A/C Wattage vs lights

<div class='quotetop'>QUOTE(kirbinster\";p=\"110712)</div>

Sorry, but you clearly have no understanding of what you're talking about.

I have no doubt that the cooling capacity of the Prius is 4500 watts. One Btu is .293 watts. That means the Prius cooling capacity is 15,300 Btu/hr, or a little over 1.25 tons.

But it does not take 4500 watts of electricity to provide 4500 watts of cooling capacity. An A/C compressor is a pump, it just moves heat around. It takes less than 4500 watts of electricity to move 4500 watts of heat out of the car. If it took 4500 watts, that would be an SEER of 1.0, or the most inefficient A/C on the planet. The SEER is probably well over 10. Even the most inefficient window A/Cs are over 10 SEER.

Do you not understand this simple concept? If not, you'll have more trouble understanding out how 1 watt of electricity into a heat pump creates more heat than 1 watt of electricity into an electric furnace, even though that furnace is 100% efficient.

 

You are right, I over simplified it to make a point. Heat pumps are generally measured in terms of coefficient of performance (COP) which is a direct measure of input heat to output heat. I think a typical number is somewhere around 5, that is for each unit of energy you put in it is able to extract 5 units of heat. I am very well aware of this, as I have an 85,000 BTU/hr heatpump for my inground pool.

A car A/C unit is very commonly sized around two tons (24,000 btu/hr) - which as you know requires a hell of a lot more energy than 300 watts. I think a 12,000 btu air conditioner is about the maximum size you can plugin to a normal 15amp 120volt circuit which would be 1800 watts.

 

<div class='quotetop'>QUOTE(jayman\";p=\"110724)</div>

Las Vegas is hot and dry - contractor units last much longer. When I replaced ours after owning our house for a couple years, they were going on their 14th season. These were cheap Carrier units that came with new construction.

The recommendation from several contractors I talked to a few years ago was to buy one level below the most efficient, as they were usually the most reliable, and you often would not make back the energy saving with increased efficiency. I think I calculated the breakeven point to be just about the same as the expected life of the unit, and I wasn't planning to stay in the house that long. One reason for the big jump in price was that to get max efficiency out of the premium units, you had to replace the air handlers too. I ended up buying the most efficient single-speed model, which had a SEER of 16.

Also at that time, contractors were experiencing a lot of problems with the top of the line two-speed Carrier heat pumps.

It was the two-speed compressors that were troublesome, at least when I was shopping.

That's comparison is obvious. Less obvious is comparing the SEER 16 vs SEER 18 model.

 

Don't have much time to spend on this, have to catch a flight.

A lot of folks appear to be misinformed about how A/C works and what the power requirements are.

Check out this long, technical URL, and scroll down to the table that lists latent heat and power consumption from 1.5 tons to 5 tons.

http://www.udarrell.com/air-conditioner-ca...acity-seer.html

My Bryant SEER 16.5 2 ton unit only needs a 20 amp two-pole breaker. The old 1.5 ton SEER 6 (Or was it 8?) in my old home needed a 30 amp two-pole breaker. A/C compressors suck huge amounts of power.

An important reason I went with a variable speed furnace and two-speed compressor is indoor comfort. Most of the time in summer here it isn't so much hot but very, very humid. The indoor humidity will quickly reach 70-80%.

I can program my Bryant Evolution system to cool with Dehumidify priority: lowest fan speed and slowest compressor speed. At least 2-3x the moisture removal.

Otherwise with a conventional system you have to keep the house ice cold and even then it will still have 55-65% humidity. Not a pleasant home to be in.

 

<div class='quotetop'>QUOTE(priusenvy\";p=\"110772)</div>

Hmmm, I'm just sitting here wondering why my 1.5 kWh battery will only run my A/C for about 10 minutes before it has to turn on the ICE to recharge the battery. If it only consumes 300 watts during a peak cooling demand, then it should run off the battery for hours. Maybe it's broken. :wink:

 

Alright, my last post was just a little bit sarcastic.

I don't know anything in depth about A/C systems, and I can't imagine why the HVAC industry would misuse the units Watts in the way that it seems they do.

However, none of that matters. You can debate what "Watts" really means in regards to "cooling capacity" all you want, but at the root of it, it all comes down to batteries, motors, and electricity.

What I do know well, is electrical stuff. (Well, I kind of know it well, I do have a Masters in Electrical Engineering, but I've been a programmer for the last 15 years, so I've kind of forgotten a lot of it )

The compressor has a motor. The motor consumes electricity, and that consumption is measured in Watts. The battery has a capacity, and that capacity is measured in kWh, which is the amount of Watts it can deliver continuously for a duration of an hour.

The battery's capacity is 1.5 kWh, and I've read that the car utilizes only 60% of it's capacity. That would result in 0.9 kWh of battery capacity.

With a full charge of the battery (8 green bars), while sitting in park with the A/C on MAX COOL, you will deplete the battery down to 2 pink bars in less than 10 minutes. I don't know the exact time, but it's not longer than 10 minutes, so lets use that for illustration.

