I've often been vehement in my condemnation of electrically assisted bicycles. I've called them a pox on cycling, in fact. You might well wonder why?
I dislike them for a number of reasons, but lets look at the simple practicalities. Electric bicycles offer the illusion of an easier ride, and make no bones, it's an illusion. There are a number of factors which come into play, but first and most important is build quality.
If you're considering an electric bike, you're probably new to adult cycling. You probably haven't ridden since you were a kid and don't believe yourself fit enough to ride far under your own power, so I'll compare the typical electric with the typical entry level department store hybrid bike.
At K-Mart, a hybrid won't set you back much more than $400 including helmet, gloves, pump, spare inner tube and plastic tyre levers, plus maybe a saddle bag to carry the last items. The average, rechargeable, electric-assist bicycle will cost between $1000 and $2000 depending on the model.
The K-Mart bike (probably a huffy) will have lightweight 700c wheels that glide over bumps but spin up easily. The overall weight will be no more than 13kg and the frame will be a tig welded alloy model with forged dropouts for the axles and rack mounts. The build quality isn't brilliant, but it's good enough to get 5 years of good leisure cycling, even with a bit of commuting. It will feel fast and easy to ride for at least half of that (or all of it if you have it regularly serviced.)
The basic electric bike, without its battery, will come in at 20kg, 25kg with the battery. It will have small, 20" wheels, a heavy, motorised hub in the rear, and this wheel will be hard to spin up without using the battery.
The build quality will be a roughly welded high-tensile steel frame with cheap brakes. There will be extra tubing required to hold the battery, and the steering geometry will be for lower speeds than the hybrid mentioned above.
At this point, the k-mart cheapy wins out already.
OK, you say, but there's an electric motor to accelerate the heavier machine. Yes, if you call that acceleration. You're still going to have to pedal to keep up with even quite sedate cyclists. You've got 10 or more extra kilos to get moving than they do, on smaller, softer wheels than theirs and a 200watt motor (the legal maximum for an electric bicycle) is barely giving you enough to pull the extra weight. You will have to pedal to get moving, probably as hard as any other cyclist.
The elctric bike has an "optimum design speed" which is chosen for a compromise between best torque at take-off and reasonable cruising speed. Above this speed you start to work against the motor as it becomes a "generator" and starts producing electricity. So, while your cycling friends are coasting downhill at 30, you're effectively speed limited to lower/mid 20s, and with that steering geometry, you're not going to want to keep up with them anyway. (Speed wobbles are not fun!)
Now, I'm not going to lie and say you won't enjoy a good ride on one of these. They are making some aspects of cycling easier. Light hills with electric assist are easier than a conventional bike, although not really any faster, and cycling is fun on a nice day over a nice course. The trouble is, how far can you go?
On a conventional bike, a beginner's range over easy terrain is 50km, say 25km out and 25 back with lunch in the middle. That's nearly twice the range of cheaper electric cycles. Maybe you don't believe you could ride that far without the electricals. On a "golden ride", where you're having a good time, you won't feel that 50km until you get home.
If you find you've gone 25km with your friends, you're going to be riding 10km without electric assistance. The last 10km, when you're on your "last legs." Your friends conventional bikes are probably also tired, but their bikes are no harder to pedal than when they set out. Yours now feels like it weighs twice as much as it did at the start and you have not much left in your legs - remember, you still have to pedal an electric bicycle to get the 40km range and that last 10 will be harder work than your friends have put in for the whole trip.
Now, what if you go to a specialty bike shop and buy a $1000 hybrid bicycle rather than the K-Mart conventional or your electric bike? The bike will be fitted to you, it will be the right size to get the most speed and range out of your legs. It will have better componentry, especially brakes and gears. It will be lighter than the K-Mart bicycle and way, way lighter than the electric bicycle, especially the wheels, the most important area to save weight. Best of all, keep it maintained, it will last you maybe 12 to 15 years and it will feel twice as fast as the K-Mart bike.
Trust me, I've had a lot of cycling experience, and I have an electrical engineering background. I've looked into electric bikes because I love gadgets, but none of them match up to the simple pleasure of the human-powered machine. Spend your money wisely, buy a quality bicycle from your local retailer, it will serve you better than any gimmicky battery-powered thing.