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Old 02-16-2018, 11:37 PM   #1
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Window AC Unit or Rooftop Fan??

We have our short bus insulated on the top and sides throughout with 3M Thinsulate. I cannot speak for other types of insulation, but this stuff has been amazing ... in the fall/winter. My concern is when we hit the road on May 1st for a year and we are ANYWHERE in the United States BUT cold regions, haha. (We plan to quite literally follow the warm weather wherever it goes during fall/winter seasons.)

That said, we are struggling between whether we want install a rooftop fan or if we should install a small window AC unit. The unit we are shopping right now is this one:

https://www.amazon.com/dp/B06WRRZLYB...META93KV&psc=0

The fan we're leaning toward is the MaxxAir 7000K model. That link is here:

https://www.amazon.com/dp/B003ZOF09Y...META93KV&psc=0


Wondering if anyone has input or suggestions based on their own experiences. (I'll take suggestions and opinions regardless, but if you have a window unit or a rooftop fan (or both) - I'd love to hear your thoughts.

Being that we are intending to be in warmer climates as much as humanly possible unless, say, a cold front blows through or something like that - I just don't know if one is better than the other as far as keeping it cool-ish in the bus. As I stated earlier, we do have what I would like to consider a strong protective layer of insulation, but I'm not so sure that's going to keep the bus cool.

We were unfortunately forced to rip out the original AC that was built into the bus originally due to corrosion and leakage and just all-around grossness. If we install a window unit we're thinking of putting it in one of the far back windows of the bus. The concern I have with this is that 1) I think it might freeze us out while we sleep. I could be wrong. However, this unit is described as intended for up to 150SF of space and we're looking at what, maybe 90SF of space all together in the short bus? Something like that. 2) I'm not convinced that even with a smaller window unit like this that it won't drain out two 100Ah batteries overnight while we're sleeping. (We do have LiFePO4 batteries, not AGM deep cycles).

While we aren't expecting nor attempting to achieve the absolute conveniences of living our regular lives like we do now in our townhouse; we also don't want to wake up dead from the heat. (Forgive the humorous attempt at that oxymoron there.)

Thoughts and suggestions? I SUPERBLY appreciate any feedback.
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Old 02-17-2018, 12:07 AM   #2
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Trying to run AC on battery power usually ends up being an exercise in frustration.

The unit you linked runs around 4 amps @ 120V = 480 Watts

Multiply that by, say, 5 hours = 2400 Wh and divide by 12 for Amp Hours

= 200 Ah.

If it has a 50% duty cycle, then your batteries would be down to 50% capacity in 5 hours.

That ignores inverter losses and any other loads.

Please ... anyone, if my math is wrong, correct it.
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Old 02-17-2018, 12:47 AM   #3
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Quote:
Originally Posted by Twigg View Post
Trying to run AC on battery power usually ends up being an exercise in frustration.

The unit you linked runs around 4 amps @ 120V = 480 Watts

Multiply that by, say, 5 hours = 2400 Wh and divide by 12 for Amp Hours

= 200 Ah.

If it has a 50% duty cycle, then your batteries would be down to 50% capacity in 5 hours.

That ignores inverter losses and any other loads.

Please ... anyone, if my math is wrong, correct it.


I am SUPER curious as to how you did that "electrical" math! (This takes of a bit off topic, but I have to know this! I feel it's very very important as "Wilson" (our bus) will be our full time home while we're out on the road. This is also why in another post you responded to earlier today (thank you very much, btw) I was asking about alternative ways to juice up the two 100Ah LiFePO4 batteries that we purchased for the "house" portion of the short bus.

So the only thing I have come up with was as follows (and I clearly have something misunderstood - NOT surprising though).

So I was under the impression that:

Watts = Amps * Volts
Amps = Volts / Watts
Volts = Watts / Amps


Again, hahahah! I am very much likely wrong. So when I saw the 4amps on the website I actually thought, "Ah, okay. So if I run the AC all night at 6 hours (typically my normal sleep cycle and I'm well rested) than that would be 4amps * 6 hours = 24 amps. I immediately told my partner (Carolyn) that this couldn't possibly be right.

I'm actually have all of the electrical belongings that we're planning on having in the bus and I have all their amps and watts and I THOUGHT I had converted them correctly.

At any rate, I hope to nail down these formulas soon so I can properly determine what I'm really going to be using energy-wise.

With alllll that said - yeah if I'm using 200Ah to run that window AC unit for one night - that absolutely will never work. Those batteries are about $900 a piece - super not cheap IMO. I'd rather not by another one because we only have two 200watt solar panels. Even if the peak time of the sun gives me a full on 6 hours to charge the batteries, I don't think I'd be able to juice the lithium batteries to 100%.

I do know from research and conversation with the company I bought the batteries from that these batteries can be drained to practically zero and it will not hurt them. I also read they last years and years and years as opposed to regular batteries.

Maybe I should be going with the fan. Have you ever used one? If so, does it work decently in the heat? With all the insulation we've put in - do you have an opinion on whether it might be fine to just go with the MaxxAir rooftop option?

