Through three examples we compile an electrical balance from three boats equipped differently. Adapt it to your particular case.
Example No.1
Day trip, return to port at night

Equipment Instant consumption Daily use Daily Consumption
Amperes Hours Amperes
Refrigeration 4 6 24
Pressurized water 8 1 8
Music 1 2 2
Automatic Pilot 5 2 10
GPS/Card Reader 2 6 12
VHF Emission 4 ¼ 1
VHF reception/watch 0.1 10 1
Sounder 1 4 4
Central Navigation 1.5 6 9
Total Consumption 24h 67 Amperes
Example No.2
Day trip, night mooring

Equipment Instant consumption Daily Use Daily use
Amperes Hours Amperes
Main cabin 2 4 8
Cabins 2 1 2
Chart table 1 1 1
Toilets 2 ½ 1
Mooring lights 1 8 8
Galley 1 ½ 0,5
Refrigeration 4 16 64
Pressurized water 8 1 8
VHF watch 0,1 24 2,4
VHF emission 4 ¼ 1
Sounder 1 4 4
Multimedia (tele, computer) 4 2 8
GPS/Card reader 2 6 12
Central navigation 1,5 6 9
Total consumption 24h 131 amperes
Navigation Longer than 24 hours

Equipment Consumption Utilisation
Instant Daily Daily
in amperes in hours in amperes
Main cabin 2 1 2
Cabins 2 1 2
Chart table 1 1 1
Toilets 2 ½ 1
Galley 1 ½ 0,5
Navigation lights 2 6 12
Deck light 4 ¼ 1
Lighting instruments 0,5 6 3
Refrigeration 4 16 64
Pressurised water 6 ½ 3
Pilot 4 12 48
Central navigation 1 24 24
VHF watch 0,1 24 2,4
VHF emission 4 ¼ 1
GPS 0, 5 24 12
Card reader 2 24 48
Graphic sounder 1 4 4
Radar watch/emission 2 5 10
Total consumption 24h 239 amperes

Once the assessment is made, we take a battery capacity at least equal to twice the daily consumption (ideally 3 times.) This enables us to not discharge more than 50% of the batteries (would not be supportable.)
Example 1:
In the case of the day trip boat, it takes at least 150 Ah.
Example 2:
For those who spend the night at anchor, it takes at least 260 Ah.
Example 3:
For more than 24 hours it must be at least 480 Ah.


A battery consists of plates (grids of lead) which enclose the active material in negative plates and lead dioxide for the positive. These plates, insulated from each other, are immersed in a solution of sulfuric acid and water (electrolyte.) When connecting a current source to this set (battery charger), an imbalance occurs: a plate becomes positive, one negative, the battery is charged. When one consumes the battery, the reverse occurs.
The battery capacity is always expressed in Ah for a given discharge time 20 hours (C20) for a starter battery or similar. In theory, a 100 Ah battery can deliver 100 A for an hour or 5 A for 20 hours. In practice, we must never fall below 80% capacity. This means that on a 100 Ah battery, one never goes beyond 80 Ah. By construction, a battery is designed for a number of charge-discharge cycles (between 100 and 1000) which depends on its technology. A charge-discharge cycle is a discharge of 80% capacity and recharge to 100% or two discharges of 50% of its capacity or four to 25%.


There are two types of battery. Starter batteries are capable of providing a large amount of energy for a short period of time (starting a diesel engine, electric windlass, bow thruster…) Service batteries supply electrical consumers for a long period of time (fridge, lighting, electronics…) They must withstand a longer and slower discharge before recharging.


These are the first models of battery. Proven technology on which caps are now sealed to prevent any maintenance. These batteries should be deeply discharged in order to return to their original capacity
Selon la règlementation, les batteries fermées et ouvertes doivent être placées dans un bac étanche avec une aération à l’air libre.


To preserve battery life, you should never go below 50% discharge. For wintering, the best solution is to charge them to maximum and leave them on board. Avoid taking them home, a battery discharges more with heat than cold! The loss of capacity is about 2% per month over 6 months this represents 12% of capacity. To reset, simply recharge them well before use. Well maintained batteries have a longer average life of 5 years. After this they may still work well but have lost much of their capacity.

Professional Advice

“The choice of a battery is often dictated by location. An important point is to change all batteries at the same time and select models of the same technology. "


According to regulations, the closed and open batteries should be placed in a sealed container with a vent to the open air.