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Peter Kennedy has been in  business since 1991 designing, installing and servicing marine electrical systems. The purpose of this blog is to offer support to both professional installers and  do-it-yourself boat owners who wish to undertake this work themselves.

Peter Kennedy has been in  business since 1991 designing, installing and servicing marine electrical systems. The purpose of this blog is to offer support to both professional installers and  do-it-yourself boat owners who wish to undertake this work themselves.

How big a battery bank do I need?

Posted by Liam Kennedy on 1/18/2019 to Batteries (Conventional)
How big a battery bank do I need?

For most people to answer to this is "as big as possible".  Charlie Wing's book the "Boatowners Illustrated handbook of Wiring"  has a very good explanation of how to calculate the capacity you need. The calculations start by working out what your daily load will be both at anchor and while underway.

You need to remember that you cannot use all of the capacity your batteries have without shortening their life considerably.  You should aim to not discharge your batteries past 50% of their capacity.  In addition if you are charging batteries from your alternator while underway you will find that the last 20% of the charging cycle is painfully slow.  All of this means that the amount you have to work with for continued operation while underway is 30% of the rated capacity of your batteries. 

So for example to provide for a consumption of 100 Amp Hours per day and charge once per day you would need to have a battery bank rated at  333 Amp Hours.  

I enclose a sample table below for you to complete in order to calculate what your actual power consumption is..  I have filled in a couple of lines with a hypothetical example.  Every boat will be different of course and you will have to make a best guess at the answers.  You should do a separate table for when underway.  Often at anchor the loads will be higher as there are more lights left on for longer times and they are not offset enough by lower navigational equipment loads.

Power consumption At Anchor
  Watts Watts / 12 = Amps Time Amp
Hours
 Lighting    
    Galley  30 2.5  3 7.5
    Salon 60 5 5 25
    Cabins    
    Head    
 Navigation lights    
    Tricolor  or    
    Running lights    
 Navigation    
    VHF    
    Instruments    
    Autopilot     
    Radar    
    SSB    
   Other    
Fans and Blowers    
    Cabin fans    
    Galley or head     
 Pumps    
    Bilge    
    Water    
    Other    
 Galley    
    Propane valve    
    Refrigeration    
 Entertainment    
    Stereo    
    TV/VCR    
 Deck    
    Windlass    
    Electric winch    
     
    Total Amp Hours
 per day at anchor.
 32.5

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