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### How Much Power Consumption

Posted:

**Thu Feb 18, 2010 10:38 pm**
by **CMS**

Hi,

I am trying to find out how much HP a 100 amp alternator will consume at max output. Balmar states 1 HP for every 25 amps but my numbers come up with roughly 2.3-2.4 hp for a 100 amp alt? Do these HO alts really consume that much energy from my sailboats engine.?

Thanks..

### Re: How Much Power Consumption

Posted:

**Fri Feb 19, 2010 1:20 am**
by **Coulomb**

The Balmar rule is a convenient rule of thumb that easy to remember, but Amps isn't the best parameter, it's Watts. Watts_out equals watts_in times efficiency. So, if you're producing 25 Amps at 14 Volts the alternator is generating 350 Watts. One HP is equivalent to 746 Watts, so efficiency in this case must be 47%. This isn't a bad number, but efficiency also varies with alternator RPM, generally best at about 3000 RPM.

Large frame alternators are more efficient than small frame units - the heavier they are, probably the more efficient they are. Efficiency on small frame alternators can be as low as 40%, and even the heaviest large frame units usually don't top 65%, unless they are a hybrid with permanent magnets to augment the field.

So yes, those alternators suck the power.

### Re: How Much Power Consumption

Posted:

**Fri Feb 19, 2010 5:26 am**
by **CMS**

Would a a 3:1 pulley ratio make the engine see more hp draw than if it was a 1:1? So if it draws .47 hp is that at a 1:1 direct drive or does that figure the 2.5:1 or 3:1 pulley ratios. I am still perplexed as to how Balmar and some others arrive at 1 hp per 25 amps (assuming 14v here) when 746 watts is one hp?

In short does a pulley ratio of say 2:1 make a .50 hp (373 watts) alt load look like a 1 hp load at the engine or is a 1hp alt load a 1hp alt load regardless of pulley ratio?

### Re: How Much Power Consumption

Posted:

**Mon Feb 22, 2010 7:39 pm**
by **CMS**

Asking again as I am trying to size an Ample alt for a small 3/8" belt engine....

Would a a 3:1 pulley ratio make the engine see more hp draw than if it was a 1:1? So if it draws .47 hp is that at a 1:1 direct drive or does that figure the 2.5:1 or 3:1 pulley ratios. I am still perplexed as to how Balmar and some others arrive at 1 hp per 25 amps (assuming 14v here) when 746 watts is one hp?

In short does a pulley ratio of say 2:1 make a .50 hp (373 watts) alt load look like a 1 hp load at the engine or is a 1hp alt load a 1hp alt load regardless of pulley ratio?

### Re: How Much Power Consumption

Posted:

**Tue Feb 23, 2010 1:35 pm**
by **Coulomb**

Pulley ratio doesn't affect the engine power required. Alternator efficiency is the governing factor.

The load on the belt is dependent on the power input to the alternator. A 3/8 belt can drive a #4023 alternator provided that there is about 150 degrees or more belt rap on the alternator sheave. A 3 inch sheave is recommended, although a 2.6 can be used as long as tension is maintained.

Side load on the front engine bearing is not a concern for a belt properly tensioned.