In this article, I write about a few different things that affect the type of battery and how much battery capacity may be needed for various purposes on board. Here you can find information about the typical consumption of electric energy, how much electricity production is needed, and a justification for the battery choice in the various subsystems of the fictitious sailboat that the articles use as a reference.
Before we can choose batteries for our electrical subsystems, we need to have an idea of how much energy the boat’s consumers use on a daily basis. It is only then that we can dimension the boat’s production of electricity and what battery capacity is needed to store produced electricity until it needs to be used by the consumers.
Energy requirements on board
When planning how much energy you want to be able to store in your batteries, you need to take a few different things into account
- How much is consumed on board in different situations?
In port, on the move, under sail, laid up on land. - What charging options are there?
An alternator on the main engine, a separate generator, shore power, solar panels, a water generator, a wind generator, or another energy source like a fuel cell. - How much energy can the various energy sources provide under the conditions you plan to be in?
Sunny, windy, under sail, moored in port, anchored, size of fuel tanks for engine and power plant.
A little further down in the article there is a table with estimated energy consumption for 2-4 people in situations when you do not have access to shore power. The figures come from our own consumption during the 3 years we lived aboard Sally at more southern latitudes where there is much sunlight for the solar panels.
In warmer climates, the air is more humid, which causes more ice to form in the fridge and freezer. The ice and the warmer climate mean that the fridge and freezer usually uses more power than specified. Cabin fans are also needed in the warm climate, but the heaters are not used at all 🙂
The columns of the table contain the following values.
- Column 1, indicates the average consumed power during the time the consumer is running.
- Column 2 & 3, indicates how many hours per day the consumer is running for the different situations.
- The last two columns contain information on consumption (Wh) per day at anchor or when we are sailing. I have not included consumption when the boat is moored with shore current because the shore current is often able to provide the energy that the boat needs.
| Consumer | Watt | Hours when anchored | Hours when sailing | Wh when anchored | Wh when sailing |
| Fridge 40l | 20 | 24 | 24 | 480 | 480 |
| Fridge box 80l | 40 | 24 | 24 | 960 | 960 |
| Freezer box | 40 | 24 | 24 | 960 | 960 |
| Water pumps & toilet | 200 | 1 | 1 | 200 | 200 |
| Lights LED | 10 | 6 | 6 | 60 | 60 |
| 2 cabin fans | 5 | 12 | 6 | 60 | 30 |
| Mobile router & phones | 10 | 24 | 12 | 240 | 120 |
| Computers & music | 30 | 12 | 12 | 360 | 360 |
| Log, echo sounder, … | 5 | 1 | 24 | 5 | 120 |
| Vhf & IridiumGo | 5 | 1 | 24 | 5 | 120 |
| Radar standby | 10 | 0 | 22 | 0 | 220 |
| Radar transmitting | 20 | 0 | 2 | 0 | 40 |
| Plotter 10 inch | 10 | 0 | 24 | 0 | 240 |
| Plotter 14 inch | 16 | 1 | 24 | 16 | 384 |
| Autopilot electrical | 50 | 0 | 24 | 0 | 1200 |
| Anchor light LED | 1 | 12 | 0 | 12 | 0 |
| Navigation lights LED | 4 | 0 | 12 | 0 | 48 |
| Sum | 0.5kW | – | – | 3.3kWh | 5.6kWh |
Electric stove and other cooking appliances
We have an electric stove that we use daily and a separate kettle that is quicker to use to boil water with. In addition to boiling water for coffee and tea, we use it to preheat water for cooking, e.g. when we have to cook pasta, rice, eggs, etc., it is faster than using the stove, which has a lower effect on a plate than the cooker which is at almost 2kW.
I have compiled the consumption when we use the electric stove with halogen plates where one plate has the power of 1000W and the kettle has the power of 1800W. We also have a baking machine that we use every 3 days to bake bread. I have not measured its consumption, however, a qualified guess is that it uses about 500 Wh when we bake bread.
The numbers are approximate, but they give an idea of how much electricity is used with an electric stove.
| Consumer | Power  | Time (min) | Wh one usage |
| Boil 1 litre of water | 1800 | 5 | 150 |
| Boil 4 servings of pasta | 500 | 10 | 83 |
| Boil 4 servings of rice | 300 | 20 | 100 |
| Heat the frying pan | 1000 | 5 | 67 |
| Frying 10 min | 500 | 10 | 83 |
| Bake a bread | 500 | 60 | 500 |
To cook 4 portions of pasta, 150+83=233 Wh of electrical energy is used.
Depending on how much and what kind of food you cook, an electric stove will draw different amounts of electrical energy. My estimate from our cooking with 8 cups of coffee a day, pasta or rice, frying, and baking bread every 3 days gives us a consumption of around 750Wh per day (2 pers).
