Watts to run AC

Not sure what you mean as the watts required to run a car air con is produced by the alternator with only a very small load on the engine. (Newer cars). For example my new Hilux does not vary in it's fuel consumption with the Air Conditioner on or off. So with that in mind the watts required are being freely generated by the vehicles alternator. Older model vehicles do suffer an increased load on the engine when running the Air Conditioner Clutch , that may equate to an increase in fuel consumption of up to 10% but it has no effect whatsoever on my new Hilux.

It's not only electrical power used to run your air-con. Engine power also ( through the effort needed to turn the compressor via the belt ) The more load you pull out of an alternator, the more power is taken from your engine. The compressor clutch pulls very little current compared to the blower fan which actually moves the air round the cab. I've been told the total horse power of a Land Cruiser air-con is just under 1/2 horse.( 380W - about ) But, not all the 380W has to come from your electrical system rather in total from the engine. Not much really.

Camper

I seem to recall having your windows closed with the AC on is similar to driving with the AC off but the windows open. Apparently the extra drag from reduced aerodynamics is more or less equivalent to the extra hp involved in running the compressor.

Mind you ... the Fender has the aerodynamics of a brick so I don't suppose it really matters in my case ..... :o)

Cheers

Rosco

Anyway there's nothing you can do about the power the watts or the whatever you feel it takes to run your air conditioner, other than turn the bleep er off..! Then again you could install a Finch Fuel System and the increase in power would easily accomdate for the power used by the air conditioner belt/clutch. Off course you could double that again by adding the Cyclone which would swirl the combustion better for more power again.

I monitored the fuel consumption closely in dads V8 Caprice with a/c on and a/c off with different drivign styles, and worked out it costs arount 10c per hour! to run the A/C - other cars may differ slightly, but as for me, the only reason i would EVER switch off the AC to save fuel, is if i was about to run out, and trying to make it to the next fuel station

I probably should have posted this on your previous thread, about converting 240 v to 12 and vice versa. An old autoelec gave me this advice about converting the figures. Roughly in round figures the watts remain the same for both voltages. So if you fan is drawing 72 watts at 240 volts it will draw 72 watts at 12 volts. Add in about 10% for inefficiencies in your invertor, your fan will be drawing around 80 watts at 12 volts in round figures. That saves on doing a whole heap of maths, which I always hated.
Thats the easy part. As the voltage reduces the current increases. At 240 volt your 72 watt fan draws 0.3 amps. At 12 volts it draws 6 amps.
An analagy using water: Volts is the hose diameter, watts is the water volume and amps are the water pressure. As the hose size decreases, you need to increase the water pressure to keep the same volume going through the hose! Okay I'm no expert on all this, and I'm happy to stand corrected. All the best for Christmas and the New Year.
Cheers Ferris

mmmm...
used to have all input figures for older style York (alias ford, chrysler and gmh0 compressors. If i recall correctly, the biggest York ( a 10 cubic inch displacement) at maximum rpm (7000?) would do around 5 ton of refrigeration capacity at airconditioning duty ratings.

this equates to around 15kWr in metric or approx 5hp airconditioner in old terms

it also has to with size of evaporator V interior of car to correctly work it out, but as rough guide I would estimate the equivalent of 2hp unit at maximum speed...giving or taking a bit for additional (that i havent factored in) thermal losses across the exterior skin of the cabin

Hi happy campers :-)) 746 watts = 1 hp.

How much watts / hp does the air con drag out of your engine, well I like the numbers of 5 to 7 hp = (3.7 to 5.2 kw) !

You can easily 'feel' this your self by taking your foot off the accelerator and turning the air con on / off when descending a steep hill. One of my vehicles tends to 'run away' a bit when descending a long steep hill so I flick the air con on and the vehicle maintains speed as if I was feathering the brakes (same thing happens if you turn on the head lights as alternators can drag the same sort of kw out of the engine).

This is a good 4wd trick when descending very steep terrain off road (stay slow, stay in control). Keeping your foot on the brakes when descending a thousand feet or so over rough rocky terain is a pain in the butt if your vehicle tends to gain speed without braking. By turning on the air con you can keep both feet on the floor and not touch a pedal.

Modern engine management systems up the idleing speed automatically when you turn on the air con.

HHMMM so now we have 12v powered airconditioners in vehicles do we ?? I don't think so Kia. If you mean the electrical load is only slightly increased by running the aircon you are right. The real power to run the compressor comes from the engine mate ! The engine gets its power from the fuel, more power consumed, more fuel consumed, pretty simple.

At the end of the day, it ALL comes back to an additional load on the engine - even the extra electrical load, which in turn requires more engine power to turn the alternator. As said previously, energy for everything in your vehicle is supplied via the fuel in, therefore, any additional energy required means more fuel needed. It may not be a direct relationship as engine efficiency changes with engine load, but essentially, there will be a difference in consumption with aircon on or off.
Gerry

Thanks to everyone for a good discussion and sorry my reply is a bit behind the post but I've been a tad crook. I got a bit interested in this because a 70 amp alternator flat out is probably using up a Kilowatt when you factor in losses in the belt drive and heat etc. If the air con is pulling 3-5 Kw and a motor is only about 35% efficient I got to wondering how much fuel goes on accessories.
Camper