Air conditioner

Some air conditioner related metrics.

Required cooling power

Required cooling power per volume
Poorly insulated room	50 Watt / m³
Average insulated room	40 Watt / m³
Well insulated room	30 Watt / m³

The above values are required cooling power, not the electrical power consumption. And since 1 Watt equals 1 Joule per second, the above numbers are the the amount of heat expressed in joules that is removed from your room every second.
The actual power consumption is much lower. The ratio between the two determines the air conditioner efficiency;

                      Watts cooling power
 Efficiency = ────────────────────────────────────
               Watts electrical power consumption

As air conditioner technology improves, this number gets higher, which reduces your electricity bill. So you want this number to be as high as possible.

Theoretical efficiency

Below the theoretical maximum performance:

      Inside temperature in Kelvin
 ────────────────────────────────────────
  Outside - Inside temperature in Kelvin

And the same for degrees Celsius:

     Inside °C + 273
 ────────────────────────
  Outside °C - Inside °C

Temperature vs performance
Outside °C	Inside °C	Theoretical maximum performance
40	27	24.1
40	24	19.6
35	27	38.5
35	24	28.0
35	0	8.8

The actual efficiency is always less than this number.

Units of measure

And yet an other SI vs imperial source of confusion;

BTU/hour vs Watt
1 Watt	=	3.412 BTU/hour
1 BTU/hour	=	0.293 Watt

When the cooling power is expressed in BTU/Hour, this somehow is always a whole multiple of 1000. My air conditioner for instance, is specified as both 9000 BTU/Hour and 2.7 kW. And 3.412 x 2700 does not precisely equal 9000 (it's 9212.4). So these are rounded numbers.

Sometimes the air conditioner efficiency is expressed as BTU/hour cooling power / Watt power consumption (or BTU / Wh). This gets you a number that's a factor 3.412 higher than the above method, in which case you have to divide the air conditioner efficiency value by 3.412 to get a more realistic number.
Always make sure you're not dealing with inflated numbers!

Efficiency

	Efficiency class	SEER
	A+++	≥ 8.50
	A++	6.10 - 8.49
	A+	5.60 - 6.09
	A	5.10 - 5.59

These numbers may actually change as regulation gets more strict.

EER

Energy Efficiency Ratio.
Efficiency measured with an outdoor temperature of 35°C and and an indoor temperature of 27°C at 50% humidity.

SEER

Seasonal Energy Efficiency Ratio.

ESEER

European Seasonal Energy Efficiency Ratio.