[sailingdog]

That's kind of like asking how far a car will go on a full tank of gas without knowing what engine is in it or how large the gas tank is, or how the driver will be driving it, and what the route they're taking is. It depends.

If you have a huge battery bank and a tiny refrigerator...might go a few days... If you have a huge refrigerator and a small battery bank, might only be a couple of hours.

You need to do an electrical budget survey of your boat, and your typical electrical usage. This will give you a rough idea of how many amp-hours your are going to use over the course of a day. Then you have to look at what the capacity of your battery bank is.

Running your battery bank down more than 50% is a very good way to shorten its lifespan...so take whatever the amp-hour capacity of your battery bank is and divide it by two or three, and then divide that by the amp-hours of your average daily usage, and that will tell you how many days, on average, you can go before having to recharge your batteries.

For example... your daily amp-hour usage is 100 amp-hours. Your battery bank consists of four (4) T105 golf cart batteries, for a total of 420 amp-hours. That means you can go roughly two days before recharging your batteries.

However, this also assumes that you have sufficient charging capabilities to properly charge up your batteries in a reasonable amount of time. 80% of your charging takes 20% of the time and the remaining 20% of the charge takes about 80% of your time. Many people skip the last 20% but this will also lead to fairly shortened battery life.