Calc. Electrical


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Electrical Load Calculation Process:

By Chuck Berrington
Vessel RelationShip
Size IP40
Excel 2000 Data File: Ip40Elec.xls 19K filesize
Directions to download: Click on link, save file to your computer, then open with Excel.

This Excel Work sheet is a guide for determining electrical loads for any yacht.  This example is the worksheet I used to size the battery bank for our IP40, RelationShip.

What we are trying to determine is the number of amp-hours consumed in a 24 hour period for all the equipment that will be operating on the boat. Thank goodness it does not all operate at the same time. Also different equipment is operating at (1) anchor and (2) when sailing. Also, more energy is consumed when sailing at night -- so that is the worse case.

Following is a description of the six (6) columns on the Excel work sheet, reading left to right.

bullet1st - A listing of all the equipment existing & expected to be on the boat.
bullet2nd - Identify & list the number of amps each piece of 12 volt equipment draws (from nameplate or manuals). NOTE, if only watts are given, one can determine amperage by dividing watts by 12 volts. 
bullet3rd - AT ANCHOR, (during a 24 hour period) Estimate the time in hours (minutes = decimal hrs) each piece of equipment will operate. 
bullet4th - this column is the product of Column 2 X Column 3
bullet5th - AT SAIL AT NIGHT, Same description as column 3
bullet6th - this column is the product of Column 2 X Column 5

Columns 4 and 6 are totaled as you can see. You select the higher of the two as being the probable maximum energy consumed in a 24 hour period.

In this case it was approximately 427 amp-hours consumed per day (24 hours).

As you know, one can only have a maximum discharge of 50% on the battery bank. Therefore, the size of the "House" battery bank for "RelationShip was 2 X 427 = 854 amp-hours.

I selected four (4) 4-D's batteries @ 200 amp-hour each which provided 800 amp-hour capacity for my house bank.

On RelationShip I also have a Group 27 battery (approx... 90 amp-hour) dedicated for engine start plus a second one dedicated to generator start. I have the capability to parallel the two group 27's in an emergency.

Therefore in summary, I have a total capacity of 980 amp-hours available.

I do not remember precisely, but at anchor the maximum was about 200 and a night sail was approximately 350 or so!

I am very comfortable with my energy system! It serves me well! The key to caring for the precious energy bank are "Smart 3 stage Regulators" on both the Charger and engine alternator and the adjusting them properly so the batteries are not cooked over time. In my case with AGM Batteries, which I highly recommend, they should be charged at approximately 14.2 volts, acceptance at 13.8 volts and most importantly float at 13.2 volts.

If you have any questions with the attached Excel worksheet or the above description, please email. I hope this will help you on your electrical load analysis project.

Regards, Chuck


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Last modified: April 05, 2006