Other high-power consumers
In addition to these consumers, there are other consumers with a large power that are not used daily, e.g. a starter motor, a vacuum cleaner, an electric mixer, a drilling machine, the bow thruster, the anchor windlass, the furlers and the windlass, which also contribute a part to the energy consumption. This consumption is more sporadic and the reserve capacity in the battery bank for service batteries is enough for them for normal use. Some of the consumers will also be placed in their own subsystems, see previous articles about the various subsystems.
How much energy production is needed?
If we don’t want to charge batteries every day, the table above gives an indication that the energy production of a modern 45-50 foot sailboat needs to have a capacity of at least 6 kWh per day, probably a little more to cover the needs of the sporadic large consumers.
Assume the boat has 500W solar panels. In sunny weather at more southern latitudes, they produce approximately 200W on average over 8 hours if they are placed horizontally and without shade. This gives an energy production of around 1.6kWh per day. The battery bank thus loses around 4 kWh per day when sailing. For anchors, they lose just under 3 kWh per day.
How big a battery bank is needed?
If we want to avoid running the engine or the generator every day, the service batteries need to have a usable capacity of at least 4kWh, which corresponds to 8kWh LFP batteries at 50% DoD. That gives us an extra capacity of 1.5kWh before 20% DoD is reached.
If the boat has a gas stove and we do not need electricity for cooking, the battery bank can be reduced with 1 kWh LFP batteries.
With LFP batteries, it is possible to have an even larger battery bank to create even greater reserve capacity to better cope with periods with little sun and wind without having to use an engine or generator to charge.
Which batteries should you choose?
Remember to see the batteries as part of a larger system where both chargers and consumers are included and to protect batteries and the boat, you must ensure that the battery solution is monitored, i.e. it turns off charging when the batteries are fully charged and that they close off the power supply when they are about to run out.
- Should the battery capacity be divided into several battery banks and if so why?
- Which type of battery should I choose for the different battery banks?
In a previous article, I described how the boat’s electrical system is built up with chargers, shore power, solar panels, batteries, and consumers and there are four different electrical subsystems that have their own batteries/battery banks, see Electrical System, Part 2 – Overview.
Here are some general tips you can follow when choosing batteries for various purposes on board.
Selection of starting battery
As a starting battery, my first choice is a separate AGM battery that can deliver sufficient current without the voltage dropping too much.
Selection of service batteries
As service batteries, my first choice at the moment is a number of parallel-connected LFP (LiFePO4) batteries equipped with a BMS that automatically balances the cell voltage within each battery, but also the voltage across all batteries. Charging is done with one or more 3-stage chargers intended for the LFP battery in question.
If several parallel chargers are needed to get maximum charging current, make sure they are synchronized and have the same measure of battery voltage and battery temperature.
If for some reason you cannot use LFP batteries as service batteries, I suggest you use AGM batteries based on lead-carbon technology or GEL batteries. Probably it will be too heavy to install the same capacity as with LFP batteries. The limitation will probably be weight/space for the lead batteries, so build as large a battery bank as you can fit and which you can charge to 100% relatively often to avoid sulphation that occurs in insufficiently charged batteries.
Because lead-acid batteries cannot be charged as quickly as LFP, charging will take longer, which is a disadvantage when charging from the engine or the generator, which gets more operating hours.
Selecting a dedicated battery
If a dedicated battery bank is needed for a special purpose. Choose the battery that best suits the type of consumer to be connected.
Is it, for example, a bow thruster or other consumer with a large power that can lower the voltage too much for other consumers. Choose an AGM battery with sufficient capacity and power.
If the purpose of the battery bank instead is to increase fault tolerance in the subsystem for 24V Service by dividing 24V Service into two different electrical subsystems, e.g. one for the real large consumers such as stoves and inverters and one for 24V consumers used during sailing, choose the same type of battery in both subsystems.
Battery selection for the various subsystems
In two previous articles there is a description of the various electrical subsystems in a fictitious 45-foot long-distance sailing yacht, see Electrical system, part 2 – Overview.
For each subsystem, I have specified an approximate battery capacity in Wh. It makes it easy to compare the capacity of the different subsystems because the nominal voltage does not affect the energy measure Wh. If the capacity is specified in Ah, the nominal voltage must also be taken into account in order to be able to compare the capacity in the different subsystems.
Own battery in 12V Start
Assumes a 12V starter motor.
Battery selection
1.2kWh AGM starter battery (100Ah 12V) which is able to supply starting current to the engine and generator.
The reason for the choice is that you get a maintenance-free battery that will last longer than a normal starter battery.
