Amp Hours Question

[RogerD]

I hooked up my kill-a-watt to my 5K BTU AC.

It used 1500 watts in 5.5 hours in cool mode and 1070 watts in 6 hours in eco mode.

Is my math right?

1500 / 12 = 125 amps used / 5.5 hours is 22.72 per hour average

1070 / 12 = 89.16 amps used / 6 houres is 14.86 amp hour average

I need to test again as last 2 days were only about 82 degrees and really need to test when like 90. Here in FL that's the more realistic summer average.

Also, I have a workshop and built an 8W x 12L x 8H insulated room that I 'm using AC with and it is bigger than a van. Also, it can be higher temp inside than outside. I didn't check to see what it was.

Anyway, just trying to get an idea on amp usage to run off batteries / solar.

 

[RogerD]

I forgot to mention. I had AC set to 73 degrees.

 

[John61CT]

Amps and Watts are an instantaneous snapshot measure of flow *rate*, makes no sense to say "used X watts (nor amps) over Y time period".

Maybe you mean watt-hours (wH) or AH?

AH per hour, or watt-hours per X hours is meaningful.

Note 110AC amps or AH are multiplied by 10 to get 12VDC amps or AH.

It is impractical to try to run aircon from batteries, all it gets you is a little time-shifting from running the genny, about 100AH bank capacity for each hour just for a very small efficient aircon unit, cooling a very small and well-insulated space.

 

[lenny flank]

The only people I have seen or heard about who are successfully running small AC units from solar have massive panel arrays and battery banks, and spent several thousand bucks on it.

 

[B and C]

A 9 degree temperature drop is not much of a test. How much cooler than the 90+ degrees are you going to want to set it at? You might try for the same temperature drop from current ambient temp. Your compressor was probably cycling on and off with that little temperature difference.

 

[John61CT]

Your compressor was probably cycling on and off with that little temperature difference.

As it should be.

Sized properly for the space, and with good insulation, 20-40% compressor-on cycling is a reasonable target.

The compressor running all the time should only happen in rare extreme circumstances.

That guideline is complicated by more efficient units varying compressor speeds rather than just turning them on and off.

e.g. Frigomar's variable speed brushless inverter tech is amazing, their 16000btu unit uses under 1400W. But it can cost $4K with install. . .

 

[John61CT]

Frigomar's variable speed brushless inverter tech is amazing, their 16000btu unit uses under 1400W.

To clarify the math, say the above averages 60% cycling at a given temperature differential.

.6 * 1400W = 840wH per hour
If real voltage is 13V, running 12 hours a day
64.6AH per hour, 775AH per 24 hours

As I stated, running from a House bank will give time-shifting from charge source of an hour per 100+AH of lead storage, not counting the "long tail" charging requirement, or any other concurrent load requirements.

That will be reduced to 80AH per hour with LFP, also no long tail issue, but at 5-7x the price per AH.

2000W of solar may run it directly bypassing the House bank for a few hours

(no startup / surge issue with that aircon unit)

but only in bright sunshine, higher heat loading, and not counting much bank recharging.

 

[B and C]

The compressor running all the time should only happen in rare extreme circumstances.

Tell that to my van's air conditioner,  It is 13K btu and in 90+ degree heat, it never cycles as it won't get down to 72.  Factory insulation, nothing I can do about it.  Insulation works well in the cold though when the heater runs.

But the A/C is old and on its' last legs.  I would like to replace it but hardly ever use it.

 

[John61CT]

Yes, unfortunately many of us are stuck with legacy sub-optimal situations, and lack the necessary funds to radically improve things anytime soon.

But specifying what is the design ideal as if from the ground up can help drive home just how significant the required compromises may be,

especially for noobs that may unrealistically believe, just because they want to do something, does not mean it is possible,

if funds are limited.

 

[RogerD]

The kill-a-watt meter keeps track of how many watts are used over X time.

In cool mode, it showed that it used 1500 watts over a 5.5 hour period.

In eco mode, it showed that it used 1070 watts over a 6 hour period.

Is there a way to extrapolate that into required battery amp hours for the time periods?

If I use 125 amps (1500 watts / 12 volts). Does that mean having 250 amp hours drained 50% would have worked for that 5.5 hour time period listed above?

 

[jonyjoe303]

If you use your AC for 5.5 hours at 1500 watts (total watts over 5.5 hours is 8250 watts), you would consume 687.5 amps, you would need a battery bank of over 1000 ah lead acid or 700 ah of lithium. 
I think the 1500 watts is peak and when the compressor cycles on and off you will use less amps. So you might use less then 697.5 amps, maybe even half that. But even half (340 amps)  is still alot of battery.

In the hurriedyear website someone runs his AC on 780 ah of  lead acid. He has lots of pictures of his setup, thats a good place to learn about using an AC with solar.   

If you heavily insulate your van, even on eco mode you will be freezing. Thats what I would do. I have so much insulation on my van, a small 2 amp swampcooler kept me cool even in the hottest part of summer. In the hurriedyear website, he uses a smaller 750 watt ( 62.5 amp peak) AC, that would be more realistic then a fullsize AC for a small heavily insulated van.

 

[RogerD]

If you use your AC for 5.5 hours at 1500 watts (total watts over 5.5 hours is 8250 watts), you would consume 687.5 amp.

Thats not what I said.

The AC used 1500 watts of power in order to run for 5.5 hours. 

The AC is 410 watts / 3.8 amps. That is the max that it will run. I never saw it get over 3.6 and the only time it got to it was when it got to the point of actually using 410 watts.

