The basic calculation
A system averaging 3.5 kW while operating for eight hours a day uses about 840 kWh in a 30-day month. At 16.5¢ per kWh, that portion of the bill is approximately $138.60. Change any input and the answer changes with it.
Nameplate power is not monthly consumption
The nameplate helps identify voltage, current, and equipment ratings. It does not tell you how long the compressor operates or exactly how a variable-capacity system behaves. Thermostat history or whole-home monitoring can provide a better runtime estimate.
Why two similar houses differ
Outdoor temperature, indoor set point, humidity, insulation, air leakage, duct losses, equipment condition, filter restriction, refrigerant performance, and sizing all affect runtime. Auxiliary resistance heat on a heat pump can also change the electric load dramatically.
Where SEER2 fits
SEER2 is a seasonal efficiency rating, useful for comparing matched cooling equipment under standardized conditions. It is not the same thing as a wattmeter reading and does not guarantee a particular bill. Installation and house conditions still matter.