Legal E bikes motors only supply power when the pedals are turning.
If power is available without pedalling, the motor will most likely have motor over the legal limit of 250w and probably only need pedalling if the battery is flat. Having looked at converting my "shopping" there are motor options from 250w (legal with torque or cadence sensors) through to 1kw or more with thumb or twist grip throttles (only legal off-road).
Worth noting (having converted my bike to an ebike) that there's a fair bit of wiggle room within the 250W rating as that is a continuous power rating. The only requirement is the motor doesn't overheat operating at that power level with 100% duty cycle. Even the big names, like Bosch, take quite some leeway within the legal definition.
There's plenty of motors marked 250W that are delivering peak powers considerably higher than this, 6-700W+ is not uncommon.
I went for a very light, relatively low powered geared hub motor, with around 40Nm of torque. On the flat it can propel me with minimal effort if I want it to (and I'm not exactly svelte!) but on steeper climbs needs user input. In reality I bought it to get me fitter and flatten the hills around here that reduce my motivation to ride. It's fitted with a full-assist throttle, but I rarely use it and the conversion has been a revelation, I *want* to go cycling again, but most of the time I'm on minimal assistance and under my own steam and my heart rate data shows I'm working almost as hard, most of the time, as I did unassisted.
I did hit a 20% gradient at the weekend, at the end of an already torturous climb and had to get off and push at that point! It did make pushing the bike easier though, as it could propel itself without my fat a*** on it!
Going off-topic here, but this is a really interesting recent study on e-MTB's:
https://formative.jmir.org/2019/3/e13643/
There's some interesting takeaway from that, riding both types of bikes “placed the vast majority of participants in the vigorous-intensity heart rate zone,”. The average heart rate of a test subject riding an e-bike was 93.6 percent of those riding conventional bikes. In addition electric bikes were identified to be an “excellent form of aerobic or cardiovascular exercise, even for experienced mountain bikers who regularly engage in this fitness activity.”
Particularly interesting was the vast majority of the test subjects said they didn’t feel like they got a workout while riding an e-bike — despite heart rate monitors and fitness trackers indicating that most participants experienced “vigorous” levels of exercise. Exercise that doesn’t feel like exercise seems like a major breakthrough to me and is close to my own experience.