Microinverter efficiency

[jay]

I'm trying to figure out if it makes sense to use a microinverter in my case.

I already have smallish solar panel + charge controller + lead acid battery installed. Currently I'm powering all of my gear from 12VDC. Now I would like to add more things to the same location like 3G WLAN router, couple of cameras and few other things. I seem to have two options:

1. Install an inverter so that I can use the wall warts that come with the gear. The microinverter datasheets always seem to claim amazing efficiency (>96% or even 99%) but I kind of suspect that. I'll believe 99% when it's just a relatively short and thick copper cable What can I expect in real life with relatively low load? I realize that doing DC -> AC -> DC must be somewhat inefficient but it would be the easiest setup.

2. Replace the wall warts with DC-DC converters. Some of the devices are 12V input anyway but I would need also 5V and 9V for some of them. I have this gut feeling that this would be more efficient than the inverter (although the DC-DC converters, that I was thinking of, have only like 85-90% efficiency in the spec). This solution is certainly less flexible than having the 240V AC so if the difference in efficiency is insignificant I'll go with the inverter approach.

[Simon]

There is no right or wrong answer and it depends on how much you want to put into it. Why not make or buy some off the shelf 12/9 and 12/5 volt converters ? the only time you actually end up with a problem is for 12V itself as the battery voltage ranges from so close that a converter would not work to so high that you much have one, maybe some sort of buck/boost topology would work, I think there is one.

For the easy method yes use a 12/240V inverter and then back again. I presume your talking a straight inverter and not a grid tie one ?

[jay]

Why not make or buy some off the shelf 12/9 and 12/5 volt converters ?

That's the option 2 in my original mail.

I presume your talking a straight inverter and not a grid tie one ?

No grid there.

I would prefer just getting the inverter but the available amount of energy from the panel is small during winter. Typical load would probably be something like 5W (max maybe 40W, very rarely). The datasheets (of few cheap & small inverters) I've seen so far are a bit vague about the efficiency. If I really get the 96% marketing value (or even 80%) then no problem. I'm hoping someone has knowledge what kind of efficiency small inverters can give under a low load. How much I can trust the datasheets / marketing material? Not sure how the manufacturers usually measure the efficiency they claim.

[David Hess]

Online UPSes can be better than 93% efficient and that includes a high power AC to low voltage DC (48 volts) power factor corrected switching regulator and a low voltage DC to AC inverter so 96% or better efficiency for an inverter does not seem unreasonable to me.

[theoldwizard1]

2. Replace the wall warts with DC-DC converters. Some of the devices are 12V input anyway but I would need also 5V and 9V for some of them. I have this gut feeling that this would be more efficient than the inverter

A average inverter is 75-90% efficient.  Real good one, $$$, are over 90%.  Then you have the losses of the wall wart.

The UPS that I have seen tested on YouTube run in the 80% range (mostly due to transformer losses).  You can usually buy small ones, <500 watt, cheap because they have a dead battery.  If you hooked up that solar, wiring it in to an UPS should not be a problem.  Be careful, because UPS inverters are not designed to give their rated power for continuous use because their original batteries only lasted 10 minutes.  Anything over about 50% of capacity for long periods will cause them to over heat and fail.



DC-DC is probably more efficient, but also a lot more hassle.  It would likely cost more

[tggzzz]

Check the efficiency at different load values.

My ill-considered suspicion is that, presuming that IR losses in cables are insignificant, one voltage changer could be more efficient than two in series.

I suspect other factors will be more significant that the last percentage point in efficiency. Off the top of my head I'd consider reliability of the whole and parts (e.g. single point of failure), maintainability/servicability, cost, simplicity of prdicting performance, etc

[Seekonk]

OPTION #3
Get a cheap inverter, hopefully blown up and power those wall warts with the inverter DC.  I use that for all my electronics like computers and DVD players etc at my camp.  I also oversize the inverters, remove the fans (they can take 120ma each) and enhance natural ventilation.

[madires]

I'd power the stuff directly with DC-DC converters because most wall warts got a bad efficiency anyway. I've replaced most wall warts with DC-DC converters powered by a central high efficiency PSU, and the whole setup needs just about 60% of the power the original wall warts need. Bonus point: you don't have to worry about cheap, crappy and dangerous wall warts.

[mtdoc]

The term "micro inverter" is typically used for small inverters designed to attach directly to one panel and used in strict grid tie situation. They typically put out 210 - 240V only.  The popular Enphase Microinverters are capable of 215 watts and cost about $160.

These type of microinverters are very efficient but are really not designed for your type of application.

If I was you, I would spend a bit more and get a Morningstar SureSine 300. They have an excellent reputation. Efficiency is about 90% (less for really small loads -see spec sheet).  They put out 115V or 220V and will give 300 watts continuous (600 watts peak).

This might seem overkill for your very small loads but the first rule of off grid solar is that loads alway grow with time...