tables and formulae for determining how large a battery bank to install on a boat given the likely loads and the amount of time they are on each day.  I have put these principles into spreadsheets incorporating Peukert's Equation and have not found the standards to be particularly helpful.  What jumps out every time I run the numbers is that most of the battery capacity is consumed by the relatively small loads that run hour after hour like refrigeration.  Less commonly considered, and pretty much ignored by the standards, you need a large battery bank to drive a large inverter. If your batteries are too small you will not be able to supply the current your large inverter needs to run the toaster, particularly after the batteries become partially discharged.  

Peukert is even going in your favor here.  Brew all the coffee you want at 1600 watts for 10 minutes each time.  It's nothing compared to keeping the pot warm for hours on end.  The bottom line is that the refrigeration loads, a few amps running for at least half the time, end up dominating the picture.  This assumes, of course, that you are not trying to heat your boat with electricity or something equally foolish.  A 1600 watt space heater will deplete large battery banks in a couple of hours or less.  A television or stereo for evening entertainment is not a big deal.  A washer-dryer is getting right up there with the refrigerator if you use it very often and without a generator.

So, on my boat I let the coffee cool after brewing and heat each cup in the microwave as I want it.  The coffee tastes good for days and my batteries stay charged.  I have beefed up the insulation on my icebox and 20 pounds of ice lasts for nearly a week in the Summer.  I use small, unvented catalytic propane heaters to cut the morning chill in the winter and make my boat wife-friendly.  If you have something larger than a 28 foot Islander sailboat the answer to your problem may differ.  If you have a generator you are willing to run a few hours a day (or whenever you want to shower) an electric hot water heater is no big deal.  

What I have concluded about battery bank size is that it depends on how you are going to use your boat.  Do you go to the island for the weekend or are you going for weeks on end?  It's easy to run lights and music for a long weekend on a 34 foot sailboat.  You basically make the battery bank just large enough so that it does not completely discharge until you are back at the dock.  If you want to go a week we had better seriously discuss how you are going to at least partially charge your batteries while you are out.  If you are cruising you have the worst case situation and it will help if you really economize.  The rule I have for cruising is that you want to be able to go indefinitely while running the engine (any engine) an hour or less each day.  My Islander will do that, microwave and all.  I have four golf-cart batteries.  Actually it will only do it for a week or two if I want to get reasonable life out of the batteries.  The cruising standard of one hour or less engine time per day is tough.  You must add something which puts out a modest amount of power for a decent amount of time, like solar panels.  But that is getting ahead of the story.

If you are refitting a 45 foot boat, space is going to be the limiting factor in determining how large a battery bank you can install.  Most boats in this range have gensets so it works.  I turned down a job a few years ago from a fellow who wanted to run his 60 foot ex-pilot boat on solar panels.  The boat had dual diesel generators, one of which was running all the time.  You could not recover the cost of solar cells for a boat that size and energy consumption over eternity, much less figure out where to put them.  If you have a large boat your personal energy should go into a smallish very quiet genset that can run 24 hours a day and then kick in the big generators when you need them.