How big an alternator do you need ?

Since a lot of this blog is about electronics I presume that you – like me – may have been busy adding all sorts of gizmos to your boat. Well, the time comes when you had better check the electrical consumption of all those useful devices and aids!. I thought I would do that and see if the battery and alternator configuration that I have is adequate.

I have two domestic batteries and one engine battery each of 60 amps and normal lead acid types. This is a perfectly standard installation and is charged using a 120 amp alternator.

So my maximum daily consumption of electricity must not exceed 120amp hours without recharging the batteries in a marina or by using the engine.

Here are three assessments of electrical consumption – I have tried to be reasonably extravagant in my assumptions about turning lots of lights on and using every piece of nav equipment at once…

Sailing Marina Anchor
energy-sailing-1 energy-marina energy-anchor

To download my free Excel spreadsheet showing how to assess your power usage while at marina, anchor, or sailing please click here

I would be interested in your experiences…


1 thought on “How big an alternator do you need ?

  1. The question is how big an alternator…surely this depends on the rate and amount of discharge of the batteries – which differs from owner to owner.

    Normally a saling boat (unlike a car) will deeply dicharge its batteries so the alternator must be big enough to recharge as quickly as possible on the basis that a sailor wants to run the engine as little as possible.

    A battery that is 50% discharged will will accept a rate of charge 25% higher than its rated capacity ie. a 200Ah battery will accept charge of 50amps. But as the battery comes up to charge the acceptance rate declines.

    By the time the battery is up tp 70% of full charge its acceptance rate will be down to 10/15% of its rated capaciy.

    The battery will begin to heat up an gas if the charger continues to run at the same rate…

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