Hello, I have a DIY solar setup, I use a reliable inverter (2500 Watts) and so far it has worked real well for about 4 years. It powers everything minus my 3 ton AC unit, but for emergencies/power outages (we had a 2 hour outage last night at 104F) I have 2 battery banks one for running the window split AC unit 12K BTUs, which is approx. 12kw and some change and the other bank for lights, small electric appliances/devices. I was monitoring the power consumption of that AC unit and on the AC side and was reading exactly what the data label says as far as amperage and volts, but when I looked at the DC side the amperage was high almost 3 times as that on the AC side, so to give you an idea, the amps rating on the unit at max settings, cool mode, (compressor/inverter) engages and high fan speed the amperage draw was around 12 amps then it tapers down and hovers around 7 amps 280-345 watts, and maintains that almost constantly from that point on. In the mean time the DC to AC Inverter is drawing 21 amps from the batteries! All cables are up to spec with some overhead and no resistance that my meter can detect, I built all the cables in the system, and they run cold to the touch and the temperature tester shows same temperature as ambient. I have checked all connections, all breakers, and everything seems to be in order. I know there will be losses but maybe I'm overlooking something and is causing the Inverter to draw too much power every once in a while. To be honest I think the AC unit could be causing this issue, is a split inverter AC unit and overall works pretty good, but I don't think it should exceed the rating on the label, should it? Does it take 3 times the DC amperage to convert to AC? Any comments are welcomed. Thanks for taking the time to read this post.
Converting DC to AC how much current does it actually require?
- Thread starter Herb
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jameshowison
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- Jul 30, 2021
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Think of this in watts, not amps.
The DC system is lower voltage (12/24/48), thus higher amps. The AC system is higher voltage (120/240) thus lower amps.
The DC system is lower voltage (12/24/48), thus higher amps. The AC system is higher voltage (120/240) thus lower amps.
Thanks for the reply, got it, thanks, is there a formula to figure this out? Wow, 3 times as much, the system voltage is 36v nominal. I'm thinking that maybe I should have purchased a DC AirCon, LOL.
Vigo
Solar Addict
Yes. It’s called Watt’s Law.
120v X 12a = 1440w
1440w / 36v = 40a (assuming 100% efficinecy, so in reality this number will be higher)
120v X 7a = 840w
840w / 36v = 23a
You are right that the dc amps will be ‘over 3 times’ the ac amps because 36 is ‘less than 1/3rd’ of 120. Plus, there will always be slightly more amps than what watt’s law would show because of efficiency losses in all the equipment, and remember that battery voltage is not static but changes with load and soc which will also change the instantaneous numbers at any given moment.
A directly 36v dc operated aircon wuld be more efficient, but the efficiency losses in the conversion are what we pay for convenience. Its pretty easy to get your hands on all kinds of 120/240 devices and run them off a diy solar system inverter that puts out 120/240. If this convenience were not in place.. it would have been impossible for you to assemble any of your system without ALREADY knowing Watt’s Law, among other things. I for one can forgive a fair good bit of inefficiency for convenience’s sake!!
120v X 12a = 1440w
1440w / 36v = 40a (assuming 100% efficinecy, so in reality this number will be higher)
120v X 7a = 840w
840w / 36v = 23a
You are right that the dc amps will be ‘over 3 times’ the ac amps because 36 is ‘less than 1/3rd’ of 120. Plus, there will always be slightly more amps than what watt’s law would show because of efficiency losses in all the equipment, and remember that battery voltage is not static but changes with load and soc which will also change the instantaneous numbers at any given moment.
A directly 36v dc operated aircon wuld be more efficient, but the efficiency losses in the conversion are what we pay for convenience. Its pretty easy to get your hands on all kinds of 120/240 devices and run them off a diy solar system inverter that puts out 120/240. If this convenience were not in place.. it would have been impossible for you to assemble any of your system without ALREADY knowing Watt’s Law, among other things. I for one can forgive a fair good bit of inefficiency for convenience’s sake!!
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