Inverter Efficency Question

[RogerD]

I recently read that if you have an inverter that is say 90% efficient, that it is really only that efficient when you pull wattage that is upward of the maximum that the inverter can produce. If you are pulling only a small amount of the maximum wattage, then the efficiency will be substantially lower.

If this is true, then getting something oversized would be a big mistake and getting multiple units properly sized for the most common tasks would be much better.

Is all of the above correct?

 

[tx2sturgis]

Yep that's pretty much the way they work.

A typical inverter is usually most efficient in the mid to upper range of it's listed capacity. 

But if it is operated right at it's max, then efficiency often drops off again. 

But you also have to include duty cycle.

If you have, lets say, an electric toothbrush that pulls 5 watts, and you operate it for 5 minutes twice a day, powered by a 1500 watt inverter, the efficiency loss is quite negligible, especially if the inverter is either turned off when not needed, or, it's powering something else you need to power, like a fridge, or a tv, or a blender, whatever.

But if you intend to only power a smartphone charger or a radio that pulls 10 watts for 5 hours, then a 1500 watt inverter is way overkill and a bit of a waste. A 100 or 200 watt inverter would be the better choice.

 

[Rabbit]

Other considerations are the added wiring complexity and cost, cost of the additional inverters, and the extra inverters taking up additional precious space. In my cargo trailer, I knew that once in a great while I'd want to pull 1500 watts or so of AC for brief periods and stumbled across a good deal on a 2000 watt unit at just the right moment. So that's what I installed to begin with, and it's still there. I figured I could easily add a smaller secondary inverter later if I felt the need. So far I never have; I'm almost never short of power and when I am it's in a big way-- in practice that little bit of extra drain from the large inverter isn't enough to even come close to mattering. Though, to be fair, I've not traveled as much as I originally anticipated and my experience level is still therefore relatively low. Others may have very different experiences.

As a side note... Almost all my low-power stuff (I can't think of a single exception offhand) runs off of 12 volts, which is drawn directly from my battery bank with no inverter involved at all. My lights, fridge, bunk heater, bi-PAP, primary electric cooking device... I even run/recharge my laptop directly from 12 volt, via a USB-C lighter plug adapter. So, now that I think about it, I wouldn't have any use for a small, secondary inverter if one was sitting there ready and waiting. This is something I'm pretty strict about when adding new equipment. For example, I recently almost bought a cordless drill. Though I eventually decided not to go there, while shopping I considered only models/brands that offered a 12-volt power-pack charger.

 

[Sky King]

They should list the idle usage & if they have a fan they tend to use more. 30 years ago I bought a 200 watt, no fan made in Ireland & it still works great. I use that, a 1000/1500 pure sine wave & a 2000/3000 non pure sine wave & have a swith I'll wire to pick the one I want to power.