Power Systems Update

Power Systems Update

Over the past few months I’ve had a few people enquire how our power systems are actually working out for us, especially as I was fairly detailed on why we selected what we did when equipping the boat. So now that we have been off the grid for over 2 months including nearly 12 days at sea here are the summaries of how much power we use and how the various parts of the electrical system are working out so far.  I’ll give forewarning, that the following isn’t overly exciting and intended more for the benefit of other boaters who are thinking of extended travel.

Power Usage at Anchor:

We are in the ballpark of what we expected and use about 80 Ah per day. On a day with no charging of electronics or inverter use we are down around 50 Ah per day. The biggest notable difference from back on the great lakes is the fridge runs a bit more often as was expected and we use the interior lights a lot more as it is dark here around 6pm, so we are glad we changed the frequently used lights over to LED.   The biggest load we didn’t account for was the convenience of a toaster and that adds 7-10 Ah on days when we use it (we don’t see the point in starting the engine to offset it), and the iPad and iPhone which adds a big draw of 7-10 Ah a day, but granted this allows us to check email and are much smaller to store than the stack of books we would otherwise have to stow.

Power usage offshore:

On passage I estimate we were averaging about 180-200 Ah per day and this allowed us to run the helm plotter, instruments, nav station repeater, the radar for 12-14 hours at night and occasional autopilot use.

Solar Panels:

Our solar panels are still working as advertised and we are glad we installed solar although it is too early to comment on the durability of the semi flexible panels.  The daily output down in the islands is lower than it was this summer but the sun isn’t directly overhead for very long so the panel efficiency drops and we have 3-4 hours less daylight down here. On those rare clear days where no clouds obscure the sun, the panels balance out the fridge, our lights, and iPhone/iPad charging. I would estimate we get up to 50 Ah on a great day, likely closer to 40 on a typical day from our 180W of solar panels with scattered cloud cover and squalls rolling through.  This leaves us with a slight power deficit each day of 10-40 Ah which is no problem with our battery capacity.

However, on passage I suspect we may have only gotten 25-30 Ah out of the solar as often we were heeled over so the panels weren’t getting any direct light or the sails were casting shadows.   Something to keep in mind if you are outfitting for offshore and expect solar to be you main power source.



Our 450 Ah of TPPL AGM batteries, have proven thus far to provide a comfortable size battery bank for our needs. Combined with our solar we can go comfortably 5-7 days at anchor without running the engine although we seldom stay in the same place more than 3 or 4 days.


On Passage, we were able to go 36 hours from full charge down to 50%  at which point we would charge back up to 80-85% (once the charge rate dropped below ~70 amps we shut off the engine to conserve fuel) and were good for another  24 hours. The batteries do seem to charge with a high charging efficiency as advertised based on my own observations using the battery monitor (I have it set to 5% compared to the 18% which seemed to work for my old wet cells), so there has definitely been some advantage over a wet cell battery, although it is too early to comment on lifespan and whether the price premium was worth it.

The biggest disappointment with the batteries has been the tapering off of the charge acceptance rate. I had found some sources suggesting that these batteries would be capable of taking a high charging current well past 90% state of charge, but I have not found this to be the case, and between 80 and 85% state of charge they no longer drive the alternator to the upper end of output (at which point charge efficiency compared to other batteries becomes a moot point).


Our alternator was our main power source offshore and is providing half our daily load now so it has been a worthwhile investment, but has been a frustrating exercise to get the regulator settings right.   Last summer with a small battery bank on the great lakes and being on the move every day or two, the alternator never saw a deep discharge that could drive it flat out for more than a few minutes. Quite the contrast to when we were offshore, so I actually found myself tweaking the Balmar regulator while on passage despite the tweaking I had done from our multi day jumps on the great lakes and from New York to Hampton.

After our trip down the coast from New  York to Hampton, I knew the alternator was being limited by the regulator intermittently for reaching the maximum operating temperature.  The problem was it would spend more time being throttled down to half output than it did at high output so my charging time was nearly twice what I had expected. To combat this I added an engine room blower on one of the two cooling ducts on the engine room which did make an improvement and I programmed the regulator to cap the alternator around 90% output which has allowed the alternator to run continuously below the temperature limiting threshold.

At 1800 rpm we see 90 -110A continuous charging current (depending what loads we are running at the same time), and at cruising rpm of 2400 rpm about 110-120A. This is pretty close to our original expectations so I am not disappointed with the performance, I just hadn’t expected so much tweaking to get it right (I’m not being critical of the regulator as each of the settings I’ve been tweaking is very much dependent on my system and the engine room insulation and ventilation).


Would we change anything?

Seeing as we never really run the engine solely for charging, I think we have a system that is more than adequate for our needs.  There is an argument to be made for having a bit more passive power generation on board as extra solar or a wind generator would keep the batteries nearly full at anchor and potentially prolong their life.  And I suppose that would allow us to be a bit more salty and sail right off anchor without starting the motor.

If we do more long distance passages I would like to have another power source to reduce the heavy dependence on the engine and diesel, but we do have enough solar to run the bare necessities in the event of an engine failure, and the alternatives are generally quite expensive each with their own drawbacks and the south Pacific isn’t currently part of our itinerary.

This article has 4 comments

  1. Thanks, Jeff…. Great update…. It looks like a wind generator would be next in line, especially where the trades are blowing!

    • Hi Walter,

      I certainly notice the wind generators on the other privately owned boats down here and it is tempting. The D400 model seems to be the most popular and is probably the way I would go, but for now I’m going to hold off and save it for my severe boaters two-foot-itus dream boat.

  2. Hi Jeff. I was not at all bored with your last post about your electrical system. Sounds to me like you have designed the system very well and it is working as expected. Not sure if I understand the issue with the charge from the alternator falling off earlier than expected but my guess is that the max charging current is being limited by max voltage setting of the regulator.  It may be reducing current to prevent over voltage condition. This may be adjustable but would need to stay within the battery spec and you might also want to watch your system voltage while charging as some electronic devices on the buss are sensitive to voltages beyond 15 or 16 volts. I have repaired a number of nav instruments that failed when someone switch the battery switch off with the engine running and the extra voltage spike fried components.  Great blog by the way, Therese and I cruised in the BVI a couple of time and were hanging on every word! Happy and safe cruising. T&B

    • Thanks Barry,

      I played with the voltages a while ago to confirm everything was per the battery manufacturers recommendation and actually bumped up the bulk charge voltage to the upper limit of target range (14.6V) from the nominal of 14.4 and didn’t see any notable change in charging amps. I keep my eye on it as we have our battery monitor at the nav station so it is easy to see what voltage we have how many charging amps. Once the battery monitor reaches about 85% charge level the charging rate drops off gradually while the charge voltage holds steady. I had read from a third party source that is researching hybrid diesel electric boat propulsion that this battery type can fast charge a little longer, but we haven’t seen that yet. Not a big deal, we aren’t short of power yet.

      I’d heard of people frying their alternators from toggling a battery switch, but never thought about sensitive electronics as well. Good to know. We don’t have a battery selector switch on our boat so the alternator is permanently connected to the big bank, but I suspect this wont be my last boat…