Power! We can never have enough.

I really hope that everyone out there is using LED lighting by now. The savings in electricity compared to regular filament bulbs are incredible.

But electricity can be a bit complicated. Amps, Amp hours, Watt, Volts and current? What?!
Now what then? It’s a mine field out there. Batteries will show how many Amp hours they have, and products show how many amps they use. Sounds simple right? Well, it is, at least if you know that Amp hours (Ah) are ampere used per hour. So if you had a 83 Ah battery and you have a product using 1 A of current, that product could (theoretically) be run for 83 hours.

It might be easy to just let an electrician deal with that stuff for you, but if you are on a budget or at sea, that might not be an option. You really should have at least a basic understanding of the electronics system on your boat.

The first step on planning your electric system is knowing how much power that you need for the equipment you have and how often you will use this equipment. Make a list of items and how much you will run them. A refrigerator will be turned on 24/7, but an anchorlight will only be turned on for perhaps 12-hour periods, certain days? Ambient lighting will probably only be used for a few hours per day.

Also it is a good idea to make several lists, depending on the situation. One for blue-water cruising, one while coastal cruising and one if at anchor/moored/harbour. (You might decide to stay a couple of days at a cosy little cove for example. It could be useful to know how long your electric supply will last.) In your list you can use the Watt to figure out how much juice you are using, more on this later.

Example blue-water cruising list*:

  • refrigerator  30 Watt for 24 hours
  • Ambient lights 10 Watt for 6 hours
  • Sailing equipment (log, gps, vhf.) 20 Watt for 12 hours
  • Navigation equipment (computer / gps) 40 Watt for 14 hours

*Note: This is an example list. This will not be the same for your boat.

Using a Watt calculator, you can now figure out how many amps you use according to how many watts you are using on your 12 volt system. (If you are using a 24 volt system, just change the numbers accordingly in the watt calculator.)

If you type in the value of 30 Watt and 12 Volt in the calculator, it will calculate the ampere used and also the Ohm*. In this case it comes up with  2.5 Amp. But, before you get ahead of yourself, this is not a total. 30 Watts of electricity on a 12 Volt system will use 2.5 Amp per hour. In other words, you have 2.5 Ah. So if you where to use this running off of a 83 Ah battery, how long could you use it? I hope you get it. Otherwise, leave a comment and I will try to explain.

* Note: You don’t need to worry too much about Ohms. It is just a measurement of resistance and, in a way, power loss. This will be solved in the future when we have access to room temperature super conductors. Before that, there is not much you can do about it.

So using the list above. The daily total power usage is 131,84 A. Sounds like a lot in my ears, (Fridge: 30 W at 12 V is 2.5 A per hour. 60 A per day. 2.5 x 24 = 60 ) I rounded up on the second decimal on some of the numbers, in case you don’t get the exact same numbers as I did.


You should have 3-4 times the required power needed in your battery bank. Not only might you need it if you can’t charge for a couple of days, it is not good for your batteries to be drained too hard. So based on these figures you will need about 400 Ah worth of batteries.

What about charging those batteries? 

Now you know how much power you are using and how much you can store in your bank, you need to know how much time it takes to charge them. Preferably you have both solar panels and a wind generator. Solar is great for sunny days with not much wind and the wind generator is great for those windy cloudy days.
If we had a 400w wind generator and a total of 200w solar panels and both wind and solar panels were working at 100% efficiency, which basically never happens, it would take 3 hours to generate the required electricity. Wow! Right? There must be a catch?
Actually yes. The optimal situation for solar panels is having the light hit, straight from above with not more than a 30 degree angle. That would make the optimal time frame for solar panels around 10 a.m. and 3 p.m. on a blazing sunny day around the equator. And if there is no wind, that means no power at all from the wind generator. On the other hand, the wind generator might be able to produce power all through a windy night.
But with a total of 600w and a 132 Amp need for power the panels and wind generator only need to work at a median average of 30% to generate the required power during a 24 hour period.

Since there is some leeway with both the panels/generator and the battery bank it will be quite possible to have no power regeneration for a day or two and with the weather allowing, able to recharge the batteries in one day. All without using the engine to recharge the batteries. Which is actually a lot more expensive than just the fuel that you use. Engines have an amount of lifetime hours, and need to be serviced. Since you will be needing your engine for better things, it is not really worth the money to use a fine piece of engineering to just create power. In that case is better to buy a small Honda, petrol powered, generator.

Honda Generator(It might be a good idea to invest in any way as a backup plan.)

I would love to have a Sterling engine for recharging purposes. They work through the difference in temperature and I visualize using the topside of the boat and a part of the boat under the waterline for the temperature differences. But this is probably a complete pipe dream. First of all they are only really effective with a big temperature difference and secondly, they are ridiculously expensive. In the meantime, I will stick to power conservation and more affordable methods.





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