Obviously rider weight has the biggest effect on range, a lighter rider will go further than a heavier one - however that's irrelevant to the owner of a bike, simply because they are who they are and weigh what they weigh. Lighter bike then becomes the relevant measurement, so if you are looking at for example two Shimano Steps motored bikes, then the one weight closer to 20kg is going to get a fair bit more range than the one weighing 24kg.
As Gary says, then it comes down to choice of equipment and personal preference, but you can equip your bike with lower rolling resistance tyres and fiddle with the gearing to maximise you cadence to the optimum of the motor, and you will get more range, but then you might also end up with a bike ill suited to the riding you want to do.
If you want your eyes opened to what an EMTB might be like in a few years, try and test a BMC Alpenchallenge Cross - this is basically a flat bar gravel bike with the Shimano E8000 system, and weighs in at under 15kg - now obviously its not an EMTB, but its not dissimilar to a fully rigid MTB, and it feels so like a normal bike to throw around and piss about on, you hardly notice the cut off point of the motor, and its a lot fo fun to ride. The reduced weight also means you are looking at a lot more range.
When they get EMTB's down to this kind of weigh we will really be talking party time.
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