Choosing an inverter

[badrano]

I'm trying to figure out how big an inverter my electrical system can handle. I have the 65 amp alternator and I'm not sure how directly that relates or limits the size of an inverter. I've got a 90 amp/hr house battery.

There's a boat parade of lights coming up and I'm thinking of powering some light-up inflatables so there is the fan and lights that will be AC powered. I still need to look to see what the fans will draw to determine the size inverter I need.

 

[Silvertip]

Can't answer your question without some load numbers. Load being whatever the current draw is for the devices you intend to power. But you can use a 10:1 conversion ratio to figure it out. For every amp of 120 VAC you intend to draw from the inverter, you need to apply roughly10 times that number of AMPs of 12 volt DC. Inverter efficiency varies so you also need to figure anywhere from 5 - 15% reduction in efficiency in the conversion. As an example, using a 75 watt light bulb I measured 7.5 amps of 12VDC INTO a relatively old inverter.

 

[badrano]

Can't answer your question without some load numbers. Load being whatever the current draw is for the devices you intend to power. But you can use a 10:1 conversion ratio to figure it out. For every amp of 120 VAC you intend to draw from the inverter, you need to apply roughly10 times that number of AMPs of 12 volt DC. Inverter efficiency varies so you also need to figure anywhere from 5 - 15% reduction in efficiency in the conversion. As an example, using a 75 watt light bulb I measured 7.5 amps of 12VDC INTO a relatively old inverter.

Part of the question then is how much can I load up on an inverter if I have a 65 amp alternator? 65A @ 12v is 780w.
If I apply a little safety margin to account for normal current draw on the alternator running the boat at night so I have nav lights, radio, gps, ignition, fuel pump, gauge back lighting....should I limit myself to a 400w inverter ie. 33A @ 12v, no more than 3 A @ 120v?

This will determine what my max AC load will be and not worry about blowing the alternator.

 

[Silvertip]

You don't have to limit the INPUT to an inverter. You need to determine what you expect to pull OUT of the inverter. Every AC device is generally labeled with its power consumption. Figure out what you are going to operated from AC and size the inverter OUTPUT accordingly. In other words, don't buy a 400 watt inverter if you intend to pull 500 watts from it. You can use an 800 watt inverter even if you only pull 400 watts from it. Keep in mind also that the battery is supplying the power for everything and the alternator is replacing what's being consumed. Even if you drew more power OUT of the inverter than the alternator was replacing, the battery will continue to supply the load even though the system is at a negative charge rate. You wouldn't want to run long with negative charge rate but you could turn something off if necessary to bring the system back positive again. You need to monitor the VOLTMETER at the helm or the AMMETER if you have one when operating all this stuff..

The numbers I just referenced are "EXAMPLES". Remember the 10:1 ratio in my earlier post. With your alternator capable of 65 amps maximum output, the most AC you can suck from the inverter would be 650 watts minus losses in the inverter and whatever 12 volt stuff is running at the time. That's a little over 5Amps at 120 VAC.

 

[badrano]

Keep in mind also that the battery is supplying the power for everything and the alternator is replacing what's being consumed. Even if you drew more power OUT of the inverter than the alternator was replacing, the battery will continue to supply the load even though the system is at a negative charge rate. You wouldn't want to run long with negative charge rate but you could turn something off if necessary to bring the system back positive again. You need to monitor the VOLTMETER at the helm or the AMMETER if you have one when operating all this stuff.

Thank you for that. I realized what I was missing. I could pull 100 amps off the battery but if the alternator could only handle 65 amps, the battery would eventually die (negative charge). It's not that I would burn up (well, it might) the alternator up if I pull 100 amps.