Calculating the Current Draw of a Typical Sailboat's 12v Domestic Circuit

The subject of boat electrics is a complex one, but the bottom line is that the current draw, battery bank capacity and charging regime must all be matched for the 12volt system to function satisfactorily.

This control panel will make it a simple matter to ascertain the current draw of each circuit

Here we deal with the first part of that equation; calculating the current draw of the domestic circuit over a typical 24 hour period.

Once this is known it's straightforward to assess the required size of the domestic battery bank.

And with that knowledge we can readily calculate the battery charging regime necessary to prevent undue strain on the batteries and keep the whole electrical system ticking over.

Assessing the Daily Current Draw

To calculate our daily domestic electrical requirement we must first make a list of all electrical equipment on board, and apply a current rating to each item.

If you've got a battery monitor installed in the system and capable of being switched to read amps you'll be able, by turning on one item at a time, to read the actual current draw for each item - otherwise you'll have to use a multimeter, or work it out.

Ratings can usually be found on equipment nameplates or in their manuals, and will be expressed in terms of power (measured in watts) or current draw (measured in amps). The relationship between power and current is expressed as:

Power (W) = Current (A) x System Voltage (V)

To derive amps from watts, simply transpose this equation and divide the wattage by the system voltage.

For example, a 6 watt navigation light bulb in a 12 volt system will draw 0.5 amps - which, if it's switched for ten hour each day when underway will have consumed 5 amp-hours (Ah).

Continuing in this vein for each item of equipment will produce a table much like that shown below, which incidentally, is the one I did for my boat Alacazam.

Click Here for a Live, Customizable Version of this Spreadsheet!

This calculation though, remains an estimate. For example:

in cold weather the fridge will draw less power than in hot weather;

in rough weather the autopilot would use more power than when it's calm;

hours of darkness will vary with latitude and time of year, affecting current draw for navigation and domestic lighting;

you'll use the watermaker more when you've got guests aboard etc, etc;

plus there are start-up currents and other losses that have been ignored.

So it's approximate, but indicates that you'll need to replace around 325Ah each day when you're sailing and 211Ah when you're at anchor.

The underway current consumption clearly presents the worst case scenario, with more power being consumed during the night than during the day. In this example the domestic battery bank will be drawn down by 175Ah during the hour night-time hours - an average discharge of around 14.6A over 12 hours.

With this knowledge you will now be able to decide on the minimum size of your 12 domestic battery bank capable of satisfying the requirements of the 12v domestic circuit.

So what's the difference between amps (A) and amp-hours (Ah)?

The best way to explain it is by example...

If an appliance drawing 5A was to run for 1 hour, its consumption would amount to 5Ah.

This would be the same as an appliance drawing 1A running for 5 hours - again the consumption would be 5Ah.

So amp-hours are simply the (average) amperage drawn multiplied by the time in hours.

Power Conservation

In our example there're several things that could be done to reduce the daily consumption:

LED (light emitting diode) lights. These draw a fraction of the current taken by a standard incandescent light and have an exceptionally long service life. I reckon if the anchor light, tricolour, cockpit light and cabin lights were replaced with LED's then at least 15amps could be shaved off the underway consumption and a similar amount off when at anchor. A further benefit of a combined anchor/tricolour LED light is that you won't have to scoot up the mast to change a blown bulb - a prospect I view with increasing dismay these days.