Amp Hours Question

[John61CT]

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.

Exactly!

Same with watts, which is just times V to get A.

That is why

> 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.

are not correct.

Only watt-hours (or AH) make sense with the "per 5.5hours"

And that is why a specialized AH/wH counter is needed, it "integrates" - like the area under a curve in calculus - all the different instantaneous watt/amp flow rate readings to give an accumulated wH/AH total per time period

> Watts divided by volts is supposed to equal amps. 1500 / 12 = 125 amps

You need wH divided by volts (13 closer IRL) to get AH.

> 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

If that 125 **AH** was at 110VAC, then at around 13VDC it would be around 10x, or 1250AH.

Stick with wH for consistency, only use AH when voltage is clearly known to all in the discussion.

Plus note the big inverter involved in the DC-to-AC conversion will also consume a fair bit of power.

 

[John61CT]

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.

I used the known Frigomar usage, because you had not yet confirmed your test results with correct units.

I will stick with that, then you plug in your numbers and post back the result as an exercise.

At 1400W while the compressor is on

and assuming the compressor is on 60% of the time while the aircon is turned on ("cycle rate")

.6 * 1400W = 840wH per hour

Clear?

Now at 13V, that 840 above becomes 64.6AH per hour.

Now, assume a runtime of 12 hours a day, which is half the time, comes to

775AH per 24 hours

 

[John61CT]

Now the IRL problem is that cycle time can be 5x or 8x greater when it gets hot out. Lower with good insulation.

Since your cycle time is not realistic, your AH per hour usage will be much much higher than your test results.

Once you understand these issues better, I suggest you closely re-read the posts above from me and tx2sturgis and let the conclusions sink in.

So that your plans are more reality-based rather than on just your hopes and desires.

Bottom line is, there is little point in running aircon off batteries, even with a huge bank, bigger than your vehicle can carry.

You will need to run your genny anyway, about the same # of hours you want to run your aircon.

Or more, if the solar + genny is not powerful enough to power the aircon **and** at the same time recharge your bank back to 100% Full each day.

 

[Weight]

I'm having a problem with your power measurements on a 5000 btu air. That just doesn't seem high enough. Especially the 120V amps.

 

[tx2sturgis]

I can make some suggestions, to help you thru the calculations, but by no means have I personally ever run an A/C unit from solar panels and batteries. There are members here who have, they can check my work, corrections are always welcome...

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To keep it a bit simpler, I suggest converting the amp-hour rating of a given battery to watt-hours.

To do this, multiply the ah rating times the voltage. Stay conservative, and use 12v not 13.8v.

If you installed one 250 ah 12v battery, that comes out to 3000 wh, or, 3 kwh.

For lead acid batteries, use half of that rating: 1500 wh, or 1.5 kwh.

Meaning, one of these batteries should (in theory) run that AC on eco mode (about 1100 wh) for about 1 hour and 20 minutes to 50% discharge. (not counting inverter loss yet)

Five of these batteries (in parallel) will then provide about 6.5 hours of use, at 7150 wh per run cycle, (again, to 50% discharge)

So, to provide about 7150 wh per day, you would need about 1200 watts of (actual) solar input for at least those 6 hours. (1200 watts times 6 hours, or 7200 watt hours)

But, this leaves nothing for inverter loss (10% or so) and battery inefficiencies, cloudy days, low sun angles, shading, recharging the batteries, refrigerator, lights, charging a laptop, etc. 

There is little surplus capacity with this setup, but with a well-insulated van, good sunny days, near perfect batteries, no additional loads, moderate weather, topping up the battery water levels often, occasional generator or shore power, and a bit of luck, it might be right on the edge of possibility.

All of these numbers are 'scalable'...meaning, change one rating or capacity, and it will change the results, of course. And some members here choose to balance the system towards more solar panels and a few less batteries, relying on the raw output of the panels to (more or less) directly supply the inverter....resulting, of course, in less battery reserve. It's all about compromise.

If you can reduce the power draw of the A/C unit, (better insulation, higher efficiency, lower duty cycle, etc) then that will pay for itself.

And keep in mind that in very hot weather, most solar panels tend to reduce output somewhat.

 

[RogerD]

I'm fully aware that the cycle times will change when it is hotter and will use more power than my initial test. That is why I said in my original post that I need to test when it's like 90 degrees out.

I follow now what you are saying about 64.6AH and only running it 12 hours per day is 775AH neede per 24 hours because you are only running it half the day.

Funny thing is my math was correct the entire time. However, I was dividing watt-hours by 12v and not 13v. Dividing by 13V actually works out to lower amps / amp hours.

 

[highdesertranger]

keep in mind that only the most efficient inverters have a 10% inefficiency rating. I would use 20% as a rough calculation number some of your junk ones can go as high as 30%. highdesertranger

 

[RogerD]

I'm having a problem with your power measurements on a 5000 btu air. That just doesn't seem high enough. Especially the 120V amps.

