Inverter myths?

[Macblaze]

Ok, maybe they are not myths anywhere but in my head.

Our boat has a Magnum MS 2812 Pure Sine Inverter/Charger... 2800 watts.
I also have a small Motomaster Eliminator 300 W inverter (modified sine wave) that plugs into the 12v socket..

In my head it makes no sense to fire up the big inverter for no reason and I often just use the small one to charge up camera batteries etc. because it's more "efficient." But I looked at the specs today (yes, I want to be out sailing that bad...) and from what I can tell from all the gobbledygook is that the Magnum has an Inverter Efficiency (peak) of 89% and the specs on the Motomaster says Efficiency (maximum) 85%.

So that means I should just use the big one despite my inner prejudice that bigger must mean it uses more power?


The Magnum specs say:
No load (120 VAC output, typical) 30 watts
(that would be .25 a right?)

There is a spec on the Motomaster that says :
No load current draw (at 12V) <0.3 A

So... anyone care to check my math (and my assumptions)?

[Shrew]

30 watts @ 120v = 0.25a

It looks like you mixing measurements. One is measuring Draw @ 12v, the other is measuring Output at 120V.

[Colin A]

Originally Posted by Macblaze

Ok, maybe they are not myths anywhere but in my head.

Our boat has a Magnum MS 2812 Pure Sine Inverter/Charger... 2800 watts.
I also have a small Motomaster Eliminator 300 W inverter (modified sine wave) that plugs into the 12v socket..

In my head it makes no sense to fire up the big inverter for no reason and I often just use the small one to charge up camera batteries etc. because it's more "efficient." But I looked at the specs today (yes, I want to be out sailing that bad...) and from what I can tell from all the gobbledygook is that the Magnum has an Inverter Efficiency (peak) of 89% and the specs on the Motomaster says Efficiency (maximum) 85%.

So that means I should just use the big one despite my inner prejudice that bigger must mean it uses more power?


The Magnum specs say:
No load (120 VAC output, typical) 30 watts
(that would be .25 a right?)

There is a spec on the Motomaster that says :
No load current draw (at 12V) <0.3 A

So... anyone care to check my math (and my assumptions)?

If it's 30W draw on the DC side (which is what I would assume) your original assumption would be correct. I have found it's usually better to have a small inveter for small loads and the big one for everything else. But there are alot of variables that can throw that off.

[Kelkara]

I don't know anything about inverters ... but ...

for charging camera batteries, if it charges via USB, then you'll get way more efficiency by plugging a USB charger directly into the 12V without using an inverter to convert from DC to AC and back to DC again ... and if you forget to unplug it, the parasitic drain is probably less than 20mA.

[Macblaze]

Originally Posted by Shrew

30 watts @ 120v = 0.25a

It looks like you mixing measurements. One is measuring Draw @ 12v, the other is measuring Output at 120V.

And...?

My understanding of the math is not up to understanding the implications.

Originally Posted by Colin A

If it's 30W draw on the DC side (which is what I would assume) your original assumption would be correct. I have found it's usually better to have a small inveter for small loads and the big one for everything else. But there are alot of variables that can throw that off.

But the specs imply otherwise don't they? .25 vs .3, 89% vs 85%

But I still want to believe you.



The specs imply the bigger unit is more efficient but...

[Paul L]

Originally Posted by Macblaze

And...?

My understanding of the math is not up to understanding the implications.



But the specs imply otherwise don't they? .25 vs .3, 89% vs 85%

But I still want to believe you.



The specs imply the bigger unit is more efeinet but

Compare watts to watts, not amps. .3 amps at 12 volts is 3.6 watts. Much less than the 30 watts wasted with the larger inverter.

[Macblaze]

Originally Posted by Paul L

Compare watts to watts, not amps. .3 amps at 12 volts is 3.6 watts. Much less than the 30 watts wasted with the larger inverter.

Ohhhhhhh... Duh. We never talk watts when we are aboard so I was just comparing amps to amps.

Thanks.

[sailorboy1]

The efficiency matters what the load is. Doubtful that the big unit is 89% when running a small load

Using my little 400w inverter for charging phones etc and watch tv uses 2 amps less than if using my 2000e inverter. I care about amps not efficiency

[thomm225]

Originally Posted by Kelkara

I don't know anything about inverters ... but ...

for charging camera batteries, if it charges via USB, then you'll get way more efficiency by plugging a USB charger directly into the 12V without using an inverter to convert from DC to AC and back to DC again ... and if you forget to unplug it, the parasitic drain is probably less than 20mA.

My $20.00 PWM Solar Controller has USB Ports also where you can charge camera batteries, phone batteries, lap tops etc

https://www.amazon.com/dp/B01MU0WMGT...a-584565296832

My 400 Watt Inverter does also but it's way better just to plug into the controller. My 1500 Watt inverter spare is still in the box

[Macblaze]

Originally Posted by sailorboy1

The efficiency matters what the load is. Doubtful that the big unit is 89% when running a small load

Using my little 400w inverter for charging phones etc and watch tv uses 2 amps less than if using my 2000e inverter. I care about amps not efficiency

Sometimes I think the manuals need manuals so you can interpret the numbers.

It makes sense that bigger loads would be more efficient on the bigger unit. I will just have to do some real time testing. Frankly, we hardly ever fire up the Magnum...it just feels like overkill, but it came with the boat and we are grateful to have it if we need it.

[senormechanico]

Inverter efficiency curves generally show the best % around a 90% load.
30 watts at idle means at 12 volts input it's drawing around 2 1/2 amps from the battery.

Use a USB outlet for charging your camera etc. if you can.

[john61ct]

Yes, quality expensive ones are more efficient.

But the actual numbers vary amazingly, need a Watts Up to actually check your unique scenario.

If you really care, avoid using them, 90% of the load devices I see plugged into them come in DC versions, or are **actually DC internally**. Certainly anything with a battery is!

[sailorboy1]

The problem with plugging DC devices that use AC and then convert them to DC is that lots of times they don’t like the way boats DC varies. On my boat the voltage of house battery varies between 12.1-14.9V. So don’t just think you can cut off the plug “brick” and connect directly to the batteries.

[Macblaze]

Originally Posted by senormechanico

Inverter efficiency curves generally show the best % around a 90% load.
30 watts at idle means at 12 volts input it's drawing around 2 1/2 amps from the battery.

This is what is frustrating about forums sometimes. Is this correct based on what I wrote or did you just read too quickly? This stuff confuses me enough as it is :-)

Originally Posted by Macblaze

The Magnum specs say:
No load (120 VAC output, typical) 30 watts
(that would be .25 A right?)

There is a spec on the Motomaster that says :
No load current draw (at 12V) <0.3 A

[sailorboy1]

30 watts divided by 12.5V DC is 2.4 amps. So if at 0 amps 120 volt AC uses 30w of power you are losing 2.4 amps DC to do nothing but be in standby waiting for an AC load. If you left that on all day that’s 57.6 amp-hours. That’s a lot of wasted power !!!!