Why would a car lose a lot of power with the air conditioning on?

The air conditioner's compressor uses lots of power, typically, around 20 to 30 horsepower. In modern cars with smaller engines and lower horse power outputs, that's a lot. Think of it this way, your average 4 cylinder engine only outputs around 150 HP, if even that much. 30 HP is 20% of that power, that is a huge loss. There's a reason air conditioning uses more fuel.

If, however, your power loss seems to exceed what is described above, you may have other problems that should be diagnosed.

Also, two cheap ways to help keep your car's power up, are regular air filter and oil changes.