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Old 06-07-2018, 12:27 PM   #1
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Inverter size for AC

Iím going to put 2 rooftop AC units on my Skoolie and power them with shore power or a generator. I have a 3000 watt inverter and Iím wondering if I need something bigger if the draw of the AC units is above 3000 Watts.
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Old 06-07-2018, 12:56 PM   #2
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Howdy and welcome Whalby!

Sometimes it helps to use "AC" for Alternating Current and "A/C" for Air Conditioning - just to keep everyone on the same page.

Roof air conditioners typically require 120VAC power (or 110VAC or 115VAC, whichever a person prefers).

Both shore power and generator provide 120VAC power (as in volts alternating current). There is no Inverter involved.

An Inverter "converts" DC 'power' to AC 'power.'
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Old 06-07-2018, 01:01 PM   #3
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Thanks! My shore power outlet will go through the inverter though is that ok or will the A/C units need a dedicated outlet that goes directly to them?
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Old 06-07-2018, 01:09 PM   #4
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Thanks! My shore power outlet will go through the inverter though is that ok or will the A/C units need a dedicated outlet that goes directly to them?
Your inverter may have an internal automatic transfer switch built in, which will run your A/C from the shore power and bypass the inverter.

Are you sure you need that much inverter? Reason I ask, is I saw another thread about solar and it seems 3000W can cause issues with solar panel/battery bank sizing.
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Old 06-07-2018, 01:09 PM   #5
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Thanks! My shore power outlet will go through the inverter though is that ok or will the A/C units need a dedicated outlet that goes directly to them?
Ahh! You have an Inverter with an internal transfer switch. Afraid I'm not sure if those transfer switches are limited (X amps) but someone here will know.

Personally, I would be inclined to install a panel in the shore power line before the Inverter and wire the A/C unit to it. That way there is no chance of accidentally running the A/C from battery (and depleting or possibly damaging them - demands on what you have there, of course).
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Old 06-07-2018, 01:11 PM   #6
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Personally, I would be inclined to install a panel in the shore power line before the Inverter and wire the A/C unit to it. That way there is no chance of accidentally running the A/C from battery (and depleting or possibly damaging them - demands on what you have there, of course).
Great point, JD. I accidentally ran my water heater from battery power and it drew them down from 100% to 70% in about 15-20 min.
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Old 06-07-2018, 01:18 PM   #7
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I got it that big so I have room to grow and it charges at 50amp which is nice. What did you hear could go wrong?
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Old 06-07-2018, 01:23 PM   #8
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I got it that big so I have room to grow and it charges at 50amp which is nice. What did you hear could go wrong?
There's some discussion of 3000W inverter here.
http://www.skoolie.net/forums/f49/so...s-22597-3.html
I haven't tried to wrap my head around it too much, I'm pretty dumb when it comes to electrical for some reason (it's a blind spot for me) but it made me start questioning my future solar setup.
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Old 06-07-2018, 01:38 PM   #9
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Basically boils down to lead-acid battery C rate for charge/discharge. You can certainly shorten the life of a battery bank by going too high or low - there is an ideal. I've always wondered where the line is between academic and real life on this topic. Clearly our (full-timers) use of our battery bank is rarely ideal.

For example (and these are made up numbers), assume you spend $500 for batteries and have less than ideal charge/discharge rates. Will they survive for 3 years instead of 4 (all other conditions being the same)? Or, will they survive 2 years instead of 10 years? I certainly do not know and would be curious to read something on that if anyone has a good link.
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Old 06-07-2018, 01:42 PM   #10
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The Dometic 13500 rooftop air conditioner claims to consume 2800 watts. At 110V AC power that's over 25A, if you're doing it from 12V DC battery power through an inverter you're going to be pulling 234A! That's a LOT and doesn't take into consideration the inefficiencies of an inverter. You'll want Lithium batteries for that to be even remotely reasonable.
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