Too many Watts = not enough power

Cariboocoot

It is unbelievably hot here. Absolutely tropical weather, which is not normal for Lat 52 Elev 3200. Not even in August. Trust me.
Most people would be crying "air conditioning!", but we all know that's not practical on an off-grid power system (you'd need about 5000 more Watts worth of panels). So I thought: fan. Just get the air moving, preferably from the 'cool' side of the house. The fan, which we've never needed before, draws 40 Watts on high according to the Kill-A-Watt. During the day with all that sunshine beating down it's a real help and there's no problem running it.

But I don't have enough reserve capacity to run it all night.

This is partially because the 'frige cycles more in hot weather, and partially because the batteries aren't living up to expectations.

So the experiment for tonight is: pull the 100 Amp/hr battery from the boat, hook it up to the old Statpower 600 Watt MSW inverter, and set the fan on 'Low' (35 Watts). This should give: 100/2 = 50 Amp/hrs @ 12 V = 600 Watt/hrs / 35 Watts = fan for 17 hours maximum. The only trouble is: inefficiencies in the system and an aged battery that probably isn't up to snuff.

So take your guesses; how long before the inverter screams "low power"? Should we have a lottery?

(For those who want to consider wiring losses, the inverter is connected via 4 gauge, 4 feet long.)

Stay tuned for the results of this practical experiment in impracticality. Or something.

AntronX

Re: Too many Watts = not enough power

(100Ah * 80% ) * 12V = 960Wh
960Wh / (35W * 1.3) = 21 hours

Cariboocoot

Re: Too many Watts = not enough power

AntronX wrote: »

(100Ah * 80% ) * 12V = 960Wh
960Wh / (35W * 1.3) = 21 hours

80% is too great a Depth Of Discharge (DOD) - especially in a non deep-cycle battery.

icarus

Re: Too many Watts = not enough power

Marc,

What we do those (mercifully) few days of hot muggy weather, is close the house at sun up,, including the window shades and winter window quilts. Then as the evening beings to cool we turn on the ceiling paddle fan and open everything,, starting with the shady side of the house, then we leave the house open all night with the fan going. Ceiling paddle fan(s) burn ~15 watts on low, up to ~45-60 on high.

Reverse the flow in the winter. Pretty cheap way (energy wise) of saving heat in the winter, adding some draft in the summer.

Bottom line, hot is hot!

Tony

Cariboocoot

Re: Too many Watts = not enough power

Tony -

I haven't got any ceiling fans, but they're on the list now! First time we've ever had this heat problem here.
And yes, the curtains are drawn against the sunshine. I'm trying to draft air through from the cool, non-sun side and switch 'round in the afternoon. It's working so far; inside is cooler than out!

And you're right: hot is hot! Even the lake is getting warm!

niel

Re: Too many Watts = not enough power

if the fan is ac powered you might find the fan drawing a bit more depending on power factor. check it in va and go by that as your watts.
yes, close up the house from early in the morning and at night, after the air has cooled acceptably, you want to draw air into the house at night when it's cooler (i put a fan in the window) as just pushing hot air around any other time would be a waste unless you are benefiting from evaporative cooling on your body or even for the house with an evaporative cooler if humidity levels are low enough. the lake heating up could thwart your efforts at night as it will shed its heat and cause a delay in an acceptable temperature to fan cool the house from being reached. that lake will also keep humidity levels higher in the area too.
i should add for you to block any solar gains through the windows as best as you can or the house will heat up fast.
best thing would be to go jump in the lake.:p

icarus

Re: Too many Watts = not enough power

All the AC paddle fans I have tested are ~pf 95. I have only done a couple however. My 12vdc paddle fan actually draws a few more watts. Personally I would go with a simple ac version as the remote on the dc version doesn't work very well.

Tony

Cariboocoot

Re: Too many Watts = not enough power

And the winner is: 10.5 hours!

Not bad. At 35 Watts that's 367.5 W/hrs. So I guess the battery is a bit old: 30 Amp/hrs before the alarm went off. BTW - the pf on the fan turned out to be 1.

And at least we had a cool night for a change.

westbranch

Re: Too many Watts = not enough power

Morning Marc, we are just back in from the lake (3000 ft elev) and had the same experience ... it was 35 C ( ~ 95 F) yesterday in the shade by noon and it got (much) hotter as we drove back home for 3 hours...

the lake is just great for an afternoon swim, but only after 5 when the sun angles down a bit otherwise you get a very well tanned (burnt) head.

We are going to take a fan out this next weekend and will try running it all night, I've also been thinking about the draw but I do have the battery capacity, but will need to match the draw to the daily input. May have to put the extra panels against the west facing wall to collect our 2 to 6 pm exposure which we currently lose... will advise.

