"Why", I'm sometimes asked, "do you specify electrical stuff that's bigger and more powerful than I had expected?"
It's a justifiable question. Batteries are expensive, heavy and bulky; why would you want any more of them than necessary? And what's with the multiple shutoff switches, subpanels, and other non-typical features of my electrical designs?
It all boils down to that old tongue-in-cheek definition of cruising: "Performing boat repairs in exotic locations". Or, more accurately, my assumption (usually accurate) that clients would rather be dancing to a live band at a beach bar, or skinny-dipping in a moonlit atoll, than lying upside-down in a 40°C engine room with a wrench and flashlight.
Cruising sailors almost always add gear over time; they rarely get rid of power-hungry systems. If the boat's electrical system is designed to be right on the edge of capacity (and reliability) on launch day, it'll be a constant source of headaches and frustrations as the boat's systems evolve over time. It's far, far easier to take the hit on battery bank size and weight up front, and not have to worry about it every time you add another gadget, light or fan.
There's also the important question of emergency reserve capacity. I'd say that most clients who are building or outfitting cruising boats now prefer to get their electrical power from solar (and sometimes wind) sources. My sizing calculations for solar panels and wind turbines take into account the expected climate of the area you plan to cruise, but it's still possible to get caught in a cloudy calm for a few days – in which case you will be very glad to have an electrical system that isn't going to let the fridge go warm if you forget to run the engine before heading into town.
When cruising sailors do have to fix stuff, they rarely have the luxury of shorepower or a fully-equipped yard. If you have to work on the boat's systems while at anchor, it is very nice to be able to shut off the battery connection to some subpanels – the charge controller panel, say, or the galley subpanel – while keeping the rest of the systems powered up so you can (a) see what you're doing, and (b) run your power tools. This is pretty easy to achieve if you split up the electrical system into multiple subpanels with multiple main battery switches, as I tend to do. It looks a bit odd on paper, at first, if you're used to a single master switch and a single giant electrical panel. In practice, though, it's far more convenient.
So yes, I do "over-design" electrical systems, at least compared to what you'll find in many mass-produced boats. There are good reasons why I do this, and you'll thank me for it when – several years into your cruise – you aren't really worrying about how much power you're using, whether you can make it through a cloudy day without starting the engine, or what battery & bus upgrades you'll have to do before you can add that Whiz-Bang Gadget of the Year.