It only used 1500 watts because it was cycling instead of using 410 watts the entire 5.5 hours (410 x 5.5).

In eco mode it only used 1070 watts total to run for 6 hours.

 

[tx2sturgis]

The kill-a-watt meter keeps track of how many watts are used over X time.

No...actually if you look closely you will see that when you press the kwh button, it reads kwh accumulated over time. Not watts or kilowatts, but kilowatt-hours.

(The 'watts' button will display the current wattage the unit is drawing at that moment)

Then you use the time feature, (second press of the kwh button on mine) and you can figure kwh consumed per 6 hours, per 12 hours, per 24 hours, or whatever the time figure is. 

If it DOES show 1.5 kwh (1500 watt-hours) in 6 hours, (rounding off) then we can calculate from there.

Trust us, the numbers have been run before, and you need a LOT of solar and/or batteries to make this thing work (plus charging the batteries and making up for inverter losses)  for even 6 hours a day. Probably a lot more panels than will fit on the roof of a typical smaller van.

It has been done, on some larger vans, with a large roof area covered in panels and a bunch of batteries inside.

 

[RogerD]

Yes, the kill-a-watt meter read 1.5 kwh (1500 watts) for 5.5 hours running in cool mode and 1.07 kwh (1070 watts) for 6 hours running in eco mode.

I have a chevy express extended van. I should be able to just barely fit (6) 200-watt solar panels on the roof. If I needed more, I could mount thin flexible panels on some form of custom sliders underneath the main panels and slide them out when parked. I will also have a portable 2K watt generator.

All I want to do is figure out what the watts used shown on the meter translate into amp hours, so I can do further testing and get an idea of what size system would be required.

 

[Weight]

Use your Kil-a-watt to see the amps being used at any point during the air-con operation. Multiply that by 10 plus some loss through the inverter. You will see you need 100 to 150 amps direct current from the battery. That is over 100 amp-hours, every hour. How long will that 230 AH golf cart bank last?

 

[John61CT]

Again, it is not meaningful to discuss

"how many watts are used over X time"

including

> 1500 watts over a 5.5 hour period
> 1070 watts over a 6 hour period

> total watts over 5.5 hours is 8250 watts
> would consume 687.5 amps
> you might use less then 697.5 amps

If the aircon uses **an average of** 1500W over 5.5 hours, then the total is indeed 8250 but **wH** not watts

At 13V DC, that is 635AH, so you would need a lead bank where 12000AH of its capacity is just dedicated to that usage.

That's 50 pairs of GC batts, about 7000 pounds.

 

[John61CT]

Yes, the kill-a-watt meter read 1.5 kwh (1500 watts) for 5.5 hours running in cool mode and 1.07 kwh (1070 watts) for 6 hours running in eco mode.

1500 watt-hours is 1.5 kwh, 1500 **watts** is not correct.

OK, that is only 115AH, so a pair of GCs will handle that, only 140 lbs.

But, in actual hot weather, that aircon unit may well use that much energy in just 1-2 hours.

If you test in realistic conditions, I think you will see this idea is pretty impractical, unless you can insulate very well, have a huge living space and lots of extra money.

And if off-grid, you will need to run the genset many hours per day anyway, no matter how large the storage bank.

Maybe a 30' long roof can support enough solar to run the aircon just a few hours **and** recharge the bank.

The setup altogether will cost many thousands of dollars just for the electrickery components.

 

[John61CT]

All I want to do is figure out what the watts used shown on the meter translate into amp hours, so I can do further testing and get an idea of what size system would be required.

So, from the examples I posted, do you now understand how the math works?

The issue of units - amps vs AH, watts vs wH, is not a trivial one.

 

[RogerD]

Use your Kil-a-watt to see the amps being used at any point during the air-con operation. Multiply that by 10 plus some loss through the inverter. You will see you need 100 to 150 amps direct current from the battery. That is over 100 amp-hours, every hour. How long will that 230 AH golf cart bank last?

In cool mode, it showed .48 amps when the compressor wasn't running and up to 3.6 amps when it was running. The amps being used fluctuates, so I can't just do 3.6 X 10.

What I do know is how many watts of power was used to run the unit for the time frame it ran.

it used 1500 watts in cool mode to run for 5.5 hours.

Watts divided by volts is supposed to equal amps.

1500 / 12 = 125 amps

I'm assuming that the 125 amps used that it took to run the unit for 5.5 hours is the same as 125 battery amp hours. Yes / No?

 

[RogerD]

So, from the examples I posted, do you now understand how the math works?

The issue of units - amps vs AH, watts vs wH, is not a trivial one.

Sort of.

You wrote...

.6 * 1400W = 840wH per hour
If real voltage is 13V, 
64.6AH per hour, 775AH per 24 hours

My test used 1500 watt hours over 5.5 hours. That was with the AC cycling. It is a 410 watt unit.

In your example, you divided the watt hours by 13V, but you also say running 12 hours a day. I don't get this part.

So for my test using the same 13V would be 1500 / 5.5 hours = 272.72 WH per hour / 13 = 20.97AH per hour.

In your example, you say..."64.6AH per hour, 775AH per 24 hours". I don't follow how 64.6AH is 775AH for 24 hours. I would have assumed 64.6 x 24 = 1550AH

In my case, 20.97 X 24 = 503AH

In eco mode, my AC used 1070 WH over a 6 hour period.  Which would drop the AH by about 35%.