Question to all about fans: do single speed fans use less power than multi speed?
(we have a multi speed fan that I will use, but have to do a K-a- W test on it...) If so I will search for a new fan...
Eric

Cariboocoot

Re: Too many Watts = not enough power

Eric;

Mine is multi-speed and I checked all three levels: Hi = 40 Watts, Med = 37 Watts, Low = 35 Watts. Yours might be different, but since this type of motor is actually 'three motors in one' instead of having its speed reduced by rheostat the energy draw goes down with the speed.

Of course a smaller fan should use less than a bigger one. Notice how we couch every statement in uncertainty these days? You just never know what kind of inefficient junk you might be getting from the store. :roll:

westbranch

Re: Too many Watts = not enough power

while I was having a coffee my wife found this and was all excited till I pointed out the 'massive' size of it !!! 5 .5 x 5.5 inches...

thought you might enjoy.;)

Eric

westbranch

Re: Too many Watts = not enough power

Marc, forgot to ask what battery capacity you have?
Eric

icarus

Re: Too many Watts = not enough power

Marc,

It sounds like on a CuFt of air basis, your fan uses more energy on slower speeds. The paddle fans I have tested seem to have a better relationship between CFM and watts.

The slow speed being ~1/3 the rpm of high, but use ~1/3 the energy.

Cheap paddle fans are ~$75. I found two in my junk shed! I knew I had rescued them from a commercial kitchen a few years ago, I just couldn't remember where I put all the parts. I just repainted and installed one.

Tony

Cariboocoot

Re: Too many Watts = not enough power

Eric - my battery bank is supposed to be 320 A/hrs. In reality it falls a bit short. I've noticed its capacity goes down in the heat too, and the 'frige's usage goes up! :grr

Tony - this whole experiment was based on using what I had kicking around to get some air moving at night without draining my main bank. On the whole, I'd say it was a success: cost nothing, for one thing. A trip into town uses up $25 in gas, so I don't go in every time I'm short of something. Make up, make do, do without!

I'm not at all surprised that the pedestal fan's efficiency goes down with the speed, especially in comparison to a paddle fan. I may try it on "Hi" tonight. Unless it doesn't get hot today. HA HA HA!

westbranch

Re: Too many Watts = not enough power

OK, local Forcast = hotter and hotter for the weekend... that is sweat not tears....

so I went down to the C Tire store and got a 2 speed window mount fan (last one)

Specs:

.85 A rating

K-a-W readings =

low speed .50 A 32 W 62 VA .52 PF
Hi speed .65 A 39 W 78 VA .50 PF

based on the VA readings I plan on using the unit on Hi speed as it seems to be more 'blow for the buck'... 78 vs 62 VA

Am I right?

Eric

Cariboocoot

Re: Too many Watts = not enough power

Eric -

To actually judge the fan's efficiency you'd have to know how many cfm's it moves at the two different speeds.

My lot was easier: limited power availability meant 'low speed so the battery doesn't die over night'.

Tuesday: Hi of 35 predicted. That's the day I go into town ... an hour down the dirt road with no AC. It gets worse once your in 100 Mile.

BB.

Re: Too many Watts = not enough power

westbranch wrote: »

K-a-W readings =

low speed .50 A 32 W 62 VA .52 PF
Hi speed .65 A 39 W 78 VA .50 PF

based on the VA readings I plan on using the unit on Hi speed as it seems to be more 'blow for the buck'... 78 vs 62 VA

Am I right?

Eric

The Watts is what the battery/inverter will drive (bad Power Factor drives up losses in the system because the AC current is ~2x what it needs to be in the inverter and wiring). And, limits the total number of devices on one inverter (inverter is pretty much limited by total current, not by total power--so if you have a "1,000 watt" inverter with PF=1 -- it will run a 1,000 watts of load. If you have an inverter with loads of PF=0.5, it can only run 500 watts of load).

Power = Voltage * Current * Power Factor

-Bill

westbranch

Re: Too many Watts = not enough power

thanks Bill, I thought I had a small understanding of PF, but...
so did some more Wiki reading... but as usual the light shines slowly... was looking at the numbers for my standing,oscillating fan and Ah Ha, I think I understand it now... the big fan has similar W but 1/2 the Amps, PF =1...

since Inverter volts does (can) not change, the effect is that the fan will use X wattage and do half the work of a similar fan with PF=1.
[ the short Wiki answer is that the other half is 'lost ' internally to ??? (did not understand it all ) ]

OR that 2 X as much amperage is needed to do the same work as a PF = 1.

Net effect is 2 X watts are needed for a PF = .5, for same work...;)

In this case is it true that if one compares the W to VA : 32/39 W to 62/80 VA, the difference is what is lost? I see that VA * PF ~= W.

Eric

BB.

Re: Too many Watts = not enough power

The short answer is that it is MATH... AC math is not simple--and even though I had a bunch of it in Engineering, I will not claim to be anywhere near an expert.

Basically... One way to look at power factor is to think that you are pulling a car with a rope... If you stand in front of the car and pull... 100% of your pull goes into moving the car forward.

If you stand at right a right angle to the car's direction of motion... 0% of your pull will go into pulling the car forward...

