# Understanding relation between energy use, watts and amps

I'm looking into air conditioners and I've been unable to fully understand the relation between watts, amps and the energy ratings (in this case, the one used in the United States, the yellow Energy Guide label).

Both are 115 V.

Air conditioner 1: 8000 BTU, it is listed as 660 W cooling power, 6.2 A cooling current. The yellow label Energy Guide lists yearly costs at $65. The energy efficiency ratio is 12. Air conditioner 2: 9500 BTU, it is listed as 920 W cooling power, 11.5 A cooling current. The yellow label Energy Guide lists yearly costs at$50. The energy efficiency ratio is 13.8.

Questions:

• Why in both air conditioners the amps is not equal to watts divided by 115 volts? 660 / 115 = 5.74 A and 920 / 115 = 8 A.

• The air conditioner 1 clearly uses less energy than air conditioner 2. However air conditioner 2 is listed as cheaper in yearly costs. Maybe it has to do with the way it cools the room, using less energy over time? But then why watts are higher, if the energy consumption is lower?

Thanks for helping me understanding this!

• Does this answer your question? Input power rating on power supplies - how does max amp draw apply to 220V Commented Jun 5, 2020 at 14:06
• in short: it's average vs peak. Commented Jun 5, 2020 at 14:06
• What are the efficiency ratings? Usually given in SEER. Commented Jun 5, 2020 at 14:25
• 8000 BTU/hr is 2344w, so 660W is probably the power consumption and not the cooling power. In this case the nominal coefficient of performance would be 2344/660 = 3.5, which is reason for a cheaper AC unit. Commented Jun 5, 2020 at 14:32
• @LShaver air conditioner 1 is 12.0 and air conditioner 2 is 13.8. I don't know what they mean. Commented Jun 5, 2020 at 15:02

Why in both air conditioners the amps is not equal to watts divided by 115 volts? 660 / 115 = 5.74 A and 920 / 115 = 8 A.

Simply because you AC units have a power factor less than unity. The "W/V = A" is only true for DC circuits and AC circuits provided the load looks resistive to the source (i.e. it has a power factor equal to 1). The compressor in these units are typically inductive in nature; newer units are powered be variable-frequency invertors with active power factor correction so they are (almost) resistive loads and the "W/V=A" formula will approximate this case well.

The air conditioner 1 clearly uses less energy than air conditioner 2. However air conditioner 2 is listed as cheaper in yearly costs. Maybe it has to do with the way it cools the room, using less energy over time? But then why watts are higher, if the energy consumption is lower?

The "energy savings" language is purely marketing. Only until you include the full context (climate, hours used per day, cost of kWh, set temperature, room insulation, ect.) can you make a proper comparison.

The objective measures are: watts, seer rating and BTU's/h. The way you want to size your unit is based on the BTU's/h that will be required. Figuring out how many BTU's/h is beyond the scope of this questing but there are calculators for this. You need, at minimum, the room size, number of windows, and you climate zone. Next, you want the unit that draws the least power for the BTU/h requirement. Also, you don't want to oversize the unit thinking that's going to work better, what happens is a big ass unit will cool the room very quickly, but won't remove much humidity. So than your stuck sitting in a cold, clammy room that's not comfortable.

Obviously other factors will play a roll in your selection besides efficiency. The brand matters too. Some units are more reliable than others. What good is little more efficiency if your system breaks down every other month?

• While the "energy saving" label is marketspeek, A/C units pretty much do have different efficiencies and for particular case the difference can be as much as twice (or more, if we talk about heating). I am not familiar with US efficiency marks, but EU ones have honest measurements to back them. Commented Feb 5 at 8:26

The Energy Guide numbers include a range of assumptions about room size, annual hours of operation, and cooling load. The manufacturers have to include values for standby power consumption as well. All of this means that while watts, amps, and volts are fundamentally related, the values reported on the label aren't. They are the result of specific testing requirements and reporting criteria that manufacturers must adhere to.

What you can assume is that in a similar application (such as the same room in your house) air conditioner (2) will cost you about 30% less to operate annually than air conditioner (1):

($65 -$50) / \$50 = 30%

Here you'll find that assumed annual operation is 750 hours. More specifics are found in Appendix F to Subpart B of Part 430—Uniform Test Method for Measuring the Energy Consumption of Room Air Conditioners, which references ANSI/AHAM RAC-1-2015 Room Air Conditioners, the testing standard that manufacturers need to follow when developing the numbers that go on the label.