It's not only using your heater: it's the effects that cold weather has on ALL cars. Read on:

Getting low MPG in cold weather is not a problem with hybrid technology, per se. If it was, there would be ZERO drivers achieving EPA numbers in the winter months, and that is not the case.

There are many reasons for cars not achieving max MPG in the colder months...

Car engines must warm up to a certain internal operating temperature before they most efficiently use the fuel. Winter months and colder temps make that transition from cold start to most efficient take a little longer.

Also: new cars go through what is called a "break-in period" where the metal engine parts are still grinding against each other more rigidly, creating more friction, which also reduces efficiency. Over time, the "extra metal" shaves off and gets lubricated better (expecially if you use Synthetic Oil) and the friction is reduced, which affect MPG numbers. A lot of us with a TCH are still in the break-in period.

The hybrid systems are also tuned to work best with a warmed-up engine. So you will be using electric assist less often until your vehicle warms up.

Winter formula fuels used in most states hold less energy and this also hurts MPG.

Generally speaking, colder weather in many areas means "rain, snow, or slush or ice" on the roads - these things add additional resistance which the tires must push out of the way to contact the road, so they hurt MPG in that way.

Tire pressures also are lowered by the cold weather too. Most people fail to properly inflate their tires all year, but ESPECIALLY in the winter when it is cold and you might have dirty tires and you just don't want to deal with spending time in the cold airing up a dirty tire. Low tire inflation adds quite a bit of rolling resistance, which forces the engine to work harder to "push" the car forward, thus using more fuel.

Additional idling uses more fuel and hurts overall MPG, and many people in cold climes by habit or necessity are warming their cars up in the morning. That is good practice to stay warm, but BAD for MPG numbers. While idling in your garage or driveway or carport, it is getting 0 miles to the gallon !!

People who use the defroster in a Hybrid are many times using the A/C compressor to help reduce the humidity in the car and clear the fog. That is a MPG killer too. The compressor can be turned off, but usually by default it comes on.

Personally, I am looking into some kind of engine block or oil heater system to help combat the cold. My current tank, with mornings being in the upper 50s and low 60s for the past week has dropped to 35.2 after 50 miles.

See this science page for more explanation:

http://tinyurl.com/3s7lo