Thanks.
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Old 02-17-2018, 02:42 AM   #4
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You should be able to get a 228ah six volt battery for nearly $400. You could run and energy efficient fan for a little while but not an air conditioner. I'm still using a generator for that. My solar is for lights and chargers.
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Old 02-17-2018, 10:33 AM   #5
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Quote:
Originally Posted by msearslive View Post
I am SUPER curious as to how you did that "electrical" math!
Okay ... Google is your friend because it will show you worked examples, but briefly:

There are a bunch of units and terms that all take their lineage from the Ohm's Law you stated in your post.

The problem is that other terms are thrown in, like Amp Hours and Watts and Watt Hours.

So for your issue we can work out your power requirement in Watt Hours, but your battery capacity is measured in Amp Hours.

So we have this situation:

4 Amp consumption @ 120V uses 480W per Hour.

Multiply that by 5 hours giving 2400 Watt Hours. That is the total power consumed in 5 hours at 100% duty-cycle.

However, that energy was supplied at 120V and the batteries supply 12V, so to get the AmpHour consumption we divide by 12

2400 / 12 = 200 AmpHours. I assumed a 50% Duty Cycle for 100 Ah

To solve your problem you will need either to plug into shore-power, or have a much bigger battery bank.

And yes, the math does my head in too which is why I asked for any corrections, but I think it's right.
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Old 02-17-2018, 12:05 PM   #6
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Think of a watt of electricity like a gallon of water, no matter what the pressure, volts, the unit stays the same so all math with electricity needs to be broken down to watts to get it right
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Old 02-17-2018, 12:10 PM   #7
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I have a similar ac in my shorty. Without venting the heat from the roof while using the ac, the whole place turns into a sauna. My bus has 2" of foam insulation top, bottom, sides and one end along with dual pane windows. The room ac running full blast only manages to keep the interior 10 or 12 degrees below the outside temperature. At temps over 95 degrees all bets are off. Frankly, in order to run that unit over night you would need a trailer full of batteries to do the job. Remember, no matter how large your battery bank, you still only get to use 50% of the charge before the battery needs to be recharged or the batteries will be damaged.
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Old 02-17-2018, 12:11 PM   #8
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And to further confuse the issue, wire size is based on amps, not watts
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Old 02-17-2018, 12:25 PM   #9
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The actual formula is P(watts)= I(amps) x E(volts). By simple algebra
amps = watts / volts and
volts = watts / amps
You have to keep in mind that you are using two different voltages in your bus. So 4 amps @ 120v = 480 watts but @ 12v that 480w is 40 amps.
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Old 02-17-2018, 12:27 PM   #10
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Appreciate the info and education on the electrical math bit. I started Googling last night, and I'm back at it today trying to make sense of this all.

I need to really remember this is one step at a time with this bus conversion. My excitement level hasn't changed, but wow - this can be a bit overwhelming when you begin to realize what all goes into this. (And math is 100% absolutely NOT my forte - at all.)

I really really don't want to by "regular" batteries for the house portion of the short bus - for many reasons. One of the biggest being that you really only get 50% of what you're purchasing. I feel like if I wanted that I'd just go to the store and buy a bag of chips - 50% chips, 50% air. (If that analogy makes any sense at all.) The LiFePO4 batteries are supposedly able to be drained practically to nothing with no negative impact to their lifespan or effectiveness. That said, those batteries are not exactly cheap. I need to decide I guess what direction I should take.

I'm now mulling over options now. When I sit down and I start doing this electrical math on all the electronics I will have in the short bus - I don't now how people do it. Here's a list of all I have originally intended to bring into the bus:

-Induction Stovetop (one burner) @ 1800w
-Two Cell Phone Chargers @ 25w (combined)
-Laptop @ 65w
-Shower Head @ 2500w
-Overhead Lighting (LED) @ 18w
-Mini Fridge @ 500w (supposedly approximately total for a 24hour period - supposedly)
-Outside electric/charcoal grill @ 1500w
-MaxxAir Rooftop fan @ 12v (i looked everywhere and could not find the watts - still looking)


Soooo, a couple things. I have a spreadsheet in Excel where I have listed all the items in the bus (like listed above), their amps, their watts, and the amount of time we anticipate using them every day. Clearly we would never use an induction stove or the showerhead (for hot showers) all day long. They'd be used minimally. For instance, we're working our way at home to get down to 5 minute showers if possible. Also, the induction stovetop cooks much faster than our stove at home so we're guessing 1 hour total a day (half hour 2x a day). Some days probably wont' even use it all. We'll cook on a grill with wood chips or charcoal or something (havent' figured that part out just yet).

My point is simply that we won't be using all this stuff all day of course.

For those of you with generators - where do you put them? We have a short bus, and in my opinion having a generator inside the unit is just not going to happen. I would imagine we wouldnt' be able to think straight much less have a conversation while it's running. I'm guessing people build a rig or something to put on the back of a hitch on the end of the bus to haul it??

Just trying to figure out options at this point.
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