PF, in the above example, is the fraction of pull that pulls in the direction of movement.

Force on the car = Force pulling * cosine of X

where X is the number of degrees off of standing in front of the care.

Notice, in this example... If you want the car to experience a force of 100 lbs... The you would pull 100 lbs if you stand in front of the car... (cosine of 0 degrees = 1)

If you are standing off to the side an pull 100 lbs:

F=100 lbs * cos 45 degrees = 100 * 0.707 = 70.7 lbs pulling the car forwards

So, in this case, you have reduced the work done to 70% of the force possible... Or if you wanted to still put 100lbs of force on the car:

F=100lbs / 0.707 = 141 lbs you must pull

Notice, that the rope now must be "stronger" to pull the same force at an angle. Sort of what happens with electrical circuits... More current must be used when the PF is less than 1.0 (voltage and current in phase vs voltage and current not in phase--the sine waves of the two are not "in phase").

There are other ways of looking at the issue with PF, capacitors and inductors (and non-linear stuff like power supplies)... Imagine you are pushing on a spring... As you push in, you compress the spring and store energy... As you move back out, the spring returns the force against your hand and supplies energy back to your hand... The total amount of work done is ZERO or PF=0.0...

Its a start--Does this sort of get you to understand Power Factor a bit?

Photowhit

Re: Too many Watts = not enough power

I'd suggest checking out the 10" O2 Cool fans sold at Wally World (WalMart). I think they are up to $15 or so this year. They draw 10-12 watts on AC but are 12 volt and run from a wall wart. These fans really move the air, go check them out! (I still run one most evenings in the summer..)That's all I used until I decided....

"...but we all know that's not practical on an off-grid power system"

Is only true if you choose not to design your system, including housing to run an AC. I built a 10 x 16 cabin with sleeping loft with 6" thick walls. I keep all heating things in a seperate kitchen/bath house. I run an 5300 btu AC on energy saver mode and cool off the room and sleep pretty comfortably.

I run this on 4 golfcart batteries and @1000 watts of panels for the last 2 years, running the AC during the day after the batteries had been topped off. This year I added @240 watts of panels and run the AC when needed with out worrying about the charging. On 80 degree nights the duty cycle is @30% after cooling down (about 2 hours of higher run times to cool off) I set the timer for 2-3 hours. If it's humid I find I'll wake up and run the Ac for another hour.

With the minimal battery setup I don't run with out thinking about the weather but heat and sunshine typically run together here in Missouri. It's been a tad easier this year as my fridges chiller went out and a week after being repaired there is a dead short... Maybe it's time to convert a chest freezer into a fridge (though I like having a freezer)

C60 charge controler, 24 volt system, Batteries are 4 years old, I had designed this for 8-12 golf cart batts or 4-8 - L16's or a fork lift battery, but choose to buy the added panels this year and see how long the golf cart batts would hang in there. They are the same batteries I was using on a 12 volt system when I was building the cabin.

I figure if I get 4-5 years out of a set of GC Batts for 2 more sets I'll pay for the 2 -170 watt panels I added this year at @ $2 a watt delivered after Tax credits and cashback (I took out 100 watts of older panels) If I make it 8 - 10 years I'll be ready for a fork lift battery and running around the clock as I approach 60.(If I haven't run into a deal before then, or the POCO chooses not to charge a $25 user fee, The green in me would rather "store" the electric by feeding the grid just too expensive with the high fee, which is rumored to be going up)

Cariboocoot

Re: Too many Watts = not enough power

AC is not practical off-grid as in:

I don't have an AC here at hand and I don't have enough PV/Batt to run one if I did. To obtain sufficient equipment would cost me another $8,000 here in BC. Not a worthwhile expenditure for something we've never needed before and would only be used a few days per year at most. The rest of the time it would count as 'excess power' - which as we know is not easily stored until needed (like December).

On the other hand, if any of you would like to write me a good cheque for that amount ...

So far my 'rigged-up-with-what-was-on-hand' fan system is working fine. The battery is even performing better now: I get 11 hours per night out of it. Wish I could say the same for the main bank; this heat is murder. It was 36 yesterday (that's 97 F).

niel

Re: Too many Watts = not enough power

photowhit,
and here i thought i was the only one to refer to walmart as wally world. we even have a bus that goes from one walmart in one shopping center and travels to another walmart in another shopping center and i call it the wally world express.:p
marc,
heat waves are h*ll aren't they? just hope it ends soon for you and everybody else on the west coast. did you try my advice and if so how did my advice fair for you? that includes go jump in the lake.:p;)

Cariboocoot

Re: Too many Watts = not enough power

Niel;

As a matter of fact, I have to go jump in the lake today: it's receding so fast that my water intake is in trouble!
Otherwise, keeping the curtains drawn against the sun and drawing air in from the cool side works (goes down to around 7 at night!). Until the day's heat makes it so there's no cool side. A bit uncomfortable between 3:00 PM and 9:00 PM.

We're surviving.