Ever wondered what it would be like to have Lance Armstrong pedal your bike for you? Well now you can find out, sort of.

About 15 companies are now offering bicycles with an electric power option -- as opposed to a purely engine-powered moped -- for around $1,000 to $4,000 -- and they are catching on with some green-thinking commuters.

The latest electric bikes from Giant, EcoBike, Currie Technologies and Ultra Motor, among others, can deliver around 500 watts of power at the flick of a wrist or a turn of the pedals. That is roughly what Armstrong could generate over shorter races in his prime.

The result is that you zip up hills or hustle along the street, silently passing all, but the most competitive two- wheelers.

One of the top sellers in the emerging market is the A2B, made by London-based Ultra Motor.

Some people buy the bike to commute, other people purchase the bike to use as a replacement for short automobile trips, said Paul Vlahos, vice-president of sales for the U.S. arm of Ultra Motor.

THE TECH

Privately held Ultra Motor has sold north of 1,000 of its electric bikes in the United States since launching the A2B model in September last year, said Vlahos. The Green Car Company in Bellevue, Washington -- close to Microsoft Corp's campus and cycling mecca Seattle -- said they have sold about 40 of the bikes this summer and are awaiting a new batch.

Priced at $2,699, the A2B is not cheap, but it is comparable to a high-end racing bike and less expensive than a standard motor scooter. It is sold at some independent bike shops, scooter dealers, specialist electric vehicle outlets and, for a short test period, about 20 Best Buy Co Inc stores in the western United States.

The machine, technically known as a light electric vehicle, qualifies as a bicycle under U.S. transport regulations, so you do not need a license to operate it and you can ride wherever cyclists are allowed. It is not the same as a moped, which generally has a gas engine and pedals fitted for emergency use only.

The A2B is meant to be pedaled, but the electric motor can be switched on at any time, with the use of a motorcycle-style throttle on the right handgrip, which generates speeds up to 20 miles per hour (32 kilometers per hour) on the flat. You can augment your pedaling efforts with the motor at any time, or switch entirely to electric power.

The aluminum frame with full suspension and disc brakes looks more like a sporty motorbike than a bicycle (see pictures at http://www.ultramotor.com/us/product/gallery). Baskets and racks for bags can be fitted to the frame to make commuting or shopping easier.

At 73 pounds (33 kg), the bike is two or three times the weight of an ordinary commuter cycle. Pedaling downhill or on the flat is reasonably comfortable, but as soon as the road pitches up, it is hard not to rev it up.

The built-in lithium ion battery will carry you around 20 miles and there is an option to put another detachable battery on the rear rack, doubling that range.

The bike has seven gears for normal pedaling, using a standard Shimano derailleur system. The Taiwan-manufactured motor, sealed into the hub of the back wheel, is brushless, like those used in PC hard drives and the Segway scooter.

The lack of friction from brushes -- which make contact with the moving rotor in conventional electric motors -- means electricity is converted to mechanical power more effectively. That gives the A2B good torque to clamber up hills and zoom from a standing start.

THE FUTURE

There have been versions of electric bicycles for more than 100 years and combinations of moped and bicycle -- such as the French velomoteur -- have achieved some popularity in Europe. But so far, electric bikes have not reached the U.S. mainstream.

The category is very much in its infancy (in the United States), said Vlahos. We think it is becoming more mainstream.

The company says the bike appeals to cyclists, but the chief market is car drivers.

The U.S. is historically so dependent on the automobile, said Vlahos. But you are starting to see cities invest more in cycling lanes. As more development comes back into an urban environment, you are starting to see a higher degree of bike- friendliness.

(Reporting by Bill Rigby; editing by Andre Grenon)