how much drag/resistance is there if the power is off, is the bike still rideable without power?
On my electrified bike everything except the frame is hefty longlife gear. (The frame is specially drawn Columbus tubing and lugs and, believe it or not, is lighter than my workalike Gazelle and Trek ali frames.) So the central motor I have now didn't feel all that much heavier than the hefty bottom bracket it replaced, and front hub motor I had was not at perceptibly heavier than the SON dynohub it replaced. Neither item causes undue drag and, if you're using the bike solely as a pedal bike, you'll soon get used to their extra weight, same as when you add an extra bottle of water for touring away from home. That sort of level, soon accustomed to. Theoretically there should be some mechanical drag but I've never noticed it.
The battery is an entirely different story. It's
heavy. Add the battery to the bike and you'll feel its weight on your arms as you tilt the bike to get on it. You'll also feel it as you break the bike away from standstill on the pedals unless you gear way down, which I don't normally do, gearing down only three stops from my normal do-everything gear of 11. Once you're up to speed, you don't feel it much. So you learn to break the bike away from standstill on the throttle and to let go once you can pedal comfortably, which in my case is about walking pace. My normal riding pace on the flat was only about 15kph (my pedal pals include ladies and gentlemen of a certain age, out for a social ride, and we talk as we ride), and it is the same with motor and battery, so the distance between walking pace and flat riding pace, through which the bike must be accelerated, is not huge. However, I live in very hilly terrain, and ride the lanes in the hilliest parts of it, because they're the safest (my town lies on main roads that are lethal, and that's putting it politely) but still all my rides average 15kph, precisely because of the motor. Besides the breakaway of a heavy, and often loaded (even on day trips I carry painting gear), bike, I use the motor to fill in near the tops of hills rather than gear down and slog it out with my blood pressure and heart rate rising dangerously. In fact, I regulate my effort by my heart rate and cut in the motor not at x speed but at x heart rate. I ride with my physician and he is impressed at how well I manage to regulate it, and perhaps more importantly, how fast my heart rate falls again at the top of the hill.
So, while either a hub or a central motor on well-equipped touring or commuter bike doesn't make enough mechanical drag to be noticeable, and will be either not at all or only slightly noticeable for weight, the battery simply by its weight is definitely a drag factor. It should be said that I always buy the highest capacity battery available for my motors, and weight is directly related to capacity (you can for instance power these motors with low capacity but lightweight Bosch rechargeable drill batteries -- quite a fad in America where weight weenies and offroad speed freaks intersect).
Neither the central nor the hub motor offers any appreciable aero drag at speed to over 50kph downhill, but then I didn't expect them to, as I had relevant experience from the time when I set my own personal (truck assisted) ton-up record on a newly resurfaced downhill lane -- on a fully equipped Gazelle Toulouse (a Dutch vakansiefiets, which is a sort of extra-luxurious fast tourer or commuter, see photo at
http://coolmainpress.com/BICYCLINGgazelletoulouse.html ) on which the only alterations I made for the speed trial was to inflate the tyres to the permitted max, raise the seat, and drop the bars and swivel them almost vertical on the toollessly adjustable stem to give me a flat back as if on drops, and loosen the front disc caliper so it didn't cause me a high speed face plant. No perceptible aero drag from the dynohub, which is pretty much the same size as a decent front motor from 8FUN/Bafang; if you were to push me, I'd say the chain cover made more aero drag than a bare chain or certainly a Hebie Chainglider would have, and certainly more than the dynohub which is after all mostly behind the tyre.
All of that said, drag, either weight or aero, is the wrong question while you're cycling in civilization (and you won't be riding an ebike away from electrical points, so civilization is the only reference). There may be some weight drag but it is irrelevant unless you ride stupidly out of your battery's range because as long as you have battery current the motor compensates for the battery's weight, and aero drag is negligible. Your questions would be better aimed at battery weight, distance achievable, battery life-cycles (the battery is the most expensive and heaviest item on your bike, and the one with the shortest life), and battery placing for best roadholding and handling. Also, you should regard not only the battery but the motor as consumable items (I burned out a quality hub motor at about 3500km -- maybe it was underspecified for the job, though it seemed plenty strong enough in use), though hub motors either bare or wired into rims are cheap (it's controls and the battery that make the full kit expensive) and central motors will eventually be cheaper than now.
I have a really good 14.5Ah battery, which I charge after every ride, even just to the shops and back, and my rides are generally 10-15 miles. The battery, used only to fill in, has never once refused to deliver all the current I asked of it. It generally arrives home still about three quarters full with all the green lights still lit (this isn't a tip about buying a smaller battery -- the rate of delivery of current, C, is related to the battery size and state of charge, so you should as a first principle buy the biggest battery available, regardless of your needs). I reckon my battery would carry me on the flat at 15kph without pedaling for 40-50km, but I haven't tried it, among other reasons because the only piece of flat road here is dangerous and short so that i would have to cross it many times to knock up 50km, and anyway it isn't an electric motorbike but a pedal bike with electrical assistance for the steepest hills. At a commuting speed of say 25kph the drain would be disproportionately greater.
BTW, the problems that happen with batteries (unexplained sudden total discharge, anyone? -- it happens to my iPhone and iPad sometimes, though mercifully never yet to my bike battery) means that you must have a transmission with a low range suited to hauling a dead battery home from wherever you are. That means that you can't do without the Rohloff box, even though at first glance it may seem you don't need as big a range with a motor as with leg power only. Furthermore, while a front hub motor will probably be safe with a Nexus or Alfine hub gearbox, a central motor will sooner or later rip apart anything but a Rohloff or (perhaps, I haven't tried it) a NuVinci HGB. Or with a front hub motor you can safely choose derailleurs. With a rear hub motor, a topology now past its sell-by date, you're forced to have derailleurs. In all cases the gear spread should be considered in view of the sort of elevations over which you may be lugging home a dead battery. Never happened to me, and when it does I'll call my driver to come get me and the bike and the dead battery, but then I'm hardly ever more than an hour or two from home, which can't be said for probably the majority of adventurous cyclists here.
