# What causes the increased power consumption of a window air conditioner in the afternoon?

I installed a new 8000 BTU window mount air conditioner on the weekend on a wall that is west facing. I plugged it into a Kill-A-Watt to monitor its power consumption and noticed in the morning, when the house and the outdoor temperature is at its coolest, it consumes 620-640 W. In the afternoon when the sun is on the unit, it consumes ~750W. Since the air conditioner is way undersized for the space it's trying to cool, it runs at max for the whole day (we shut it off at night since it's in our room and it's crazy loud).

In the morning, the indoor temperature is probably ~22ish C and the outdoor temperature is around 20C, and the afternoon the outdoor air temperature is around 29 C.

It didn't sound like the cooling fan had multiple speeds to account for an increased power consumption when the fins are hot. Is there anything else (electrically related) that could be causing the power consumption to increase by about 100 watts in the afternoon?

The air conditioner has to work harder when it has to work against a higher temperature difference. This changes the relative pressures inside the cooling unit, which puts more back pressure on the compressor, which puts more load on the motor, which draws more power.

The temperature coefficient of the wire resistance is a tiny irrelevant fraction of the difference. Copper does change resistivity a tiny amount over the temperature range you mention, but in either case the copper wires are only responsible for a very small fraction of the overall electric power going into the air conditioner.

• Put some sort of thermometer outside in the same sunlight as the unit. Graph the outside temp vs power consumption and I bet you will see they track each other. Jul 5, 2011 at 14:06
• en.wikipedia.org/wiki/Boiling_point - As Olin said, with higher temps, the compressor and motor would have to work harder to liquify coolant Jul 5, 2011 at 14:09
• Aha, I hadn't thought about the compressor and the gas in the system. I also initially discounted the wire since 100W would have meant a fairly significant resistance increase from the temperature. Jul 5, 2011 at 16:37

The resistance of a metal increases linearly with temperature.

Thus spake Wikipedia.

This means that as it gets hotter, so the resistance of the wires, motor windings, etc all increases.

The formula to calculate the temperature coefficient of resistivity is:

$$\R(T) = R_0[1+\alpha(T-T_0)]\$$

where $$\T\$$ is its temperature, $$\T_0\$$ is a reference temperature (usually room temperature), $$\R_0\$$ is the resistance at $$\T_0\$$, and $$\\alpha\$$ is the percentage change in resistivity per unit temperature.

(again Wikipedia)

As the power consumption formula is $$\P=R\times I^2\$$ this means that as the resistance increases, so does the power consumption for the same amount of current.

So for a resistance of say 100Ω at 20°C, rising to 120Ω at 30°C (dummy figures for demonstration - not actually meaningful), and a device that draws 5 amps you would see:

20°C:

$$\P = 100 \times 5^2 = 100 \times 25 = 2500W\$$

30°C:

$$\P = 120 \times 5^2 = 120 \times 25 = 3000W\$$

Obviously for your air conditioning unit the values will be wildly different.

• and some of the components are in the airflow being cooled, others are on the hot side. Jul 5, 2011 at 13:54
• @dwelch In an air conditioning unit there are very few electrically conducting items in the cold airflow - a few detectors etc. The majority of the power consuming items (motors for the fans etc) are in the passive (which will be being heated by the sun) areas, or hot areas. Jul 5, 2011 at 13:56
• Agree and understand Matt. You have the fan motors of course which are power consumers, but not compared to the motor driving the compressor...The point was to try to connect your metal gets hotter to the question at hand and how to use it. It seems pretty clear though the poster answered their own question, the unit works harder when the sun is on it and the room it is cooling, not so much because of hot metal, but because it is an air conditioner fighting higher ambient heat. Jul 5, 2011 at 14:12
• The difference in resistance for the conductors in the motor will be tiny for the differences in temperature that are being mentioned here. The difference in outside ambient is only 9 degrees and the temperature coefficient for copper is only 0.0039/degree. 100ohm will only rise to 103ohm as a result of the ambient temperature change.
– uɐɪ
Jul 5, 2011 at 14:14
• @Matt: It still doesn't matter. The power lost in the wires is small compared to the power the motor uses. The change in the power lost to the wires between cool and warm wires is even smaller. This whole wire temperature thing is a distraction. It can't come remotely close to accounting for the 20% power change the OP observed. The really funny thing is that the wire temperature actually works in reverse. Hotter means more resistance with means less current and less power ultimately drawn by the whole unit. Jul 5, 2011 at 15:24