It depends on how tight your house is, it's installation & etc.
My 24,000 BTUh Sanyo is cooling 1,200 sq ft down to 75 (it's 85 outdoors & very Humid).
The whole house is very comfortable and the Sanyo is just idling along at 420 to 440 watts.
From 5 feet away, It 'feels' like it's pumping out 4,000 to 8,000 BTU.. (A wild guess).
We might spend 70 or 80 cents on cooling today! (20 cents per kWh).
My system has a clear shot right down a hall to the master bedroom,
and it does the job..
Sanyo 24KHS72 AC/HP DIY install project - Page 7 - EcoRenovator
If you look at each of your rooms and can tell what size of window AC
would work in that room, and add them all up, (plus the hall)
you should get a good idea of how many BTUs you need.
Nice thing about inverter units, they don't use a lot of power,
if only a little cooling (or heating) is needed to maintain a temperature.
So, if you get one that's 12,000 BTU too large, it's not going to run
very hard and run up your Electricity bill..
The compressor and fan motor slows down and uses less power than
a 5,000 BTU window AC..
WikiAnswers - How many watts does it take to crank a 5000 btu air conditioner
I'm getting cold in here (down the hall in a small bedroom) and I see
the power meter says 420 watts right now..
From here, I can't hear it. I can hear the oil burner down in the basement,
but the Sanyo makes very little noise..
In the living room, the refrigerator is louder than the Sanyo!
It does make some fan noise when it's running hard in the morning
(heating) as it uses about 1500 watts for a while before dropping down
to 450-480 watts, to maintain the temp..