Photo: Could solar power be the future of electric bicycles? The large solar panel mounted over the back wheel of this experimental bike powers an electric motor connected by a chain drive to the back wheel, helping the rider when he doesn't feel like pedaling. Using clean, green solar power would remove the problem of having to charge electric bikes with electricity generated from fossil fuels—and help to extend their range significantly. Photo by Warren Gretz courtesy of US DOE/NREL.
At the top end, the 20mph speed limit isn't an absolute, like the speed of light. It's just the most you'll get from the motor. I was able to get it up to 28mph on a flat road (there are no other roads where I live) in Turbo mode and 8th gear, but it took hard work to get there and maintain the speed. The easiest comparison is my mountain bike, where I've hit 23mph on that same stretch of road—and that took a heck of a lot of effort. Exactly how much effort does it take to ride? I have a 15-mile route that I ride on both my mountain bike and cyclecross bike. According to my Apple Watch, I generally burn around 1,400 or so calories on a brisk ride. I'll probably average around 13mph or so on my Marlin and between 16 and 17.5mph on my XO2. I averaged 14.9mph and burned a hair over 1,000 calories with the Bulls Cross E8.
Power-assist: Also known as pedal-assist bikes, these are the bicycle equivalents of hybrid cars: they're designed to be pedaled quite a lot of the time and electrically powered either when you're tired or when you feel like a bit of electric help (when you're going up hill, for example). Unlike full-power bikes, they don't have hub motors; instead, there's a separate electric motor mounted near the rear wheel and driving it either through the gear sprocket or simply by pressing against the rear tire. Where a hub motor is difficult or impossible to pedal without any power (because you're effectively turning it into a generator), power-assist motors turn easily with little or no resistance when you pedal. That gives power-assist bikes much greater range than hub-motor ones (as much as 80–145km or 50–90 miles).
As electric bike options continue to expand, more brands are integrating the battery more seamlessly. That makes them look sleeker (and more like a real bike). Batteries are expensive, so make sure there’s a good way to lock the battery to your bike if you’ll be keeping it outside. Overall weight is important. Some battery and motors can add 15 pounds or more to the bike. With assist, you won’t feel that much when you’re riding, but you will if you have to carry your bike up stairs or lift it onto a bike rack.
The federal Consumer Product Safety Act defines a "low speed electric bicycle" as a two or three wheeled vehicle with fully operable pedals, a top speed when powered solely by the motor under 20 mph (32 km/h) and an electric motor that produces less than 750 W (1.01 hp). The Act authorizes the Consumer Product Safety Commission to protect people who ride low-speed electric vehicles by issuing necessary safety regulations. The rules for e-bikes on public roads, sidewalks, and pathways are under state jurisdiction, and vary.
E-bikes mostly use motors and battery options from a few major suppliers: Bosch, Yamaha, Shimano, and Brose. A few other brands exist, but are less reliable or powerful. Some, like the Yamaha system, have more torque, and others, like Bosch’s Active Line, are nearly silent. But, generally, all four make good options. Look for motor output (in torque), which will give you an idea of total power. Just like car engines, more torque equals more power off the line and more boost to your pedaling. But watt hours (Wh) is perhaps a more important figure to use—it takes into account battery output and life to give a more accurate reflection of power (higher Wh equals bigger range).
Parliament has decided to approve the speed pedelec – a type of super electric bike that can reach speeds of up to 45 km/hour – for riding on cycle paths. Danish Parliament has decided that as of July 1 2018 those operating the super bikes only need to have turned 15 and wear a helmet, while the licence and number-plate demands will no longer be in play. 
I bought the 52v 13ah for my 1000w BBSHD build and it is working great. My bike tears it up pretty good with this battery pack. Hopefully I don't run into any problems but so far so good. One thing I learned the hard way - if you use a 52v pack with the 1000w bbshd your battery meter will no longer be accurate. That was one tough ride home with my heavy fat bike!
I do not recommend hacking a bike into s-pedelec, i.e. make a 25km/h bike go at 45 or faster. If you have an accident you will be in trouble. Just get a legal version. In addition you will have good insurance. Of course, in countries like France or the UK that so far do not know S-Pedelecs, you will have to make a difficult moral choice.... At least, make sure that your rig is safe...
A new European product safety standard EN 15194 will be published in 2009. EN 15194 contains several new requirements for ebikes to be sold in European Union and European Economic Area, including weight and voltage limitations. EN 15194 also defines a specific name for EU approved electrically-assisted cycles, EPAC - "Electrically Pedal Assisted Cycle".
And let’s not forget the economic advantages of owning an e-bike. The annual cost of running a new family car is, on average, about $9,000 per year. Running an electric bike costs around $400 per year. And while filling a gas tank costs around $30, recharging an electric bike battery costs only about 50 cents. A tank of gas may get you further, but not 60 times further!