Advantages
There are several advantages to having a separate battery dedicated to a starter battery.
- Increased operational reliability by having a small and simple electrical subsystem that works independently of all other electrical subsystems on board. It increases the chance that it will always work when the engine needs to be started.
- Can use a battery designed to supply enough power to the starter motor.
- Protect other consumers against too-low voltage that occurs in connection with starting or too-high voltage that occurs during a so-called ”load dump”. See the Electrical system, part 3 – Alternator and charging
Disadvantages
- More complicated and thus more expensive installation than using the same battery bank for starting and 12V Service.
Own battery in 12V Service
Assumes there are consumers using 12V.
Battery selection
LFP battery 2.5kWh (200Ah 12V) or AGM lead-carbon or GEL battery with a capacity of 7.5kWh (600Ah 12V).
The reason for choosing LFP is to get a battery bank that holds as much useful energy as possible per kg of battery and still be safe and has a long life.
Advantages
The advantages of a separate battery bank for 12V Service are
- Can use battery technology that provides long life and higher capacity per kg of battery.
- For operational safety, consumers that need 12V have been separated into an electrical subsystem that works independently of the other subsystems.
- Operational reliability, with a 7.5 kWh capacity, the battery bank can be used as a starter battery in an emergency.
- Since the boat is equipped with a large battery capacity in 24V Service, a DC/DC converter from 24V to 12V could be used to supply 12V consumers with electricity.
With a DC/DC converter, you lose some energy, and it always draws some current when it is running. Another disadvantage is that there is no emergency start battery.
The advantage of the DC/DC converter is that you can concentrate all the capacity in one battery bank for 24V Service.
Disadvantages
- More complicated and thus more expensive installation to charge a 12V Service from all different energy sources (shore power, engine, power plant, solar panels, wind power, etc.).
- Probably you need more total installed battery capacity because you split the battery banks into 12V and 24V respectively. It is not certain that you use the same amount of 12V/24V consumers all the time. This means that the batteries in the different battery banks for service are discharged at different rates.
Own battery 24V Service
Assumes that the majority of consumers need 24V.
Battery selection
Battery bank with LFP batteries 10kWh-20kWh (400Ah to 800Ah 24V, weight 125kg-250kg) or AGM lead-carbon or GEL batteries with the capacity 20kWh-30kWh (800Ah to 1200Ah 24V, weight 500kg-750kg).
By choosing LFP, you get a battery bank that holds as much useful energy as possible per kg of battery and is still safe, and has a long life.
The higher capacity is recommended if you have replaced gas with an electric stove and oven. If you cannot use LFP, you should not install an electric stove because the capacity requirement is not covered by a reasonable amount of AGM batteries for this size of boat.
If the boat is equipped with a bow thruster in the stern, you have the option of having a separate battery bank for the stern thruster.
If the battery bank for 12V/24V Service is located near the bow thruster, it can be used instead. However, ensure that the battery bank has sufficient capacity and can withstand the double power output compared to what the bow thruster requires at full power. As long as the batteries have more than half their capacity left, the voltage should not drop below 12V/24V when the bow thruster is in use.
Advantages
- No need to use a DC/DC converter from 12V to 24V to supply 24V consumers with electricity.
- Can use thinner cables in the electrical system, which saves weight, and space and is cheaper.
- You can have a battery built to withstand deep discharge and many charge cycles without being damaged.
Disadvantages
- I see no disadvantages. If you have many 24V consumers on board, I believe that there needs to be a 24V battery bank that can withstand deep discharge and many charge cycles.
Own battery 12V/24V Bow thruster
Assumes that there are one or more large 12V or 24V consumers in the fore part of the boat.
Battery selection
3kWh – 4kWh (150Ah-200Ah 24V, 80-120kg) AGM battery based on lead-carbon technology.
The reason for choosing AGM based on lead-carbon technology is to get a battery that can withstand high current draw and many charging cycles.
The bow thruster has high power and thus uses a large current, which lowers the battery voltage down to 11V/22V or lower, which can interfere with other equipment that requires 12V/24V to work.
Battery capacity and power depend on how much power the bow thruster has and how much it is expected to be used.
Today it is common for sailboats to have double rudders and no stern thrusters. They may need to use the bow thruster in the bow more often and perhaps for longer because the propeller power from the main engine cannot be used to turn the boat.
Advantages
- Do not disturb equipment sensitive to low voltage.
- Safety, you always have a charged battery for the bow thruster and there is no risk of discharging the service batteries during difficult maneuvers.
- You can have a battery built to be able to deliver high power (large current).
- Can use thinner cables for the bow thruster because the batteries are located closer.
Disadvantages
More complicated and more expensive installation to charge a separate bow battery.
Sailing Sally 