"Electric-assisted bicycle" means a bicycle with two or three wheels, a saddle, fully operative pedals for human propulsion, and an electric motor. The electric-assisted bicycle's electric motor must have a power output of no more than one thousand watts, be incapable of propelling the device at a speed of more than twenty miles per hour on level ground, and be incapable of further increasing the speed of the device when human power alone is used to propel the device beyond twenty miles per hour.[66]
Photo: This typical electric bicycle, a Sanyo Eneloop (now discontinued), had a range of about 30–55 km (17–35 miles) and a top speed of around 24 km/h (15 mph). Note the 250-watt hub motor on the front wheel and the 5.7Ah lithium-ion battery pack (black, marked "Sanyo," just in front of the back wheel). Picture by kind permission and courtesy of Richard Masoner, originally published on Flickr under a Creative Commons (CC BY-SA 2.0) licence.
The recommendation depends a lot on what you plan to use the bike for and how you plan to ride. That said, I would recommend at least a 750 (if not 1,000) watt motor, and, if you want reasonable range, 48 or 52v cells with a minimum rating of 19ah. For shorter rides, you can use a smaller battery. Multiply battery voltage by amp hour rating to get battery watt hours. Divide the watt hours by the motors power rating to get a general idea of how many hours of heavy use you might get. I’d also recommend a mid drive as opposed to a hub drive, or to find a hub drive wheel rated to take your and the bicycle's combined weight.
None of this would matter if the VanMoof Electrified S2 (and its close relative the X2) wasn't fun to ride, but it is a blast. Like the Brompton, it pulls off the neat trick of powering you along but giving the illusion that you're doing the work, reacting quickly and cleverly to the speed of your pedalling and the difficulty of any incline you're on. 
As is the case with any e-bike assembled from Chinese parts, Wing has its share of clones that can be found for sale on sites like IndieGoGo and Kickstarter. For example, the Danish company Strom Bikes has a model for city biking that also looks incredibly similar to Wing’s Freedom — and is apparently several hundred dollars cheaper too. This is also commonly found with electric scooters, with startups buying Chinese-made models and rebadging them for sale in the US.

Motor: We have a blog post showing some motor differences: https://www.ebikekit.com/blogs/news/its-whats-inside-your-motor-that-counts Cabling is probably the #1 issue when something goes wrong. Good connectors, good cables, and good assembly (bike shop!) are crucial Strain reliefs on all cable joints! Where you have a junction box or a connector, the stress is concentrated over a small area near this stiff spot on the cable. This can cause small breaks on the internal wires, causing a disconnect or a short. This is a major reason for failure. Strain reliefs are more expensive – you need special tooling, skilled labor, and appropriate materials to...
The Riese & Müller Load Touring HS is billed as “the ultimate minivan of e-bikes,” and it holds up to that claim. With a low center of gravity (aided by the 20-inch front and 26-inch rear wheels), the Load is easy to handle. Tektro hydraulic disc brakes add control, and front and rear suspension provide comfort. The Bosch motor offers an assist up to 275 percent of your effort until you hit 28 mph, when it cuts out. Two 500Wh batteries give you 12 hours or more of range at full power. It’s capable of toting up to 220 pounds of pets, people, and less-animate cargo. R&M also sells a double child seat for kids up to age 6 and a child-seat fastener for your youngest passengers.
5. Prizes: The approximate retail value of all prizes is $1361. The prizes are: Performance E-BikeKit or Heavy Duty E-BikeKit. You are not guaranteed to win a prize and your chance of winning is dependent on the total number of eligible entries received. Actual/appraised value may differ at time of prize award. The specifics of the prize shall be solely determined by the Sponsor. No cash or other prize substitution permitted except at Sponsor's discretion. The prize is nontransferable. Any and all prize related expenses, including without limitation any and all federal, state, and local taxes shall be the sole responsibility of the winner. No substitution of prize or transfer/assignment of prize to others or request for the cash equivalent by winners is permitted. Acceptance of prize constitutes permission for Electric Bike Technologies INC. to use winner's name, likeness, and entry for purposes of advertising and trade without further compensation, unless prohibited by law.
There is a lot of information on the internet about electric bikes. Hopefully you have found lots of useful information right here, on this website. Based on this information, you may already have a preference for a specific electric bicycle and / or (technical) components. Your friends that ride e-bikes can also advise you based on their own experiences. But there is only one way you will know if you really make the right choice: take a test drive. During a test drive you will discover how close the riding experience is to your expectations.
This article did a good job of trying to justify the assist speed limits but fell short of detailing how that can ever be effectively enforced. In reality the only enforcement that is going to work is applied speed limits to use of bikes and ebikes. For example, it makes sense that a bike lane on a street that the speed limit of vehicles is the speed limit of the bikes/ebikes. On sidewalks and shared pedestrian paths the speed limit probably does need to be in the 15-20mph range to match traditional bikes speeds on those paths.
To qualify as an electric-assisted bicycle under state law they need to have a seat and fully operable pedals for human propulsion, meet federal motor vehicle safety standards, an electric motor that has a power output of not more than 1,000 watts, maximum speed of not more than 20 mph (electric motor and human power combined), disengages or ceases to function when the vehicle’s brakes are applied, two or three wheels