Lets further assume that the bars on the battery scale are linear, so if the entire range of bars represents 0.9 kWh, then the 6 bars you consume in 10 minutes represents 0.675 kWh.

So, 10 minutes is 1/6th of an hour. Simple math will determine that 0.675 kW for 1 hour equals 4.05 kW for 10 minutes.

Therefore, the A/C system must consume in the neighborhood of 4 kW of electricity when it is operating at it peak cooling demand.

So you guys can argue all you want over what "Watts" means when discussing "cooling capacity" and what "SEER" means, is, or should be. It doesn't matter, the battery tells you every thing you need to know.

The only way you could convince me that the A/C system consumes only 300 Watts is to demonstrate to me how to run the A/C off the battery, while in park, for 2.5 hours without the ICE turning on to recharge the battery.

 

<div class='quotetop'>QUOTE(priusenvy\";p=\"110772)</div>

I made a dumb (and obvoius) mistake in units conversion. SEER is Btu/watt-hrs. So if the SEER is 10, a cooling capacity of 15,300 Btu/h takes 1.53 kw, or around two horsepower. That makes a lot more sense. 4500 watts is way too high, but 2-300 watts was way too low.

 

Marlin You've hit the nail on the head! Air conditioning takes a lot of energy - it is that simple!

 

<div class='quotetop'>QUOTE(priusenvy\";p=\"110823)</div>

I've lost track. Is an SEER of 10 a good, bad, or average number?

Again, I know virtually nothing about A/C systems, home, car, or otherwise. However, I would be leary about assuming that the efficiency of a car A/C system is necessarily equal to the effeciency of a home system, or even in the ballpark for that matter.

The compressor on a home unit is 2'x2'x3' and the one in a car is about the size of a large coffee can. You would find similar size differences in the evaporator coils. It may well be that size constraints trumped efficiency during the design of the Prius A/C system (or any car A/C system for that matter).

 

<div class='quotetop'>QUOTE(thorn\";p=\"110832)</div>

The compressor runs off the hybrid battery, not the 12V battery. As such, I doubt there would be any user servicable fuses that have anything to do with power from the hybrid battery.

Toyota seems to use "Air Conditioning" in the most generic sense. They seem to use it to describe the complete process that maintains a desired temperature, regardless as to whether it is cooling or heating air.

My guess is the 15A fuse that also lists the "Multi-Information display", is for the control electronics, and the 10A one is for the blower.

 

<div class='quotetop'>QUOTE(Marlin\";p=\"110834)</div>

10 is low. In fact, I'm not sure you can buy a central air system rated below 13 now, at least in the USA. I think that's some kind of floor set by the EPA. If your unit is ten or more years old, it might as low as eight. 18 is about as high as you'll see for the most efficient units with two-speed compressors.

 

<div class='quotetop'>QUOTE(Marlin\";p=\"110834)</div>

I don't know where you would obtain numbers for the Prius A/C, or any other car for that matter.

Something sort of close would be a roof A/C for a motorhome. Here's a link to a Carrier A/C for a motorhome. It's rated at 15,000 Btu/h and consumes 1.6kw of electricity. That works out to an EER of a little over 10 (subtract the power for the blower fan). 15,000 Btu/h is very close to the 4500 watts of cooling capacity from that Prius manual page.

http://www.pplmotorhomes.com/parts/rv-air-...r-condition.htm

 

If you talk to most HVAC contractors and then try to pull a fast one on them by getting into a serious discussion of Superheat, Sensible Heat, Latent Heat, they'll run away screaming.

In most of the HVAC problems I've had as a homeowner, sloppy or incorrect installation was to blame, not the unit itself.

You just know you're dealing with a "slap it together" sort of person when you ask them how they will size the main ducts, air handler, evaporator, and condenser. And all they ask for is the square footage of your home. They won't even show up until they arrive with the components.

That, right there, means you will have a system operating nowhere near peak efficiency. More often than not it will be dramatically oversized. Read the URL I provided earlier, it's from a retired HVAC guy who got fed up with the scam artists in his profession.

By not matching the air handler, evaporator coil, ductwork, and condenser/compressor, you're nowhere near SEER 16. Maybe SEER 8. Just by having a variable speed furnace or air handler you will gain much improved indoor air quality, and at least 2-4 SEER units.

Friends of my parents who have a home in Mesquite, NV, have had one since 1997 and it seems flawless. They really had to hunt around for a knowledgible installer though.

True, I would never replace a SEER 16 with a SEER 18. But I would replace a SEER 8 with a SEER 18 in a heartbeat. If every home in the country had a very efficient A/C and heating system, we could get rid of a lot of this concern about brownouts, building new nuclear power plants, etc.

Ironically, the points you raised about HVAC can also be applied to our Prius: in most cases the car will NOT save you enough money to be "worth" it, there is a question mark about future repair costs, and good luck finding a tech who knows anything about it.