If you read anywhere that you can calculate exactly how much solar power you need on your boat, know in advance the site you are reading is misleading you.
That's not to say that other sites go out of their way to provide mis-information, but determining exactly what you need is virtually impossible in advance due to so many variables;
- How much sun will you have every day to power your panels?
- How many lights do you have on every minute of the day and night every day?
- What if you host a party and you are going in and out of your fridge and it's running twice as long as usual?
- How many visitors will you have that increase demand?
- What happens if you add new equipment?
- What changes when something is running on shore power and then runs off your inverter? How much energy use is different?
As you can see, the variables for determining what you use will be in a state of flux most times as we tend not to all do exactly the same thing every day. But, this shouldn't discourage you in setting up a system that will work for your situations. Let me start by saying I've never heard anyone say, "I put too much solar power on my boat." So your only worry will be to have too little.
So, you can get an estimate two different ways. You can use a worksheet and go through your boat to determine how much power each piece of equipment and lights might use and estimate your daily usage (download a worksheet here), or you can purchase a battery monitor that will provide that information to you after it's installed and working with your system for a little while.
The thing to remember in both cases is; if you intend on cruising to an area different from where you get your estimates, your demands change and so do your needs. Home Port and the Bahamas do not have the same energy requirements (unless of course you're lucky enough to live in the Bahamas...).
Once you see what your batteries are providing and how often they need topping off, you can determine what wattage of solar input is required to keep the house bank full and useful for your specific cruising style.
Here is a quick example from the worksheet method to give you an idea of how this works.
Most solar panels will provide you with the watts they deliver, typically per hour in full sunlight. So a 100 watt panel gives you 100 watts in a perfect sunny day. Assume you don't get this, but for our illustration we'll go with it.
100 watts x 5 hours of peak sunlight provides us with 500 watt hours per day. Convert this to Amp-hours by dividing by the voltage of your system. Most are 12v system = 500/12 = 41 AH per day.
After adding up everything on your worksheet, you determine you use 60 AH per day. So one 100 watt solar panel won't keep your house bank happy unless you have more sunlight or less energy use. You can add another panel or you can make adjustments another way.
Just remember, using the worksheet is a rough estimate and can be way off if the type of cruising you do and the areas you cruise change routinely. Always better to get your estimate, then count on using at least 30% more than estimated and cover it with your panel size.