A recurrent theme among boaters is the energy footprint of one’s boat. We don’t pay much -or nearly enough- attention to our consumption on land because the infrastructure is very well established and the energy is plentiful. It might not always be affordable but it is generally easy to access: your smart phone is low on battery, you plug in and don’t think about it. On a boat, it is different, unless you are plugging in at a marina every night: your boat carries its own energy reserve in form of batteries, diesel and butane (or propane). Let’s ignore the gas for now as it is typically only used for cooking, but we’ll touch on the challenges and issues we faced with propane and butane in a coming post. Batteries are a great way to store electrical energy for later use. They are charged when there is a surplus of energy created by an energy source, e.g. a solar panel, a wind turbine, a hydro generator, or an alternator on the diesel engine. The extra energy is stored in the batteries for later use, i.e. when the energy sources are not providing enough energy to sustain the load. What is the load? It is all that needs electrical energy to run: lights, navigation electronics, water pumps, fridge, and many more.

Ubi is wired as a 12-volt system. The battery bank is composed of 6 12-volt batteries In parallel. The batteries are similar to a regular car battery (but not the ones in a Toyota Prius or in a Tesla, those are lithium-based, whereas we are still with the older generation of lead-acid batteries). One battery is dedicated to the engine: its exclusive role consists on starting the diesel engine. It otherwise doesn’t feed into the electrical system of Ubi. The 5 other battery are called the house batteries: they provide energy for all other systems, i.e. everything but the engine. Each battery is 80 amp.hours, meaning that in theory it could deliver 80 amp of current during 1 hour, or 10 amp for 8 hours, or 1 amp for 80 hours… the amperage times the duration can be up to 80, per battery. So, still in theory, our house battery bank would provide us 400 amp.hours, which would be substantial. But, in practice, there is a hard physical limitation due to the chemistry of the lead-acid batteries: you can only drain them so much before they start consuming their internal parts and/or drastically reducing their capacity, in other words the charge they can hold. Typically, we aim to not drain our battery below 80%. This means that we only can use up to 20% of the 400 amp.hours, i.e. 80 amp.hours. And this number assumes batteries in perfect conditions; and since we didn’t not install these batteries, we do not know what they have seen or been through before we acquired Ubi. When the battery bank has been depleted to 80%, it is well time to recharge it or else it will sustain some damages. Our main source to recharge the batteries is the sun. We decided when we refitted the boat in Martinique that we would go with the largest solar panels we could practically install on our stern arch. We got 2 LG panels of 300 Watts each (amusingly, the back side of the panels are not coated with the regular white cover and the seller told us it would allow some extra rays of sun to be reflected from the sea onto the back of the panel and therefore get a bit over 300 Watts… although it might be the case, we didn’t include this extra boost in our energetic model). They output 17 Volts and so, under the best possible conditions (bright sun directly overhead and no loss), they would provide a combined 35 amp of current to the boat. However, the conditions are optimal only a small fraction of the day. To account for the variation of the incidence angle of the sun to the panel during day time, let’s assume that we average a 1/4 to a 1/3 of the optimum during 10 hours. The solar energy gives us between 90 and 120 amp.hours per day to use. When at anchorage, we draw less energy than when at sea. This difference is mainly due to the electronics and the auto-helm that draw a combined 4 amps on average. We have a battery monitor that measures the instantaneous consumption and keeps track of our overall consumption. This device is very useful to know where we stand, how charged the batteries are and whether or not we have a surplus energy to use. In a typical day, the solar panels would completely charge the batteries and the battery monitor would show that we reached full charge. When the sun is setting and the solar panels are not generating enough electricity, the batteries seamlessly take over the load. It is the time of the day where we cook dinner and use lights. These are not large loads: 1 amp for the stove solenoid (safety electrovalve that shuts off the propane flow outside of the boat); about .5 amp per flood lights in the saloon area and .2 amp per reading lights. We converted all the lights to LED for that smaller footprint at night. The big load is the fridge, which turns on its cooling system when the temperature in the fridge goes below the threshold set by the thermostat. It then consumes about 5 amp. Depending on the outside air temperature and the sea water temperature, the fridge would run anywhere between 10 to 30 minutes per hour (see note below on how we measured this duty cycle), all day long. All in all, when the sun starts to shine of the panels, we are typically down 30 amp.hours in our house batteries. On a sunny day, we would be back to fully charged by 10am. We can then use the extra energy to recharge our devices (computers, phones, drone, cordless drill, etc.) and also run larger loads like the water maker or the water heater. We monitor them closely to make sure we do not deplete our batteries with these power hogs but we can typically expect to run them a few hours per day. However, on rainy days, the solar panels are not providing as much. We then focus to recharge the batteries before charging the devices and we might very well not produce fresh water nor heat it up that day. Both of which are fine as we generally keep our water tanks full for that reason and we don’t really need a hot shower every day under the tropics -to be clear, we still shower but not necessarily with hot water ;).