The unit is 410 watts / 3.8 amps. It was the lowest watts / amps 5000btu I could find. It also has a soft start, so it doesn't spike when starting. I think it had an energy saving rating of 12.9.

Most cheap 5000 btu / $100 ish units use more energy. I paid a premium for it, as it was $175.

However, that is the max that it will run at.  I also never saw it reach 3.8 amps, even when pulling 410 wats. The highest amps I ever saw was 3.6.

When the compressor isn't running, the fan runs if in cool mode and it pulls like 56 watts and .48 amps.

In eco mode, the fan doesn't run when the compressor isn't.

It was also only 82 degrees outside and I had it set to 73. I need to test it when it is like 90 degrees and the numbers should be higher.

 

[RogerD]

keep in mind that only the most efficient inverters have a 10% inefficiency rating.  I would use 20% as a rough calculation number some of your junk ones can go as high as 30%.  highdesertranger

Since the max watts is 410. Could I just get a really good 500-watt inverter just for the AC?

 

[Weight]

A good 500 watt inverter should be ok at 400+ watts. I would have a bit more caution, maybe 750 watt. Your biggest problem is enough battery ah capacity that you don't pull the voltage down.

 

[highdesertranger]

yeah I would rather have a 750-1000 watt inverter no need to cut it so close. also a soft start doesn't mean there is zero surge at start up it means there is lees of a surge than a non soft start. highdesertranger

 

[RogerD]

yeah I would rather have a 750-1000 watt inverter no need to cut it so close.  also a soft start doesn't mean there is zero surge at start up it means there is lees of a surge than a non soft start.  highdesertranger

I watch the kill-a-watt meter when I turn it on and have never seen it use more than 100 wats.

When it starts, only the fan comes on / not the compressor.

 

[John61CT]

The best battery value by far is Duracell (actually Deka/East Penn) FLA deep cycle golf cart batteries, 2x6V, around $200 per 200+AH pair from BatteriesPlus or Sam's Club. Deka labeled same batts also sold at Lowes.

70 pounds per 200AH pair.

Figure the first pair for general loads if not too heavy.

Then if you **really** need to run aircon without running the genny - why? - figure each further pair gives you ~2 hours.

But you **must** replace any AH you take out *1.2, and for longevity get the bank back to 100% Full "most cycles", at least 3-4 hours genny run before the solar day starts and while charging another 4-5 hours, all aircon supported by the genny, which **must** be large enough both all the aircon **plus** all the other loads **plus** at least some charging.

Look at fuel costs too, as I said the genny will likely be running at least as many hours as the aircon per day.

So again, why bother using the battery bank to do so? Just adds expense weight and space used.

 

[John61CT]

I watch the kill-a-watt meter when I turn it on and have never seen it use more than 100 wats.

When it starts, only the fan comes on / not the compressor.

Link to aircon unit please.

 

[highdesertranger]

the Kill-a-watt meters are not accurate for start up surge. highdesertranger

 

[RogerD]

A good 500 watt inverter should be ok at 400+ watts. I would have a bit more caution, maybe 750 watt. Your biggest problem is enough battery ah capacity that you don't pull the voltage down.

Yea, I totally understand the battery bank issue.

I'm really starting to think the best / easiest way will be to use lithium batteries since you can draw them down much further and they weight a lot less. I'm just not thrilled about the price.

Before I do anything, I'm going to buy a zero breeze mini AC and test it. I see a bunch of negative reviews, but I also see good ones. I will wait a while and hope they come out with a new version before getting one. It certainly looks worth testing.

 

[RogerD]

Link to aircon unit please.

https://www.frigidaire.com/Home-Comfort/Air-Conditioning/Window-Mounted-AC/FFRE0533S1/

 

[tx2sturgis]

I watch the kill-a-watt meter when I turn it on and have never seen it use more than 100 wats.

When it starts, only the fan comes on / not the compressor.

But when the compressor DOES kick on, the in-rush current surge is a LOT...you will need a minimum 1000 watt inverter, and of course, some very heavy cables. Running wattage of the A/C unit at half the inverter rating will mean it loafs along with little trouble.

I would recommend a pure sine inverter...these are not cheap...a good one will likely cost about three to five times the price of the A/C unit.

 

[John61CT]

Some aircon unit designs have a startup current requirement much **lower** than the peak ongoing continuous requirement.

In other words, optimised soft start built in like the marine unit I mentioned.

Probably not ones costing under a grand, but hey you never know, these days it's just tweaking the algorithm of cheap control electronics.

It would be very helpful for those running mains aircon units off 12V to specify what lower-powered inverters they find capable of running which specific models.

I have a small relatively light DC generator coming, hope to post results of using that one day.

 

[Weight]

The problem with kill-o-watt. They don't react fast enough to register the sudden inrush current. Even with a soft start there will be some inrush.