My air conditioner is controlled by a thermostat like most are. When the set temperature is exceeded, the system turns on the air conditioner using the R and Y wires of the controller. As I understand it, this causes the furnace controller board to activate a relay which turns on the outside airconditioning fan and compressor, and the inside fan is enabled as well (I also connect R to G to enable the circulation fan.)

What is curious to me is the drop in power draw as it gets later in the day. I would have thought an air conditioner is either on or off - drawing the same amount of current (ignoring blockage in air intake etc - assume these are constant). Look below:

Power Draw Furnace+AC

Each block is when the thermostat turned on the air conditioner. Green indicates the compressor / fan outside, blue is the power draw from the furnace.

Why does the power draw slope down like that?

At 18:00 it seems to be at its highest, then each time it turns on, the power draw reduces even during a single cycle, dropping from a maximum of about 3.2kW down to 2.66kW.

The only thing I can think of is that it correlates with temperature, and that if it is cooler outside the compressor does not work as hard. See temperature for same time frame:

Outside Temperature

UPDATE: See below for an overlay of line voltage, outside temperature and the power draw by the aircon. I cannot see a correlation between line voltage and power draw other than the voltage dip as the compressor turns on, which is to be expected. The only correlation I see is with temperature but I am not sure I understand why. Red arrow shows slope in power draw, blue arrow shows similar slope in temperature and purple arrow shows line voltage - fluctuating randomly.

Overlay of Voltage, Temperature and AC Power Draw

  • 2
    \$\begingroup\$ The line voltage and outside temperature may change during the day, and both can affect the power through the compressor. Measuring those may give you more insight. \$\endgroup\$ Commented Aug 19, 2022 at 23:37
  • \$\begingroup\$ I updated my question with an overlay of the line voltage. Does not correlate to the power draw as far as I can tell. \$\endgroup\$
    – pwnell
    Commented Aug 20, 2022 at 0:38
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    \$\begingroup\$ The current draw of a motor depends on how much resistance there is to its motion. For an air conditioner compressor, it makes sense that that would correlate with the pressure in the coolant loop in some way — and it makes sense that that would depend on both the outside temperature and on how long it's been since the system was last running. But I'm not good enough on the physics to give a lucid explanation of exactly why that's so :) \$\endgroup\$
    – hobbs
    Commented Aug 20, 2022 at 0:49
  • 1
    \$\begingroup\$ This is actually a mechanical engineering question, but essentially an AC works by compressing gas into liquid, cooling the liquid back to ambient, and then evaporating the liquid to produce refrigeration. The higher the ambient temperature, the more work the compressor has to do to compress the gas into a liquid and the less efficient the unit will be. This is logical, if the compressor didn't have to do more work to get larger temperature differences you could get free energy out by cooling more than you needed and using the excess delta T to power an engine which could run the compressor! \$\endgroup\$ Commented Aug 20, 2022 at 1:11
  • \$\begingroup\$ Measure the condenser temperature your current will follow that, the hotter it is the higher the head pressure the more power required. Much easier then measuring the head pressure. Charles's & Boyle's law in action. \$\endgroup\$
    – Gil
    Commented Aug 20, 2022 at 5:36

1 Answer 1


As the outside air cools down the effective consumed power of the ac will go down. This is specified in a table of the datasheet for the external compressor unit.

Here is an example of such a table from a heat pump unit.

enter image description here


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