the btu on a casement aircon is much much different from the btu on a portable aircon......casement aircons are way more efficient....but for electric consumption u should mainly look at the wattage on the electrical equipment.....as rule of thumb portable aircon is only 50% as efficient at best depending on a number of factors...so u need twice as much btu to achieve the same cooling as a casement aircon....for example casement aircon 5000 btu,portable aircon needs at least 10,000 btu to cool the same room......this is why portable aircon are seldom labelled as energy efficient units......
first of all the entire unit of the portable aircon is inside the room,which means a huge amount of heat is generated by the unit while the compressor is running which is additional load of heat the aircon must carry in order to remove heat from inside the room to exterior,this contributes to a 30% loss in efficiency.
window casement aircon most of the unit is outside of the room(including the compressor)which means the only part of the aircon thats in the room is the aperture that blows the cold air in,this means the room cools way faster,
second are the exhaust pipes of the portable air con,depending on whether u have 2 hoses or single hose......single hose are more inefficient,single hose aircon only blows hot air out of the room,since there is no intake pipe to take in air from outside the room,the air con sucks in air from inside the room(which is already cooled)to cool the compressor and blow the hot air out of the room......this creates a vacuum like effect in the room,since theres less air inside the room,the lower air pressure sucks in air from outside to room to fill the inequilibrium.....that means hot air is gradually leaking into the room from outside thus further wasting ur cooling power....
thirdly the insulation of ur exhaust pipes.......
