The efficiency of a portable AC, like any AC system, will depend on several factors like the units’ BTU’s, size of the room, how well the room is insulated and how much sun or shade there is. Keeping a fan running (especially a ceiling fan) while the room is occupied can also help reduce energy usage. With more air circulation it’s possible to keep a thermostat about 3 degrees higher and still feel the same comfort. Also placing the portable AC as close to the window as possible will help efficiency. The longer the distance that warm air must pass through the exhaust to outside, the more warm air which can escape through the exhaust hose back into the room. The exhaust hoses may not be perfectly insulated. You can improve this by putting some black foam insulation tubing around the exhaust hose.