Dc vs ac efficiency

mountainman · June 24

I've been reading on dc ceiling fans claims of 70% more efficient than ac fans with 300+ cfms per watt. 20 watts on high at 6600 cfm. Has anyone tried one of those fans?

Estragon · June 24

I have one in my bedroom. I don't remember the numbers, but definitely much more efficient than an ac equivalent. It's ~3' across and does the job with a couple amps at 12v on high IIRC.

mike95490 · June 24

Many AC fans (appliances) have given no thought to efficiency, but when a DC circuit and electronic controls are designed, the lower the power, the cheaper it can be made.

Estragon · June 24

I just finished putting in an over-the-range microwave. It has an "eco" button that turns off the display. Aside from saving a bit of power for the LEDs, it figured that it would save me having to look at a display with the wrong time showing

mountainman · June 25

I thought my little 30 watt 900 cfm fan was cooling. How would you keep the cover on the bed with 6600? The way this one works it uses ac and converts it to dc . So to use it 12 volts dc inverted to 120 ac and back to dc. Seems like there would be a lot of losses. Wish you could buy 30 watt psw.lol

Dave Angelini · June 25

It seems to me that if you are having issues with running a normal AC powered ceiling fan in summer, your system is definitely going to have problems in winter. It is all about winter offgrid

Raj174 · June 25

We installed two these 56" fans at our house. They are expensive at $180, but take very little power to run. Attached the pdf on the fans specs.They use AC, but convert to DC.

Rick

mountainman · June 25

If every thing was as eff. as those fans, solar arrays could be the size of tvs. I spend summers in virginia off grid. Before winter comes I head out the hills. Where power lines are.

Estragon · June 25

This is the one I have:
http://www.backwoodssolar.com/products/fans-large-small/40inch-12-volt-ceiling-fan-white

It doesn't give CFM, but it's pretty quiet and moves enough air to do the job without blowing the covers off the bed!

AFAIK, it's a DC motor so no conversions.

mountainman · June 25

@Rick where did you purchase yours? That's better than some of the prices I'm seeing. In my energy audit my noisey little dc fan 24 hrs a day. Used more than every thing else combined.

mcgivor · June 25

FWIW When temperatures at night never drop below 27°C I use a 12VDC brushless table fan, rated at 12W, powered by a 45Ah battery, charged by a 60W panel via a cheap 10A PWM controller, a little stand alone system, it works flawlessly, actually cobbled together with components on hand, the fan itself cost a little over $10.

Raj174 · June 25

Got it on sale at Home Depot a couple years ago. The regular price is currently $229, but they do go on sale at 10 to 15% less. I don't know why it says 15 inch, it actually a 56 inch fan.
Here's the link:
https://www.homedepot.com/p/Home-Decorators-Collection-15-in-Brown-Ceiling-Fan-with-Light-Kit-and-Remote-Control-51558/301909748

mountainman · June 25

@ mcgivor My first fan was from a honda civic powered by a 100 watt panel wired directly works great, but didnt do much of anything at night lol. then tried a 12" dc table top. Not much air! Now I have the endless breeze 12 volt dc 2.52 amps on high it really blows but sounds like an airplane. Night temps are 15 to 20c. Personally That's cool enough to not need a fan at all but, My wife has a different opinion. Thanks @Estragon I'll check it out. When I get my cabin finished, to home Depot. Any one heard of a tv with really low consumption.

jonr · June 25

High efficiency requires large, slow moving fans - like 56 or more inches.

Raj174 · June 25

Our 55 inch LED LG TV LB5900 series draws 45 watts. I think that's on the low end for it's size, or at least it was a couple of years ago. Spec sheet says wattage is to be determined, but I measured it wit a kill-a-watt meter. Spec sheet attached. You can tell, I've been down the efficiency road before. The heat pump water heater is probably our biggest cost saver.

mountainman · June 25

For my rv a 55" tv is larger than I would need . A 32" would be a better fit. I like the 56" fans efficiency but a 40" should be abut the right size for a small bed room and cut my use by more than 50%. That's almost 400 whrs for tv. While still staying under my daily 1 kwh limit.

Estragon · June 25

Getting one with adjustable brightness/modes and putting it in a darkish place might help.

mountainman · June 25

A friend just told me of his 32 vizio 21 watts. With a 120 watt samlex inverter .5 amp idle would it be 21÷.85 eff.= 24.7 + 6 watts about 31 whrs?

Thom · June 25

https://www.homedepot.com/p/Hampton-Bay-Winfield-54-in-Indoor-Liquid-Nickel-Ceiling-Fan-with-Light-Kit-and-Remote-Control-AG804-LN/202608829 I have that fan .

Thom

myocardia · July 8

mountainman said:

would it be 21÷.85 eff.= 24.7 + 6 watts about 31 whrs?

Absolutely not. It would be 24.7 watts, times the amount of time used, in hours, to find your watt hours. So, 24.7 watts for 15 minutes= 24.7 X 0.25= 6.175 watt hours. Watch it for 2.5 hours, and it becomes 24.7 x 2.5= 61.75 watt hours. You know how you don't count your car's idle gasoline usage against it while you're driving down the interstate, only the amount of miles driven and it's miles per gallon? You don't count an inverter's idle power usage against it while it is inverting for the exact same reason: it is not idle.

Photowhit · July 8

myocardia said:

mountainman said:

would it be 21÷.85 eff.= 24.7 + 6 watts about 31 whrs?

Absolutely not. It would be 24.7 watts, times the amount of time used, in hours, to find your watt hours.

@mountainman is only adding on the extra energy needed to drag the trailer with the car, not the car's idle. In this case the inverter likely uses 6-8 watts at idle, that is NOT being considered. only the extra energy needed to run the load (trailer). If it's 85% efficient then the load when running will be roughly 31 watts. The total load will be the 31 watts AND the idle of the inverter...

mountainman · July 11

I am currently using a TV that uses 31 watts on my kilowatt meter. 31/.85 36.47 + 8 watts for inverter 44.47 watts on DC meter from battery.

jonr · July 11

The efficiency of an inverter changes significantly with load in a non-linear way. A single percentage plus a constant (a straight line) isn't going to match the actual curve.

mountainman · July 11

I read in the inverter manual highest effiency at 80% load. I'm in the 10% range 31 on a 300 watt.

jonr · July 11

31/44.47 is a typical/reasonable 70% efficiency @ 10% load. Peak efficiency load point varies, but 50% load would be more typical.

mountainman · July 12

I only use an inverter for TV so I could use a smaller one. 120 watts is the smallest psw I can find. That would put me at @25% load. I'm not sure If it would be of any advantage from a 300 watt @10%?

Estragon · July 12

It seems to me the difference in efficiency between a 120w psw inverter and a decent 300w one wouldn't amount to enough to worry about in this application. There might even be more consumption with the 120w one if it doesn't have asleep mode, and the 300w one does.

jonr · July 12

Depends on the model, but around 90% would be reasonable, maybe saving 10 watts.

Dc vs ac efficiency

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