Boat Power Consumption on 12v Systems

Before you begin reading, for an in-depth discourse on 12 volt electrics systems for boats, we would perhaps suggest obtaining a copy of “The 12 Volt Bible for Boats” which is generally lauded as a very good guide for using 12 volt power in a marine capacity, to fully understand what may be involved in setting up a 12v supply. You can purchase the book here.

GPS Chartplotters require a 12volt power supply, usually external and taken from the boat supply.

12 Volt Power Consumption and Amp Hours

As an idea of consumption vs battery capacity, it helps to  think of the following scenario:

Lets say you are running a 12volt Chartplotter and it draws 2amps.  Now if you had the plotter switched on for the full hour, it would have drawn 2amps in total for that hour, or 2 Amp Hours (2Ah), if you then leave the chartplotter running for a full day, or 24 hours, it would have drawn and consumed 48 Amp Hours in total (2ah x 24h = 48Ah)

For power supply: 12v batteries usually have an amp hour (Ah) rating which, on a simple level, can be thought of as the number of amps they will supply for an hour before being fully discharged. A typical small marine leisure battery of about the size of an average car battery is usually rated around 80Ah. This means the battery could supply an appliance drawing 80amps for about an hour before the battery would be discharged, or an appliance drawing 1 amp for 80 hours.

So to apply a typical 80ah battery to running the above Chartplotter example at it’s 2amp draw, The battery could run this Chartplotter for 40 hours before the battery was completely discharged and requiring charging (80ah divided by 2amp draw = 40hours). If you were likely to be running the battery to empty on a regular basis, you would have to ensure that it is a “deep cycle” battery, which is designed to take heavy discharging and re-charge – though you would also need to ensure the battery is getting a full charge as I mention in the next paragraph, to prevent the battery degrading over time.

Establishing Battery Bank requirement and Re-charging your battery bank from renewable sources (Wind or Solar)

If using Solar or Wind power to maintain the charge of a bank of batteries on a boat, you should identify what the draw on the electrical system will be, in amps, from all the electrical items you will be running, Eg : Lights, VHF, Plotter, Fridge, Heaters, Microwave and so on. Most appliances show their Amp draw and multiplying that by the number of hours run on each will give you an idea of the total amount of Amp Hours (Ah) being drawn from the battery bank. (Some appliances are only rated in Watts; to calculate the Amps draw, you take the Watts rating and divide it by voltage to get Amps)

To use another example of a larger boat to illustrate this – Let’s say that you’re:

  1. Still running the your chart plotter at 2amps, and this runs for the full hour.
  2. In addition you have a 12volt fridge rated at 8 amps when running, but it only runs for 15 minutes in an hour
  3. The combined total of your running lights draw 0.5amp and are left on over the hour, and
  4. Your VHF also uses 0.5amp in standby and receive mode and is left on over the hour
  5. You also run a 12v heater which draws 80amps but is only run for 15 minutes over the hour.  This means that in one hour, you are drawing:

A power consumption total of:

  1. Chartplotter – 1amps over the hour
  2. Fridge 8/4 = 2amps over the hour
  3. Running Lights 0.5amps over the hour
  4. VHF = 0.5amps over the hour
  5. Heater 80/4 = 20amps over the hour.

This adds up to 24ah of consumption in a typical hour. On identifying this figure you would then need to establish for how long you would normally be sailing and using all this electricity for before being able to charge the batteries at a shore power point, that would determine the size of your battery bank.

For example, if, in the above scenario, you regularly sail for 5 hours while using all this electricity, you would multiply 24amps x 5hours and identify a requirement for at least 120Ah capacity in your battery bank if you have no charging system between visits to shore to “hook up” to a battery charger. Which leads us on to onboard charging.

Most Solar Panels or Wind Turbines give an output in Watts – So to establish what wattage input you will require from your charging source, you can use the familiar (to those who work with electronics regularly!) equation of (Watts = Volts x Amps) – You know that you have 24amps being drawn from the calculation above, and you know that you have a 12 volt system on the vessel – so by multiplying 24 x 12, you establish that you will require 288 watts of input per hour to “keep up” with the electricity being consumed – a Fairly hefty bank of solar and a good sunny day, a good quality turbine and a decent breeze, or a combination of the two!  Obviously this amount of input would only be required if you wanted to keep the battery banks continuously topped up – If you were simply looking to extend the time you could spend at sea before requiring to return to shore power to fully recharge the battery bank, a lower Solar/Wind power input wattage could be used, and using the (Watts = Volts x Amps) equation you would be able to calculate how many amp hours you would be able to extend your power supply by.

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