When we are underway, the navigation electronics and the auto-helm must be on so our energy footprint is higher. And this is visible in the early morning as our batteries are typically down to around 60 amp.hours. In sunny days, we would be able to recharge and produce some more to make water; otherwise, we either reduce our consumption or we fire up the engine to recharge the batteries via the alternator. The former could be done by shutting off the fridge or steering by hand, but these are uncomfortable and only temporary solutions that can be deployed for a few hours only. The latter is always possible but the engine is noisy and it goes against the charms of a nice sail.

We are generally happy with our electric footprint and especially with the fact that we don’t need to use fuel for our energy. After the units cost of the solar panels, the system gives us more free energy than we typically need. And we have our diesel engine as our backup. So we do not need to plug in at marinas nor do we need a generator. Of course, we are very conscious about our energy needs and when it is appropriate to use energy: the kids have heard enough of “turn off the lights you don’t use” to “do not charge our iPad when the sun is not shining”.

But the engineering minds in us are always thriving for ways to improve any system. In our case, we could gain on two aspects: more battery capacity and more generation capacity. The 20% limitation of the lead-acid talks to battery capacity issue. By upgrading to lithium batteries, we could virtually use 100% of the charge. So we could either have a smaller battery bank for the same capacity, or much more capacity with a similar volume of battery. Regarding the generation, the limitation comes by the natural phenomenon that the sun only shines half of the day (on average) at a given location: when there is no sun, we depend on our batteries -or on our engine. We could install other forms of energy harvester. We see a lot of boats with wind generators and some with water generator. The wind doesn’t stop at night and it would be a good supplement under sail at night. And the water generator would convert some of the boat speed into energy to feed the electronics under way.

We have decided to not pursue these improvements for now for various reasons. First, lithium is expensive and it would require new equipment to control these batteries. Also it is not readily available in French Polynesia (long delivery time due to boat shipment). Second, we don’t have a great place for a wind generator as we maximized our stern arch with solar panels. And we don’t want to reduce their production with the parasitic shade of a wind generator. Finally, the cost of the water generator seems to us better spent in lithium batteries. We have considered extending our solar arch over the bimini but it is not necessary until we have more battery capacity. Unless a donor miraculously sponsors us to convert to lithium (interest donors, please send us a note to ;), we are going to wait until we reach a place where the lithium batteries are more readily available.

We see many boats running generators a few hours per day to top off their batteries. They typically have a larger electric footprint, generally because of a freezer or air-conditioning. Yes, these amenities make life easier but to us they are too close to what we can get on land and somehow -and it might be weird for some- we are not attracted to these comfort items. Why would we be on a boat if we would live like in a house? Comfort is such a personal perspective that, like religion, it can be discussed but you should not hope to convert anyone to your own personal believes. That is why each boat is it own special setup and that what makes for endless sundowner discussions.

Note on the fridge duty cycle.

It is very hard to determine how much time the fridge is running per day. It hums a bit when the compressor and the pump are working but we got used to their noise and we only take notice when they stop. We estimated it was working half of the time here in Tahiti and less when we were in colder waters and colder weathers (since the fridge’s refrigerant is cooled by sea water, rather than by a fan like a land-based fridge). To determine the percentage of time the fridge is on, we designed a small electronic system with an Arduino board to keep track of the ON – OFF ratio of the fridge. The board was simply sensing the input voltage at the pump and would update a variable holding the average: average=(cnt*average+(input==12V))/(++cnt). We introduce a 1-second delay in the main loop. Once the counter read 86,400 seconds (i.e. 24 hours), the average stood at 0.43. So the fridge was running 43% of the time during that full day, so about 10 hours and 20 minutes.

Of course, this average would change with the target temperature, the air temperature, the sea water temperature, the amount of food in the fridge (thermal inertia), the amount of time we open and close the lid of the fridge, etc. But at least, it gives a good idea of the duty cycle.

We plan to build a permanent monitor so we’ll be able to keep track of this average and detect a faulty condition. More on this in a later post.

10/13 Update.

On Makatea, we rafted with S/V Tao and Olivier had the gauges to check the refrigerant level in our fridge. It was low and therefore the fridge wasn’t performing well. We added some more coolant (thanks to Robbie for the coolant he found us in Morro Bay) and it was running fine again. Olivier told us that our cold plate in the fridge is supposed to run as much as possible during the day while there is plenty of energy and then just ‘coast’ during the low-energy times. We tried that approach: in the morning, we set the thermostat to -10 degrees Celcius and when the sun is low we put it back to 0 degree C. This way the fridge, doesn’t run at night, making it our more quiet nights for us but, best of all, we consume very little during the night. The bad news in all this is that it makes less reasons to upgrade to lithium.

But there might be hope, somehow. Our starter battery recently died so we need to replace it in a way or another. On one hand, we can get a cheap lead acid battery; on the other hand, we can experience the comfort of lithium, for a premium price.

Leave a Reply

Your email address will not be published. Required fields are marked *

For security, use of Google's reCAPTCHA service is required which is subject to the Google Privacy Policy and Terms of Use.