Air Conditioner power ratings. - Page 1
Forum section: electronics / general

author: aargee
author_rank: Frequent Contributor
timestamp: December 09, 2020, 10:36:43 pm
content: Given Dave's post on his AC installation, it has brought up a question about some quoted specs that AC rated power is not the electrical power load but the 'cooling capacity'."When we are talking 3.5 kw it is cooling capacity not anything to do with electricity consumption." (Sic)Maybe my physics is lacking (likely) but how is this equated then? Should there be a 3.5kW circuit for a 3.5kW air conditioner?

author: mag_therm
author_rank: Frequent Contributor
timestamp: December 09, 2020, 10:52:13 pm
content: Read about Heat pump coefficient of performance.But I agree that appliances should be rated on the nameplate expicitly stating the input kVA and current.We have some smaller units here and the electrical supply requirement is not stated in the user instructions and I could not find it on internet.That is crazy, in my opinion.

author: Siwastaja
author_rank: Super Contributor
timestamp: December 11, 2020, 10:32:08 am
content: From a very practical viewpoint (inside your house), you can think about it exceeding 100% efficiency.Strictly, you can't talk about "efficiency", that's why different terms like "coefficient of performance" are invented, but yes, you will get very actual 3.5kW cooling with way less than 3.5kW of power put in. This is because totransferheat, less power than the heat flow itself is needed. For example, to transfer heat in the naturally occurring direction requires no power at all, it flows by itself. Heat pumps can force heat transfer, and that requires energy to do it, but still less than the energy transferred. There are formulae that dictate the maximum heat transfer coefficient of performance as a function of temperature difference; real heat pumps will always perform worse than that, but COPs in range of 500% are fairly typical.If you look at the complete system, i.e., both inside and outside your house, the device always generates heat according to how much power it draws. In cooling application, this is lost outside; in heating application, it plays in your favor.

author: Rick Law
author_rank: Super Contributor
timestamp: December 15, 2020, 10:25:53 pm
content: Quote from: aargee on December 09, 2020, 10:36:43 pmSee if this helps you think about the issue...Efficiency can never be 100%. Waste is typically in the form of heat.If you are heatingandyour heater is in the house, the waste heat is inside the house so it could be perfectly efficient. That said, some energy can be absorbed (or released) by the material as friction slowly disassembles whatever is rubbing against each other. It takes energy to scrap away tiny layer of your motor shaft if even a tiny amount of fraction exists. Depending on the chemistry involved, breaking chemical bonds and/or rearranging molecular structure takes or releases energy. If you bend a metal paper-clip at the same point a few times and touch the bend, you can feel the heat created by the repeated bending. So, more heat (or slightly less) heat can be dumped into the environment.If you are coolingandyour entire air conditioning system is inside the house, the compressor (typically greatest heat generator inside an AC) in a perfect system will create as much heat as the evaporator is taking out, plus the waste heat from <100% efficiency. So this fully in-the-house "cooling system" would be like a typical kitchen refrigerator: it take out heat from one part (inside the fridge) and dump the heat right back in your kitchen, plus waste heat... Cooler for inside the fridge, but acts like a heater in your kitchen warming up the kitchen.Thus, with the AC system, generally, your evaporator (heat absorbed by the coolant as it evaporates) will be in the inside and the compressor (dissipating the absorbed heat in the coolant) will be on the outside. The inefficiency (waste heat) would be created by the fan (and other electronics) that blows air over your evaporator in the side, and the fan on the outside that blow the heat dissipated by the coolant.

author: bdunham7
author_rank: Super Contributor
timestamp: December 15, 2020, 10:41:10 pm
content: Quote from: aargee on December 09, 2020, 10:36:43 pmIt's a units thing. This never came up when we used BTU/hr or tons!Keep this in mind: A standard horse (the four-legged manure-generating kind) is 0.746 kW. The sun is (maximally) 1.0kW/m2. Nobody is suggesting you need a circuit breaker on your horse or a basking turtle.A watt is generically a term for 1 Joule per second. This can be physical work or thermal or electric, it is just a rate of a unit of energy per unit time. The fact that the air conditioner is using the same basic unit of expression for both its thermal capability and electrical consumption is just a confusing coincidence unless you know more about its inner workings. There is a relationship between the cooling kW and the electrical kW, but it varies between each individual unit and their exact operating conditions.

author: james_s
author_rank: Super Contributor
timestamp: December 15, 2020, 11:20:15 pm
content: Very roughly speaking you can assume the electrical consumption to be pretty close to 1kW per ton of refrigeration capacity. 1 ton = 12,000 btu/hr = 3.5kW. As someone already mentioned, it's just a units thing, you're not getting free energy, you're just measuring different things.

author: G7PSK
author_rank: Super Contributor
timestamp: December 18, 2020, 04:14:40 pm
content: I have tree air conditioners/heat pumps. two are rate at 3.5Kw output and one at 15 KW out. Not measured the power consumption of the larger unit as it was installed by an air con company just a couple of weeks ago and is hard wired in with the power supply outside. The two smaller units have 13 amp plugs fitted as supplied and I installed them myself. I have used them with a killawatt power plug and they vary between 400 watts to a 1000 watts consumption and typically use around 1.5 to 3 units of power over an 8 hour period, they are rated by the manufacturer as having 1KVA input the large unit is rated at 5KVA input come the summer when I can safely run it with the cover off the power supply I will use a clamp meter to check on actual power draw.From what I have read you can expect about three and half time the heat power equivalent out for that put in.

author: james_s
author_rank: Super Contributor
timestamp: December 18, 2020, 06:54:05 pm
content: The power consumption will vary with head pressure, which will vary depending on the temperature of the condenser. The colder it is outside when heating or the hotter it is outside while cooling, the higher the power consumption will be.

author: G7PSK
author_rank: Super Contributor
timestamp: December 18, 2020, 07:04:47 pm
content: Quote from: james_s on December 18, 2020, 06:54:05 pmalso varies according to fan speed on the units I have as the units are inverter powered the fan speed is fully variable.

author: Ground_Loop
author_rank: Frequent Contributor
timestamp: December 19, 2020, 02:39:48 am
content: My 4 Ton heat pump is pretty clear about the electrical requirements.