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Old 04-09-2011, 15:35   #1
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Sizing a Generator (120v ac)

I know that most generators are chosen based on start-up loads of air conditioning or other motor driven gear. If you don't need to consider startup loads, then what is the sweet spot for sizing a generator? For instance, if you have a 2400w, or perhaps a 4800w load, then what size generators would be considered the best, for fuel consumption and longevity.

Thanks in advance,
Chris
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Old 04-09-2011, 16:05   #2
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Re: Sizing a generator (120v ac)

70% of rated load for efficiency. 1,800 rpm for noise and, especially, longevity. Inverter-type generators for stable output. Victron did a detailed generator study with tons of data.

Bill
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Old 04-09-2011, 16:17   #3
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Re: Sizing a generator (120v ac)

You will gain a huge amount of efficiency if you pair your genset with an inverter which is capable of adding inverter power to cover short-term start-up loads. Victron inverter/chargers do this, and I believe Mastervolt and some others will, too.

Then you can ignore the startup loads and size the generator so that it is running decently loaded -- as diesel engines like to be -- at your actual sustained loads. I think diesel generators are usually running with reasonable efficiency and low risk of bore glazing at 30% of their rated capacity. The greater the load as a percentage of rated capacity, the more efficient the genset will run in terms of diesel fuel consumed per unit of power delivered. On the other hand, you don't want them running at or near their rated capacity for a long time -- accelerates wear. So the "sweet spot", as you say, would be probably 70% to 80% of rated capacity, as the maximum sustained load you would expect to encounter.

The beauty of the Victron and other modern inverter/chargers is that you can set the maximum power draw from any AC power source. So you can positively prevent the genset from being overloaded by setting the maximum power draw to, say, 90% of the rated capacity. When this is reached, the inverter/charger will start cutting back the charger. When that fails to prevent an overload, the inverter will start pulling power out of the batteries to make up the shortfall.

This totallly solves the inrush current problem with air conditioning -- very cool.
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Old 10-09-2011, 12:58   #4
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Re: Sizing a Generator (120v ac)

Do those inverters match phase from the generator, so they work like that with any AC source?
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Old 11-09-2011, 07:25   #5
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Re: Sizing a Generator (120v ac)

Quote:
Originally Posted by sdowney717 View Post
Do those inverters match phase from the generator, so they work like that with any AC source?
Yes, they have to in order to supplement the source. It is possible they have design limits, make sure the inverter you are buying will work with the source. (50hz or 60hz, etc.)
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