Saturday, February 25, 2017

Electric Power Strategy

Since talking about the original design of our power system back when we were specifying the boat, I really haven't said much more about how it's working, what changes we are making, and why.
I've posted this article on our solar system, this article on reworking my main engine alternator system, and this article on the relative efficiency of generating power via the main engine vs a generator.  I have several more articles is process for other modifications, but realize I've never stepped back and provided an overview of what I'm doing, where it's heading, and why.  So here's the big picture.

Solar Panels

As you read through this, think in terms of cruising for extending periods of time, underway some days, but not all, and anchored at night or for several days.  Notably absent are marinas and shore power, so the boat needs to be completely self-sufficient from a power perspective.  Loads need to be powered, and batteries need to be recharged.

If you return to a dock each day and plug into shore power, this is a much easier problem to solve.  All you need is enough power to last until you return to the dock, and shore power can recharge everything.

Similarly, if you run a generator all the time, you will have plenty of power.  As boats get larger, this becomes the norm, and if you are in a climate where you want air conditioning all the time, a constantly running generator is the practical solution.

For other boats, the power we consume comes from an alternator on our engine while underway, from a generator run periodically, or from power stored in a battery bank which of course needs to be recharged periodically.  Increasingly common are solar arrays that generate power as well.  Operating comfortably like this is our goal.   We ran for over a month last summer, never tying up to a dock, and never plugging into shore power.

All of these elements interact in infinite ways depending on how the boat is used, and how it is equipped.  It's important to understand that there is no right answer here, however I always find it interesting to see how other people have solved the problem, and hopefully our approach will prove equally useful to others.

The diagram below shows our power system.  We have ways to generate power; a generator, main engine alternator, and a solar array.   We have a way to store power in the batteries.  And we have things that consume power, some more than others.  It's worth spending a little time on the things that consume power, since they have a large influence on the over-all power system design.

Here's what we have discovered and done so far.

First, we have and continue to pay close attention to our electric loads, and select appliances that are as power efficient as possible without giving up the features we want.  I think we have been pretty successful so far, and have an at-anchor battery load that 1/2 to 1/3 what other boats of the same model report.

Relatively small loads can run off the batteries.  This includes things like lighting, entertainment systems, and various boat systems.  Heavier loads can also run off the batteries just fine as long as they are not prolonged loads.  Our house water pump, microwave, and coffee maker are good examples.  They draw a lot of power, but only for a limited time, so the total power consumption isn't huge.  The greater the cumulative load of everything, the larger batteries you will need and/or the more frequently they will need to be recharged.  Everyone needs to find their happy compromise between convenience of having gadgets, and the inconvenience of powering them.

For heavier, longer lasting loads, running off batteries usually doesn't make sense.  The batteries get drained quickly and need recharging, so it tends to make sense to just run the generator from the onset, powering the load directly, and taking the opportunity to charge the batteries at the same time.

This raises a couple of interesting questions for future articles.  For example, what is the dollar cost of generating power from a generator?  This article compares the fuel efficiency of generating power from a generator vs and engine alternator, but it is comparative only and doesn't quantify the costs in dollars.  And a related question is the cost to store and retrieve power from a battery.  It's not free.  But that can be the subject of another future article.

Batteries eventually need to recharged, and larger loads need to be powered.  We have a solar system, and it contributes nicely, but is limited by the space we have available for solar panels.  On a sunny day it more or less covers our standby loads while at anchor, but there is nothing left over to contribute towards recharging our batteries from the night before.  That still requires some other form of charging, even though the solar makes it less frequent.

That leaves us with two forms of power generation;  alternators on our main engine, and a generator.  Of the two, the generator is the more powerful with a 20kw output.  The main engine alternator is about 5kw.  If we are getting underway, we will typically just let the alternator recharge the batteries.  It can almost always do it before we reach our next destination.  But if we are staying at anchor for the day, we will run the generator and take advantage of the available power to do laundry, heat water, and of course charge the batteries.

