My experience as an e-bike commuter is that using an e-bike is still good exercise. From what I have read about in media, scientists seem to agree. There are several reasons for this:
- The motor assist requires that you pedal, and is proportional to how hard you pedal. Therefore, you always need to pedal when going uphill, and if you want to get uphill faster, you need to pedal even harder.
- Even when going flat or slightly downhill, you need to pedal if you want to move. And if you want to move faster, you must pedal harder.
- There is a legal speed limit (EU 25 km/h) where the assistance cuts off. If you want to go faster, you must pedal as hard as on a regular bike (or harder, since an e-bike is heavier than a typical bike).
- Setting motor assistance to the highest setting, at least on my bike, will result in a slightly uncomfortable cutoff at 25 km/h. It also eats battery faster. Therefore I usually set motor assistance to lowest setting (but not completely off), except on very steep hills.
When arriving at work after 45 minutes e-bike commute, I feel like I've had a good workout, and usually take a shower, but I'm not so sweaty that I absolutely must take a shower before seing anyone else. But it's all up to you how hard you decide to pedal.
The good thing about e-bikes is that they are a fun and healthy way to move around on, for just about everyone with “normal” health. As long as you have good enough health to risk riding a regular bike, you're good to go. But if you are extremely fit, and want to get maximal workout per hour, you should probably not add a motor to the mix.