Backyard Aquaponics
A place to discuss aquaponics

It will actually be far, far more than that. Inverters have an overhead of power use that is there all the time- the electronics, fans etc have to be energised. Small el cheapos from China are very inefficient and can consume tens of watts in standby, and even good quality Aussie manufactured units will use way more than 3W when operating. Efficiency when operating loads of 5 or 10W will often be well under 50%, IE the internal consumption of the inverter will be a lot more than your load.
When selecting very low power equipment - up to a few tens of watts, it's a much better idea to use DC, it will be far more efficient.

Thanks guys, this helps a lot. Anyone know how long a few of those 12v pumps would run on an inverter with 2 deep cells? I will have solar, but worry about a "worst case" scenario...hurricane knock out power for several days, lots of clouds, little/no solar available.

IF you are going DC use a relay because DC air pumps are not rated for continuous use.

Stuart, so you're saying I can use a DC air pump, but would need a relay to keep it from overheating? Any suggestions on quality 12v pumps? I've seen a few, but they were really expensive.

I was advised by DC air pump supplier that the relay would close (turn on the pump) when mains power was lost. This means that the air pump is sitting there doing nothing until needed.

I would get advice from people in the US because the products available to you are likely to be different from what we can get here.

What you need is a 12V 5 pin relay, which is energised to have contacts open with a 12V plug pack transformer plugged into your 120V mains supply. When the power goes out, the relay contacts close, completing the battery - pump circuit. It will turn off again when the mains power resumes.

There are a bunch of threads that give really good instructions on how to do this.

Thanks again guys...don't know what I'd do without you. Cheers.

hey coach did you end up working this all out?

Once you know the power draw (watts) of your load, you can then calculate how many batteries and solar panels you'll need.

hit us with your questions if have any

I've been using this small 150 watt modified sine wave inverter from Jaycar to run my airpumps:

http://www.jaycar.com.au/productView.asp?ID=MI5127

I just spoke to a Jaycar tech over the phone and he confirmed it is 90% efficient.



I'm sorry but this is incorrect.

I connected an inline watt meter to my load and measured around 3.4 watts power draw when 1 of the airpumps was connected to the inverter ie:

solar battery bank > solar charge controller > wattmeter > inverter > airpump

This would confirm my theoretical calculations:

Power draw of 90% effecient 150w inverter + 3w load
= 1.1 * 3w
= 3.3 watts total power consumption




This has not been my experience in practice with the above mentioned inverter.

I think it depends entirely on your selection of inverter. If you oversize your inverter to the load, you may have this issue.. So choose an inverter which is appropriate for your load.




Again, I disagree. This is too generalised a statement. It depends if your DC device is more efficient than the 240 volt equivalent. I run a lot of straight DC gear, like extremely effecient 5 watt 12 volt pumps. I have 4 of these running as we speak, and they work very well. In this case, yes, it makes sense to run straight DC pumps without an inverter, avoiding the costs of the inverter and associated losses.

BUT.. when it came to choosing a DC airpump, I could not find one as effecient as the 240 volt airpumps available on the market. Don't take my word for it though, see if you can find a 12 volt equivalent air pump which is more effecient than the 240v one I've mentioned. I think you will struggle.

so.. my point? In this case, it worked out MORE efficient to run a 3w 240 volt air pump (or a couple of them) through a 150 watt inverter, than any DC equivelant air pumps.

Yes, I was speaking more generally, referring to larger sized inverters that can handle a decent sized water pump plus an air pump. As you say, smaller sized inverters will certainly be more efficient for smaller loads. I was also assuming equal efficiencies for the AC and DC air pumps, as I had not looked up any comparative efficiency factors. When I was looking for a DC air pump, there just did not seem to be anything of suitable capacity for my application available anywhere - all were too small.



That will only be the peak efficiency, which generally occurs at reasonably light loads, it falls off above and below that peak.




What did you get when you put a wattmeter in between inverter and airpump? I'd suspect less than 3W, if the meter can accurately measure such low loads.

You've clealy got a reasonably efficient unit from Jaycar- the 300W one I bought about 10 years ago from them was terribly inefficient at low power levels, especially because it had a fan running full time, even with no load. Modified sine waves dont work all that well with some motors, but should be fine for a vibrator and diaphragm type air pump.

Yeh there is no fan in this little inverter that I'm aware of, at least I haven't heard one run.

I haven't measured power drawn between inverter and airpump. I've got another 240v plug in (inline) wattmeter which should be able to measure that.

Can you please explain your logic here though - in theory, why would it measure less than 3w?

Yes I can confirm the smaller aquarium air pumps seem to run smoothly without issues on a modified sine wave inverter.

The power rating is meant to be the max consumption in normal use, and I've found that many electrical devices use less than their rated consumption- due to being a compulsive measurer of such things with my off grid PV and wind system data logging
However, I'm not sure how accurate the average power meter is at very low levels, and what effect the chunky wave form will have on the accuracy either.
The last 240V power meter I tried sure gave some inaccurate readings at times, and that was with a good quality 4kW true sine wave inverter.

Jono, you could also try :
solar battery bank > solar charge controller > wattmeter > inverter > <nothing plugged in>

just to see what the inverter uses with no load.

At least that way you are measuring with the same instrument.