But the decision becomes more complex when we are both underway, and when we want to do laundry or run some other larger load like our electric oven or air conditioning.  One option is to run off the alternator and inverters to power these devices.  The other option is to run the generator to power the loads, removing all loads from the alternator.

This brings us to the first enhancement we wanted to make, and the ensuing ripple effect of changes.

While underway, it is a great time to do laundry.  But it required running the generator, yet it's a very light load for the generator.  And our inverter system was 120V only, so we didn't have the option of running off the main engine alternator.  This spawned a project that I will write about in the future where we expanded our inverter system to run a few select 240V appliances - the washer, dryer, and oven to be specific.

Our inverters are capable of powering 7kw which will run two out of three appliances at the same time, and perhaps all three at once.  But our main alternator is only about 5kw, so we needed more alternator output or heavy loads will drain the bateries.  This triggered the first ripple effect and a project to increase our alternator output to about 7kw to match the power of the inverters.

Many people, including me, have increased their alternator and inverter capacity to operate some of these larger loads while underway.  Our initial focus was on being able to do laundry while underway without running the generator.  Laundry by itself isn't enough of a load to keep the generator busy, and not too large a load to power with our existing alternators, so it seemed like a good starting point.   The end result has been quite successful, I think.
One thing I have been wondering through the whole process is how far to take the alternator+inverter approach vs just running the generator.  The next questions I was facing was whether to allow for powering some or all of my air conditioning units via the alternator+inverter.  This question brought about a set of experiments that I ran last summer and reported in this article comparing the fuel efficiency of generating power via alternators vs a generator.  As a result of the experiments, it's unlikely that I will expand my alternator capacity since larger loads are more efficiently powered by the generator.

One related side effect benefit of moving a few 240V appliances to our inverters is that we now have a way to run them while on 50hz shore power.  Our washer, dryer, and oven will only run on 60hz power, or at least that's what the manufacturer says.  I haven't tried it, so am going on what they say.  That created a problem when we get to locations with 50hz shore power.

Our 120V service is 100% serviced through inverters so that always runs at 60hz.  The 240V service, however, was direct connected to either the shore power or generator.  The generator is 60hz, so that would work fine, but not sure power.

By moving the laundry and over to our inverter service, it too now always run at 60hz, so it at least partially solves the 50hz problem.   However, on 50hz shore power our inverters never switch to charge mode and just keep inverting, drawing their power from the batteries.  So there is a need to keep the batteries charged and the inverters fed from shore power.

Back when we first designed our electrical system we considered this and added a universal shore charger.  It's a 100A charger that can run on either 50 or 60hz, and pretty much any input voltage that you throw at it.  So when on 50hz shore power, the charger provides DC that keeps the batteries charged and powers the inverter, and the inverter in turn powers the 120V loads and the 60hz sensitive 240V loads.  It works well, but with the increased inverter load from the laundry, there are times when the charger won't keep up.  So, in the same way we needed to increase alternator output to support the washer and dryer loads, a project for this year is to add a second charger to double its capacity and provide some redundancy when running on shore power.  That will happen sometime before we leave North America.

Hopefully this provides a little more context for some of my more recent projects, and a few articles that I have not yet written.


  1. I was looking at your diagram and how do you get 240vac from the autotransformer which is supplied by 120vac. I know it's a mistake.


  2. That's the cool thing about an auto-transformer. The conceptual schematic in this post doesn't really show it, but the follow up that I'm currently working on shows the detailed wiring.

    In a nut shell, the auto transformer takes the 120V input and creates another, out of phase 120V output. The original 120V becomes L1 in the 240V circuit, and the auto-transformer output becomes L2. So the original 120V ends up powering both the L1 and the L2 legs of the 240V circuit.

  3. Thanks for the explanation. I totally forgot what an autotransfirmer does and why you are using it. It's does make sense for the efficiencies that you are trying to achieve.


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