What is an electricity balance and how to calculate it?
A boat's electrical balance consists of estimating the electrical consumption of each device in Ah (amperes hours or number of amperes consumed per hour).
Then to cumulate all the consumptions by carrying them over one day. This calculation takes into account the daytime hours (no need for lighting, heat that triggers the fridge...) and the nighttime hours, taking into account also the days of navigation (electronic instruments, autopilot...), as well as those spent at anchor.
Once all the consumers on board are listed, they are added together and a daily consumption in Amperes hours or Watt hours is obtained depending on the chosen value.
Reminder of the calculation formula: P=U x I
Power (Watt) = U (voltage in volts) x I (Current in Amperes)
Example of an electrical balance sheet:
- AutoHelm ST2000 AutoHelm Pilot 2A consumption for 6h = 12Ah
- Incandescent masthead light 1Ax 8h = 8 Ah
- Refrigerator 12v 40W (40W/12V=3A) works 5h (5h x 3A) = 15 Ah
- 2A phone charger for 6 hours = 12 Ah
- GPS navigation unit 1.5A for 6 hours = 9 Ah
- Laptop computer 5ah for 2 hours = 10 Ah
- Incandescent 8A interior lighting for 3 hours = 24 Ah
- Water pumps 8A for 0.5 h = 4 Ah
- Total = 94 Ah
We round it up to 100 Ah for an electricity balance typical of a coastal cruiser.
The question of the desired autonomy
Then we must ask ourselves the question of the autonomy we are looking for without using a charging mode (engine, generator, hydro generator, wind turbine or shore charger). In other words, you have to decide if you want to be able to stay at anchor without a power unit or engine for 1 or 2 days or more!
Let's say that being a little misanthropic in navigation, we want to have two days without engine, group or port, so we will have to multiply our assessment by 2 (days), or 200Ah for our example.
Determine the capacity of the battery fleet
To determine the capacity of the batteries, there are two scenarios:
- The case of lead batteries (which support 50% discharge) 200 Ah x 2 = 400Ah.
- The case of Lithium batteries (which support 100% discharge) = 200Ah.
All we have to do now is choose a battery type among those of the trade and to compose a park:
