About Power

The Tuamotus Diaries #50, Day 98 – September 3rd 2016


… electrical power, that is.

Pesto hasn’t been connected to an external source of electrical power from the time we left Mexico, almost 6 months ago. Every electron utilized by our systems since had to be Generated and Stored in-house. So, I thought I’d drop a few words here on how things are going on this front.

We generate energy in three different ways onboard:

  • when the Main Engine is running
  • with our dedicated diesel Generator
  • with our Solar Panels

The main engine counts with a large alternator which outputs over 50amps (24v). It’s quite effective to keep the batteries full and all systems going when we are motoring, but its overall contribution for the long lapses of time we stay at anchor is negligible.

The Generator plays an important role in our generation matrix. Particularly during passages, there’s often something shadowing the solar panels – clouds, the sails or even birds – affecting their contribution substantially. Ironically, this is when we need energy the most, with lots of systems in operation (autopilot, electric winches, fridge/freezer, etc). During the Puddle Jump we had to run the generator every other day, and some instances even daily, to an average of 1 to 2 hrs per day. In the Marquesas we also ran the generator a lot, I would say probably once every three days on average. There, the tall mountains fringing the bays created shadows – either directly or by grabbing clouds – which affected the solar panels as well.

But once we got to the Tuamotus, with unobstructed skylines providing maximum exposure to the sun, the Solar Panels came on their own.

We have four 140W panels on the railings and another 275W one over the dinghy davits, on the stern. They all converge to an Outback 80A Charger which then sends the load to the house battery bank – 10 wet cell batteries with 110Ah (12V) capacity each.

Since arriving here, the panels practically took over most of the electricity generation needs on Pesto. Whenever the day is sunny, even partially cloudy, the panels will top up the batteries before sunset. We have come to a point that I am only using the generator in two instances: either when it remains fully overcast for over 2 days, or to desalinate water, which we do once a week on average.

Now that we have switched our freezer off, our battery charging status has gotten even better. This is how it’s been working

  • at wake up, just after sunrise, the batteries are at a rate of charge between 91% and 92%
  • the batteries are reaching 100% charge between 11:30 and 11:50am every day. From sunrise until then, the Charger delivers an average of 8amps per hour (24V) to recharge the batteries, on top of anything that might be drawing energy from them.
  • Once the batteries are full, we recharge all of our electronics – most notably the laptop which we use to watch movies (a nightly family tradition onboard)
  • During the peak sun hours the surcharge of electricity is such that we are even able splurge a bit: some times using the slow cooker (7 amps draw on 24V) to prepare meals, running the blender for smoothies (20amps/ 24V when on), or the Vacuum Cleaner – all without reducing the battery bank’s level from 100%
  • from 16:30-17:00 on, the panels can’t keep the batteries at 100% anymore and Demand exceeds Supply. From here on we go on “economy mode” and become judicious to what is drawing energy from the bank


The measurements above were taken during 5 consecutive days of typical Trade Wind conditions – sunny with cloud covers between 25% and 50%. A few other measurements that came out of this exercise and I am sharing here for whatever they may be worth:

  • our Fridge is drawing an hourly average of ~1amp (24V). The compressor is working an average of 25 minutes per hour (the Fridge is fully loaded with all the stuff we had to take off the Freezer, and I adjusted the thermostat to work at a lower temperature, between 34F and 36F)
  • our masthead LED light, which remains on all night, draws 0.15amp (24V)
  • Paulo’s fan, which also remains on all night, equally draws another 0.15amp (24V)
  • I also detected a “residual” draw of 0.9amp which is nagging me and will likely become a “project” some time in the future


The kids, who have acquired the habit of checking the status of our battery bank regularly, have noticed the improved performance of our system since we got to the Tuamotus, and again after we switched off the Freezer, and are celebrating the results. They really enjoy the fact that we are topping up our batteries directly from the sun on a daily basis, and question me every time I come close to the Generator’s switch.