You need to learn some basic electricity and then it's not so hard.
Volts X Amps = Watts (Ohms Law - very big deal)
If you learn this you'll be able to compute the raw numbers. Not so hard is it?
On land you think in 120 volts (other
parts of the world are 220 volts) but on the boat it is 12 volts (maybe 24). So you see your shore amps are ten times more powerful than your boat amps but a watt is a watt!. We also need to think DC volts instead of AC volts too but that is another lesson. for now you can convert between them but it's not perfect.
If you have 2
panels that can under ideal conditions generate 46 watts that translates into 92 watts or 7.67 amps at 12 volts. Your fridge we don't know but maybe it draws 5.5 amps and runs 50% of the time. If it's dark 50% of the time you might be close to running that fridge from the solar
panels with enough for
electric lights at night.
Batteries measure power the same way but
battery capacity is measured in Amps X Hours. The idea is how many amps can you run the batteries for how many hours.
On shore you have a meter and you use what you want and get a bill. On the boat you use what you want but you have to repay the batteries for what you used. Your solar panels won't run the fridge but they will charge your batteries. The idea is you can charge your batteries when you choose so you can use power when you want even in the dark or when not running the
engine. There needs to be a balance between what goes in and what comes out. To make it a little more complicated you want to not draw the batteries below 1/2 full and you want to recharge them back 100% regularly. Doing this means the batteries last a long time. Not doing this means the batteries eventually go dead sooner or they don't last a long time.
There are some additional complexities in that
charging is not 100% efficient and batteries can't be charged instantly. They take time to absorb the power being added back. This limits how quickly you can charge or the charge rate.
In the end it is less about how much battery capacity you have and more about how much
charging you can do and really does not matter how you charge but only that you recharge fully.
On the power calculation you are both the consumer and the power company. You need to look at power from both sides at the same time. So, how much can you run is at the end of the day related to how much you can charge back the batteries. You are fortunate in having some solar power but it's not a huge amount. You'll need to run the
engine once in a while and less often is usually desired.
To get into the serious numbers you need to figure out how much power you will need and when. On a very conservative
budget the fridge is about 1/2 of the
electrical power you need. You measure usage in watts over time and from the voltage you can compute amp hours. If you look at how much power things take you find that the 1200 watt microwave
oven at 120 volts now uses 100 amps in 12 volt power and that assumes a 100% efficient conversion from 12 volts to 120 volts (see above). Your fridge may only need 60 to 80 amp hours per day so this microwave is going to use all the power it takes to run the fridge all day in about 1/2 hour. The use of electricity is about "lifestyle choices".
You start with how much power things take and then look at how long they run, then you can compute a daily energy load that needs an equal energy supply. The problem is easy when you
plug into
shore power every night and have an almost infinite supply to recharge with and the crew is asleep, but not so easy in the cool quiet anchorage you just can't stand to leave for 3 days.
Start by thinking in these terms and then you can get started thinking in more technical details of how this really does
work. Ohms law is the truth everything else is just the ramifications and the cold reality of stuff isn't perfect nor are people. When you have to be the power company too it does get a little more complicated. It's about thinking in different ways and looking at what things cost in terms of
work, engine
noise, convenience, and enjoyment. Oh yes, you have to pay for it all too.
I suppose you hoped it could be easier. It really is once you get your
head straight on how it really works. Start with thinking about the ideas and less about the details until you start to see the bigger picture of it all. It's less about the magic in a solar panel and more about so what amount of power can it make? Power is everything. Given enough amp hours you can make everything work so long as you repay the short term loan with the battery bank.
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