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I have a 950w generator, with an a/c outlet that puts out 20 amps, in which I am currently plugging a 20a charger, but this is inefficient. How can I utilize all the extra voltage that's going to waste?
Not sure, I never have that problem as I have things to make that need AC power. So when the generator is on I am being productive with tools of various types that need AC to run them or charge them. Then the excess from that can go to charging the main house battery.
A 950 watt generator is closer to about 7 amps. Might be close to 10 or so on peak.

A 20 amp charger for 12v batteries is going to pull about 2-3 amps, maybe 5 amps (at 120v) at the beginning of charging a depleted battery.

These are all round numbers.

You don't have any 'voltage going to waste' ...actually your genset and your charger seem to be closely matched.
If you don't already have some solar I would suggest getting some to top off your batteries after using the generator to bulk charge first thing in the morning. I was able to really cut down my generator run times by simply using a portable 100 watt system and still keep healthy batteries. I also was able to charge a jumper pack while running the generator so that later on I could charge my small electrical appliances without starting the generator if there were clouds or rain and the solar was needed to keep the house battery at a healthy charge. This allowed me to fine tune my small dual battery bank.
You could probably use a higher amp battery charger but not by much.

I tried to run my 50 amp Iota charger on my Honda 1000Ei but it would overpower my generator on start up.

You didn't say what size battery bank you're charging. The battery charger should be matched to the battery bank for good charging and then the generator sized to accommodate the charger.

But no matter how closely matched, you're still going to be looking at 5 hours from 50% to full no matter how well matched. That's why a lot of us use the generator for bulk charge and then go to solar the rest of the day.
(05-06-2019, 02:52 PM)tx2sturgis Wrote: [ -> ]A 950 watt generator is closer to about 7 amps. Might be close to 10 or so on peak.

A 20 amp charger for 12v batteries is going to pull about 2-3 amps, maybe 5 amps (at 120v) at the beginning of charging a depleted battery.

These are all round numbers.

You don't have any 'voltage going to waste' ...actually your genset and your charger seem to be closely matched.

Then where is all the wattage going, or are you suggesting my battery is being charged at close to 950 watts?
Is this a inverter generator with a eco mode? If so then turn on the eco mode and the generator will lower the output to match the charger and be as efficient as possible.
(05-06-2019, 03:43 PM)Hircarra Wrote: [ -> ]Then where is all the wattage going, or are you suggesting my battery is being charged at close to 950 watts?

If the load pulls more wattage, the generator will burn more gasoline.

If the load is modest or very small, the generator will throttle back and burn less gasoline (or LP if it uses that).

The wattage doesn't 'go' anywhere, except into the load you have plugged into the generator.
(05-06-2019, 04:01 PM)tx2sturgis Wrote: [ -> ]If the load pulls more wattage, the generator will burn more gasoline.

If the load is modest or very small, the generator will throttle back and burn less gasoline (or LP if it uses that).

The wattage doesn't 'go' anywhere, except into the load you have plugged into the generator.

Then are you suggesting the load on my generator is close to 950 watts? The OP question is implicitly: How do I use the 950 watts?
My 'suggestion' is to run the generator as you are now.

If you intend to max out the load on the generator constantly, it will run hotter, burn more fuel, need more maintenance, and will make more noise. If it is running at maximum wattage, any additional load will probably either shut it down or trip the overload circuit or the breaker. Not a good plan.

Car engines might have a rating of say, 350 hp, but you don't work them at 350 hp ALL the time, only sometimes.

But, it can be done, by using a larger capacity battery charger, OR, you can use some of that 'surplus' to power another battery charger, or other loads such as fans, refrigerators, laptop chargers, etc.
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